The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom by Charles Darwin, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
[6. CRUCIFERAE.—Brassica oleracea.
VAR. CATTELL’S EARLY BARNES CABBAGE.
The flowers of the common cabbage are adapted, as shown by H. Muller, for cross-fertilisation, and should this fail, for self-fertilisation. (4/1. ‘Die Befruchtung’ etc. page 139.) It is well known that the varieties are crossed so largely by insects, that it is impossible to raise pure kinds in the same garden, if more than one kind is in flower at the same time. Cabbages, in one respect, were not well fitted for my experiments, as, after they had formed heads, they were often difficult to measure. The flower-stems also differ much in height; and a poor plant will sometimes throw up a higher stem than that of a fine plant. In the later experiments, the fully-grown plants were cut down and weighed, and then the immense advantage from a cross became manifest.
A single plant of the above variety was covered with a net just before flowering, and was crossed with pollen from another plant of the same variety growing close by; and the seven capsules thus produced contained on an average 16.3 seeds, with a maximum of twenty in one capsule. Some flowers were artificially self-fertilised, but their capsules did not contain so many seeds as those from flowers spontaneously self-fertilised under the net, of which a considerable number were produced. Fourteen of these latter capsules contained on an average 4.1 seeds, with a maximum in one of ten seeds; so that the seeds in the crossed capsules were in number to those in the self-fertilised capsules as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed 3.88 grains, were, however, a little finer than those from the crossed capsules, fifty-eight of which weighed 3.76 grains. When few seeds are produced, these seem often to be better nourished and to be heavier than when many are produced.
The two lots of seeds in an equal state of germination were planted, some on opposite sides of a single pot, and some in the open ground. The young crossed plants in the pot at first exceeded by a little in height the self-fertilised; then equalled them; were then beaten; and lastly were again victorious. The plants, without being disturbed, were turned out of the pot, and planted in the open ground; and after growing for some time, the crossed plants, which were all of nearly the same height, exceeded the self-fertilised ones by 2 inches. When they flowered, the flower-stems of the tallest crossed plant exceeded that of the tallest self-fertilised plant by 6 inches. The other seedlings which were planted in the open ground stood separate, so that they did not compete with one another; nevertheless the crossed plants certainly grew to a rather greater height than the self-fertilised; but no measurements were made. The crossed plants which had been raised in the pot, and those planted in the open ground, all flowered a little before the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Some flowers on the crossed plants of the last generation were again crossed with pollen from another crossed plant, and produced fine capsules. The flowers on the self-fertilised plants of the last generation were allowed to fertilise themselves spontaneously under a net, and they produced some remarkably fine capsules. The two lots of seeds thus produced germinated on sand, and eight pairs were planted on opposite sides of four pots. These plants were measured to the tips of their leaves on the 20th of October of the same year, and the eight crossed plants averaged in height 8.4 inches, whilst the self-fertilised averaged 8.53 inches, so that the crossed were a little inferior in height, as 100 to 101.5. By the 5th of June of the following year these plants had grown much bulkier, and had begun to form heads. The crossed had now acquired a marked superiority in general appearance, and averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31 inches; or as 100 to 91. The plants were then turned out of their pots and planted undisturbed in the open ground. By the 5th of August their heads were fully formed, but several had grown so crooked that their heights could hardly be measured with accuracy. The crossed plants, however, were on the whole considerably taller than the self-fertilised. In the following year they flowered; the crossed plants flowering before the self-fertilised in three of the pots, and at the same time in Pot 2. The flower-stems were now measured, as shown in Table 4/29.
TABLE 3/29. Brassica oleracea.
Measured in inches to tops of flower-stems: 0 signifies that a Flower-stem was not formed.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 49 2/8 : 44. Pot 1 : 39 4/8 : 41.
Pot 2 : 37 4/8 : 38. Pot 2 : 33 4/8 : 35 4/8.
Pot 3 : 47 : 51 1/8. Pot 3 : 40 : 41 2/8. Pot 3 : 42 : 46 4/8.
Pot 4 : 43 6/8 : 20 2/8. Pot 4 : 37 2/8 : 33 3/8. Pot 4 : 0 : 0.
Total : 369.75 : 351.00.
The nine flower-stems on the crossed plants here average 41.08 inches, and the nine on the self-fertilised plants 39 inches in height, or as 100 to 95. But this small difference, which, moreover, depended almost wholly on one of the self-fertilised plants being only 20 inches high, does not in the least show the vast superiority of the crossed over the self-fertilised plants. Both lots, including the two plants in Pot 4, which did not flower, were now cut down close to the ground and weighed, but those in Pot 2 were excluded, for they had been accidentally injured by a fall during transplantation, and one was almost killed. The eight crossed plants weighed 219 ounces, whilst the eight self-fertilised plants weighed only 82 ounces, or as 100 to 37; so that the superiority of the former over the latter in weight was great.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
Some flowers on a crossed plant of the last or second generation were fertilised, without being castrated, by pollen taken from a plant of the same variety, but not related to my plants, and brought from a nursery garden (whence my seeds originally came) having a different soil and aspect. The flowers on the self-fertilised plants of the last or second generation (Table 4/29) were allowed to fertilise themselves spontaneously under a net, and yielded plenty of seeds. These latter and the crossed seeds, after germinating on sand, were planted in pairs on the opposite sides of six large pots, which were kept at first in a cool greenhouse. Early in January their heights were measured to the tips of their leaves. The thirteen crossed plants averaged 13.16 inches in height, and the twelve (for one had died) self-fertilised plants averaged 13.7 inches, or as 100 to 104; so that the self-fertilised plants exceeded by a little the crossed plants.
TABLE 3/30. Brassica oleracea.
Weights in ounces of plants after they had formed heads.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants from Pollen of fresh Stock.
Column 3: Self-fertilised Plants of the Third Generation.
Pot 1 : 130 : 18 2/4.
Pot 2 : 74 : 34 3/4.
Pot 3 : 121 : 17 2/4.
Pot 4 : 127 2/4 : 14.
Pot 5 : 90 : 11 2/4.
Pot 6 : 106 2/4 : 46.
Total : 649.00 : 142.25.
Early in the spring the plants were gradually hardened, and turned out of their pots into the open ground without being disturbed. By the end of August the greater number had formed fine heads, but several grew extremely crooked, from having been drawn up to the light whilst in the greenhouse. As it was scarcely possible to measure their heights, the finest plant on each side of each pot was cut down close to the ground and weighed. In Table 4/30 we have the result.
The six finest crossed plants average 108.16 ounces, whilst the six finest self-fertilised plants average only 23.7 ounces, or as 100 to 22. This difference shows in the clearest manner the enormous benefit which these plants derived from a cross with another plant belonging to the same sub-variety, but to a fresh stock, and grown during at least the three previous generations under somewhat different conditions.
THE OFFSPRING FROM A CUT-LEAVED, CURLED, AND VARIEGATED WHITE-GREEN CABBAGE CROSSED WITH A CUT-LEAVED, CURLED, AND VARIEGATED CRIMSON-GREEN CABBAGE, COMPARED WITH THE SELF-FERTILISED OFFSPRING FROM THE TWO VARIETIES.
These trials were made, not for the sake of comparing the growth of the crossed and self-fertilised seedlings, but because I had seen it stated that these varieties would not naturally intercross when growing uncovered and near one another. This statement proved quite erroneous; but the white-green variety was in some degree sterile in my garden, producing little pollen and few seeds. It was therefore no wonder that seedlings raised from the self-fertilised flowers of this variety were greatly exceeded in height by seedlings from a cross between it and the more vigorous crimson-green variety; and nothing more need be said about this experiment.
The seedlings from the reciprocal cross, that is, from the crimson-green variety fertilised with pollen from the white-green variety, offer a somewhat more curious case. A few of these crossed seedlings reverted to a pure green variety with their leaves less cut and curled, so that they were altogether in a much more natural state, and these plants grew more vigorously and taller than any of the others. Now it is a strange fact that a much larger number of the self-fertilised seedlings from the crimson-green variety than of the crossed seedlings thus reverted; and as a consequence the self-fertilised seedlings grew taller by 2 1/2 inches on an average than the crossed seedlings, with which they were put into competition. At first, however, the crossed seedlings exceeded the self-fertilised by an average of a quarter of an inch. We thus see that reversion to a more natural condition acted more powerfully in favouring the ultimate growth of these plants than did a cross; but it should be remembered that the cross was with a semi-sterile variety having a feeble constitution.
Iberis umbellata.
VAR. KERMESIANA.
This variety produced plenty of spontaneously self-fertilised seed under a net. Other plants in pots in the greenhouse were left uncovered, and as I saw small flies visiting the flowers, it seemed probable that they would be intercrossed. Consequently seeds supposed to have been thus crossed and spontaneously self-fertilised seeds were sown on opposite sides of a pot. The self-fertilised seedlings grew from the first quicker than the supposed crossed seedlings, and when both lots were in full flower the former were from 5 to 6 inches higher than the crossed! I record in my notes that the self-fertilised seeds from which these self-fertilised plants were raised were not so well ripened as the crossed; and this may possibly have caused the great difference in their growth, in a somewhat analogous manner as occurred with the self-fertilised plants of the eighth generation of Ipomoea raised from unhealthy parents. It is a curious circumstance, that two other lots of the above seeds were sown in pure sand mixed with burnt earth, and therefore without any organic matter; and here the supposed crossed seedlings grew to double the height of the self-fertilised, before both lots died, as necessarily occurred at an early period. We shall hereafter meet with another case apparently analogous to this of Iberis in the third generation of Petunia.
The above self-fertilised plants were allowed to fertilise themselves again under a net, yielding self-fertilised plants of the second generation, and the supposed crossed plants were crossed by pollen of a distinct plant; but from want of time this was done in a careless manner, namely, by smearing one head of expanded flowers over another. I should have thought that this would have succeeded, and perhaps it did so; but the fact of 108 of the self-fertilised seeds weighing 4.87 grains, whilst the same number of the supposed crossed seeds weighed only 3.57 grains, does not look like it. Five seedlings from each lot of seeds were raised, and the self-fertilised plants, when fully grown, exceeded in average height by a trifle (namely .4 of an inch) the five probably crossed plants. I have thought it right to give this case and the last, because had the supposed crossed plants proved superior to the self-fertilised in height, I should have assumed without doubt that the former had really been crossed. As it is, I do not know what to conclude.
Being much surprised at the two foregoing trials, I determined to make another, in which there should be no doubt about the crossing. I therefore fertilised with great care (but as usual without castration) twenty-four flowers on the supposed crossed plants of the last generation with pollen from distinct plants, and thus obtained twenty-one capsules. The self-fertilised plants of the last generation were allowed to fertilise themselves again under a net, and the seedlings reared from these seeds formed the third self-fertilised generation. Both lots of seeds, after germinating on bare sand, were planted in pairs on the opposite sides of two pots. All the remaining seeds were sown crowded on opposite sides of a third pot; but as all the self-fertilised seedlings in this latter pot died before they grew to any considerable height, they were not measured. The plants in Pots 1 and 2 were measured when between 7 and 8 inches in height, and the crossed exceeded the self-fertilised in average height by 1.57 inches. When fully grown they were again measured to the summits of their flower-heads, with the following result:—
TABLE 4/31. Iberis umbellata.
Heights of plants to the summits of their flower-heads, in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants of the Third Generation.
Pot 1 : 18 : 19. Pot 1 : 21 : 21. Pot 1 : 18 2/8 : 19 4/8.
Pot 2 : 19 : 16 6/8. Pot 2 : 18 4/8 : 7 4/8. Pot 2 : 17 6/8 : 14 4/8. Pot 2 : 21 3/8 : 16 4/8.
Total : 133.88 : 114.75.
The average height of the seven crossed plants is here 19.12 inches, and that of the seven self-fertilised plants 16.39, or as 100 to 86. But as the plants on the self-fertilised side grew very unequally, this ratio cannot be fully trusted, and is probably too high. In both pots a crossed plant flowered before any one of the self-fertilised. These plants were left uncovered in the greenhouse; but from being too much crowded they were not very productive. The seeds from all seven plants of both lots were counted; the crossed produced 206, and the self-fertilised 154; or as 100 to 75.
CROSS BY A FRESH STOCK.
From the doubts caused by the two first trials, in which it was not known with certainty that the plants had been crossed; and from the crossed plants in the last experiment having been put into competition with plants self-fertilised for three generations, which moreover grew very unequally, I resolved to repeat the trial on a larger scale, and in a rather different manner. I obtained seeds of the same crimson variety of Iberis umbellata from another nursery garden, and raised plants from them. Some of these plants were allowed to fertilise themselves spontaneously under a net; others were crossed by pollen taken from plants raised from seed sent me by Dr. Durando from Algiers, where the parent-plants had been cultivated for some generations. These latter plants differed in having pale pink instead of crimson flowers, but in no other respect. That the cross had been effective (though the flowers on the crimson mother-plant had NOT been castrated) was well shown when the thirty crossed seedlings flowered, for twenty-four of them produced pale pink flowers, exactly like those of their father; the six others having crimson flowers exactly like those of their mother and like those of all the self-fertilised seedlings. This case offers a good instance of a result which not rarely follows from crossing varieties having differently coloured flowers; namely, that the colours do not blend, but resemble perfectly those either of the father or mother plant. The seeds of both lots, after germinating on sand, were planted on opposite sides of eight pots. When fully grown, the plants were measured to the summits of the flower-heads, as shown in Table 4/32.
TABLE 4/32. Iberis umbellata.
Height of Plants to the summits of the flower-heads, measured in inches: 0 signifies that the Plant died.
Column 1: Number (Name) of Pot.
Column 2: Plants from a Cross with a fresh Stock.
Column 3: Plants from Spontaneously Self-fertilised Seeds.
Pot 1 : 18 6/8 : 17 3/8. Pot 1 : 17 5/8 : 16 7/8. Pot 1 : 17 6/8 : 13 1/8. Pot 1 : 20 1/8 : 15 3/8.
Pot 2 : 20 2/8 : 0. Pot 2 : 15 7/8 : 16 6/8. Pot 2 : 17 : 15 2/8.
Pot 3 : 19 2/8 : 13 6/8. Pot 3 : 18 1/8 : 14 2/8. Pot 3 : 15 2/8 : 13 4/8.
Pot 4 : 17 1/8 : 16 4/8. Pot 4 : 18 7/8 : 14 4/8. Pot 4 : 17 5/8 : 16. Pot 4 : 15 6/8 : 15 3/8. Pot 4 : 14 4/8 : 14 7/8.
Pot 5 : 18 1/8 : 16 4/8. Pot 5 : 14 7/8 : 16 2/8. Pot 5 : 16 2/8 : 14 2/8. Pot 5 : 15 5/8 : 14 2/8. Pot 5 : 12 4/8 : 16 1/8.
Pot 6 : 18 6/8 : 16 1/8. Pot 6 : 18 6/8 : 15. Pot 6 : 17 3/8 : 15 2/8.
Pot 7 : 18 : 16 3/8. Pot 7 : 16 4/8 : 14 4/8. Pot 7 : 18 2/8 : 13 5/8.
Pot 8 : 20 6/8 : 15 6/8. Pot 8 : 17 7/8 : 16 3/8. Pot 8 : 13 5/8 : 20 2/8. Pot 8 : 19 2/8 : 15 6/8.
Total : 520.38 : 449.88.
The average height of the thirty crossed plants is here 17.34, and that of the twenty-nine self-fertilised plants (one having died) 15.51, or as 100 to 89. I am surprised that the difference did not prove somewhat greater, considering that in the last experiment it was as 100 to 86; but this latter ratio, as before explained, was probably too great. It should, however, be observed that in the last experiment (Table 4/31), the crossed plants competed with plants of the third self-fertilised generation; whilst in the present case, plants derived from a cross with a fresh stock competed with self-fertilised plants of the first generation.
The crossed plants in the present case, as in the last, were more fertile than the self-fertilised, both lots being left uncovered in the greenhouse. The thirty crossed plants produced 103 seed-bearing flowers-heads, as well as some heads which yielded no seeds; whereas the twenty-nine self-fertilised plants produced only 81 seed-bearing heads; therefore thirty such plants would have produced 83.7 heads. We thus get the ratio of 100 to 81, for the number of seed-bearing flower-heads produced by the crossed and self-fertilised plants. Moreover, a number of seed-bearing heads from the crossed plants, compared with the same number from the self-fertilised, yielded seeds by weight, in the ratio of 100 to 92. Combining these two elements, namely, the number of seed-bearing heads and the weight of seeds in each head, the productiveness of the crossed to the self-fertilised plants was as 100 to 75.
The crossed and self-fertilised seeds, which remained after the above pairs had been planted, (some in a state of germination and some not so), were sown early in the year out of doors in two rows. Many of the self-fertilised seedlings suffered greatly, and a much larger number of them perished than of the crossed. In the autumn the surviving self-fertilised plants were plainly less well-grown than the crossed plants.
7. PAPAVERACEAE.—Papaver vagum.
A SUB-SPECIES OF Papaver dubium, FROM THE SOUTH OF FRANCE.
The poppy does not secrete nectar, but the flowers are highly conspicuous and are visited by many pollen-collecting bees, flies and beetles. The anthers shed their pollen very early, and in the case of Papaver rhoeas, it falls on the circumference of the radiating stigmas, so that this species must often be self-fertilised; but with Papaver dubium the same result does not follow (according to H. Muller ‘Die Befruchtung’ page 128), owing to the shortness of the stamens, unless the flower happens to stand inclined. The present species, therefore, does not seem so well fitted for self-fertilisation as most of the others. Nevertheless Papaver vagum produced plenty of capsules in my garden when insects were excluded, but only late in the season. I may here add that Papaver somniferum produces an abundance of spontaneously self-fertilised capsules, as Professor H. Hoffmann likewise found to be the case. (4/2. ‘Zur Speciesfrage’ 1875 page 53.) Some species of Papaver cross freely when growing in the same garden, as I have known to be the case with Papaver bracteatum and orientale.
Plants of Papaver vagum were raised from seeds sent me from Antibes through the kindness of Dr. Bornet. Some little time after the flowers had expanded, several were fertilised with their own pollen, and others (not castrated) with pollen from a distinct individual; but I have reason to believe, from observations subsequently made, that these flowers had been already fertilised by their own pollen, as this process seems to take place soon after their expansion. (4/3. Mr. J. Scott found ‘Report on the Experimental Culture of the Opium Poppy’ Calcutta 1874 page 47, in the case of Papaver somniferum, that if he cut away the stigmatic surface before the flower had expanded, no seeds were produced; but if this was done “on the second day, or even a few hours after the expansion of the flower on the first day, a partial fertilisation had already been effected, and a few good seeds were almost invariably produced.” This proves at how early a period fertilisation takes place.) I raised, however, a few seedlings of both lots, and the self-fertilised rather exceeded the crossed plants in height.
Early in the following year I acted differently, and fertilised seven flowers, very soon after their expansion, with pollen from another plant, and obtained six capsules. From counting the seeds in a medium-sized one, I estimated that the average number in each was at least 120. Four out of twelve capsules, spontaneously self-fertilised at the same time, were found to contain no good seeds; and the remaining eight contained on an average 6.6 seeds per capsule. But it should be observed that later in the season the same plants produced under a net plenty of very fine spontaneously self-fertilised capsules.
The above two lots of seeds, after germinating on sand, were planted in pairs on opposite sides of five pots. The two lots of seedlings, when half an inch in height, and again when 6 inches high, were measured to the tips of their leaves, but presented no difference. When fully grown, the flower-stalks were measured to the summits of the seed capsules, with the following result:—
TABLE 4/33. Papaver vagum.
Heights of flower-stalks to the summits of the seed capsules measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 24 2/8 : 21. Pot 1 : 30 : 26 5/8. Pot 1 : 18 4/8 : 16.
Pot 2 : 14 4/8 : 15 3/8. Pot 2 : 22 : 20 1/8. Pot 2 : 19 5/8 : 14 1/8. Pot 2 : 21 5/8 : 16 4/8.
Pot 3 : 20 6/8 : 19 2/8. Pot 3 : 20 2/8 : 13 2/8. Pot 3 : 20 6/8 : 18.
Pot 4 : 25 3/8 : 23 2/8. Pot 4 : 24 2/8 : 23.
Pot 5 : 20 : 18 3/8. Pot 5 : 27 7/8 : 27. Pot 5 : 19 : 21 2/8.
Total : 328.75 : 293.13.
The fifteen crossed plants here average 21.91 inches, and the fifteen self-fertilised plants 19.54 inches in height, or as 100 to 89. These plants did not differ in fertility, as far as could be judged by the number of capsules produced, for there were seventy-five on the crossed side and seventy-four on the self-fertilised side.
Eschscholtzia californica.
This plant is remarkable from the crossed seedlings not exceeding in height or vigour the self-fertilised. On the other hand, a cross greatly increases the productiveness of the flowers on the parent-plant, and is indeed sometimes necessary in order that they should produce any seed; moreover, plants thus derived are themselves much more fertile than those raised from self-fertilised flowers; so that the whole advantage of a cross is confined to the reproductive system. It will be necessary for me to give this singular case in considerable detail.
Twelve flowers on some plants in my flower-garden were fertilised with pollen from distinct plants, and produced twelve capsules; but one of these contained no good seed. The seeds of the eleven good capsules weighed 17.4 grains. Eighteen flowers on the same plants were fertilised with their own pollen and produced twelve good capsules, which contained 13.61 grains weight of seed. Therefore an equal number of crossed and self-fertilised capsules would have yielded seed by weight as 100 to 71. (4/4. Professor Hildebrand experimented on plants in Germany on a larger scale than I did, and found them much more self-fertile. Eighteen capsules, produced by cross-fertilisation, contained on an average eighty-five seeds, whilst fourteen capsules from self-fertilised flowers contained on an average only nine seeds; that is, as 100 to 11: ‘Jahrb. fur Wissen Botanik.’ B. 7 page 467.) If we take into account of the fact that a much greater proportion of flowers produced capsules when crossed than when self-fertilised, the relative fertility of the crossed to the self-fertilised flowers was as 100 to 52. Nevertheless these plants, whilst still protected by the net, spontaneously produced a considerable number of self-fertilised capsules.
The seeds of the two lots after germinating on sand were planted in pairs on the opposite sides of four large pots. At first there was no difference in their growth, but ultimately the crossed seedlings exceeded the self-fertilised considerably in height, as shown in Table 4/34. But I believe from the cases which follow that this result was accidental, owing to only a few plants having been measured, and to one of the self-fertilised plants having grown only to a height of 15 inches. The plants had been kept in the greenhouse, and from being drawn up to the light had to be tied to sticks in this and the following trials. They were measured to the summits of their flower-stems.
TABLE 4/34. Eschscholtzia californica.
Heights of Plants to the summits of their flower-stems measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 33 4/8 : 25.
Pot 2 : 34 2/8 : 35.
Pot 3 : 29 : 27 2/8.
Pot 4 : 22 : 15.
Total : 118.75 : 102.25.
The four crossed plants here average 29.68 inches, and the four self-fertilised 25.56 in height; or as 100 to 86. The remaining seeds were sown in a large pot in which a Cineraria had long been growing; and in this case again the two crossed plants on the one side greatly exceeded in height the two self-fertilised plants on the opposite side. The plants in the above four pots from having been kept in the greenhouse did not produce on this or any other similar occasion many capsules; but the flowers on the crossed plants when again crossed were much more productive than the flowers on the self-fertilised plants when again self-fertilised. These plants after seeding were cut down and kept in the greenhouse; and in the following year, when grown again, their relative heights were reversed, as the self-fertilised plants in three out of the four pots were now taller than and flowered before the crossed plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
The fact just given with respect to the growth of the cut-down plants made me doubtful about my first trial, so I determined to make another on a larger scale with crossed and self-fertilised seedlings raised from the crossed and self-fertilised plants of the last generation. Eleven pairs were raised and grown in competition in the usual manner; and now the result was different, for the two lots were nearly equal during their whole growth. It would therefore be superfluous to give a table of their heights. When fully grown and measured, the crossed averaged 32.47, and the self-fertilised 32.81 inches in height; or as 100 to 101. There was no great difference in the number of flowers and capsules produced by the two lots when both were left freely exposed to the visits of insects.
PLANTS RAISED FROM BRAZILIAN SEED.
Fritz Muller sent me from South Brazil seeds of plants which were there absolutely sterile when fertilised with pollen from the same plant, but were perfectly fertile when fertilised with pollen from any other plant. The plants raised by me in England from these seeds were examined by Professor Asa Gray, and pronounced to belong to E. Californica, with which they were identical in general appearance. Two of these plants were covered by a net, and were found not to be so completely self-sterile as in Brazil. But I shall recur to this subject in another part of this work. Here it will suffice to state that eight flowers on these two plants, fertilised with pollen from another plant under the net, produced eight fine capsules, each containing on an average about eighty seeds. Eight flowers on these same plants, fertilised with their own pollen, produced seven capsules, which contained on an average only twelve seeds, with a maximum in one of sixteen seeds. Therefore the cross-fertilised capsules, compared with the self-fertilised, yielded seeds in the ratio of about 100 to 15. These plants of Brazilian parentage differed also in a marked manner from the English plants in producing extremely few spontaneously self-fertilised capsules under a net.
Crossed and self-fertilised seeds from the above plants, after germinating on bare sand, were planted in pairs on the opposite sides of five large pots. The seedlings thus raised were the grandchildren of the plants which grew in Brazil; the parents having been grown in England. As the grandparents in Brazil absolutely require cross-fertilisation in order to yield any seeds, I expected that self-fertilisation would have proved very injurious to these seedlings, and that the crossed ones would have been greatly superior in height and vigour to those raised from self-fertilised flowers. But the result showed that my anticipation was erroneous; for as in the last experiment with plants of the English stock, so in the present one, the self-fertilised plants exceeded the crossed by a little in height. It will be sufficient to state that the fourteen crossed plants averaged 44.64, and the fourteen self-fertilised 45.12 inches in height; or as 100 to 101.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
I now tried a different experiment. Eight flowers on the self-fertilised plants of the last experiment (i.e., grandchildren of the plants which grew in Brazil) were again fertilised with pollen from the same plant, and produced five capsules, containing on an average 27.4 seeds, with a maximum in one of forty-two seeds. The seedlings raised from these seeds formed the second SELF-FERTILISED generation of the Brazilian stock.
Eight flowers on one of the crossed plants of the last experiment were crossed with pollen from another grandchild, and produced five capsules. These contained on an average 31.6 seeds, with a maximum in one of forty-nine seeds. The seedlings raised from these seeds may be called the INTERCROSSED.
Lastly, eight other flowers on the crossed plants of the last experiment were fertilised with pollen from a plant of the English stock, growing in my garden, and which must have been exposed during many previous generations to very different conditions from those to which the Brazilian progenitors of the mother-plant had been subjected. These eight flowers produced only four capsules, containing on an average 63.2 seeds, with a maximum in one of ninety. The plants raised from these seeds may be called the ENGLISH-CROSSED. As far as the above averages can be trusted from so few capsules, the English-crossed capsules contained twice as many seeds as the intercrossed, and rather more than twice as many as the self-fertilised capsules. The plants which yielded these capsules were grown in pots in the greenhouse, so that their absolute productiveness must not be compared with that of plants growing out of doors.
The above three lots of seeds, namely, the self-fertilised, intercrossed, and English-crossed, were planted in an equal state of germination (having been as usual sown on bare sand) in nine large pots, each divided into three parts by superficial partitions. Many of the self-fertilised seeds germinated before those of the two crossed lots, and these were of course rejected. The seedlings thus raised are the great-grandchildren of the plants which grew in Brazil. When they were from 2 to 4 inches in height, the three lots were equal. They were measured when four-fifths grown, and again when fully grown, and as their relative heights were almost exactly the same at these two ages, I will give only the last measurements. The average height of the nineteen English-crossed plants was 45.92 inches; that of the eighteen intercrossed plants (for one died), 43.38; and that of the nineteen self-fertilised plants, 50.3 inches. So that we have the following ratios in height:—
The English-crossed to the self-fertilised plants, as 100 to 109.
The English-crossed to the intercrossed plants, as 100 to 94.
The intercrossed to the self-fertilised plants, as 100 to 116.
After the seed-capsules had been gathered, all these plants were cut down close to the ground and weighed. The nineteen English crossed plants weighed 18.25 ounces; the intercrossed plants (with their weight calculated as if there had been nineteen) weighed 18.2 ounces; and the nineteen self-fertilised plants, 21.5 ounces. We have therefore for the weights of the three lots of plants the following ratios:—
The English-crossed to the self-fertilised plants, as 100 to 118.
The English-crossed to the intercrossed plants, as 100 to 100.
The intercrossed to the self-fertilised plants, as 100 to 118.
We thus see that in weight, as in height, the self-fertilised plants had a decided advantage over the English-crossed and intercrossed plants.
The remaining seeds of the three kinds, whether or not in a state of germination, were sown in three long parallel rows in the open ground; and here again the self-fertilised seedlings exceeded in height by between 2 and 3 inches the seedlings in the two other rows, which were of nearly equal heights. The three rows were left unprotected throughout the winter, and all the plants were killed, with the exception of two of the self-fertilised; so that as far as this little bit of evidence goes, some of the self-fertilised plants were more hardy than any of the crossed plants of either lot.
We thus see that the self-fertilised plants which were grown in the nine pots were superior in height (as 116 to 100), and in weight (as 118 to 100), and apparently in hardiness, to the intercrossed plants derived from a cross between the grandchildren of the Brazilian stock. The superiority is here much more strongly marked than in the second trial with the plants of the English stock, in which the self-fertilised were to the crossed in height as 101 to 100. It is a far more remarkable fact—if we bear in mind the effects of crossing plants with pollen from a fresh stock in the cases of Ipomoea, Mimulus, Brassica, and Iberis—that the self-fertilised plants exceeded in height (as 109 to 100), and in weight (as 118 to 100), the offspring of the Brazilian stock crossed by the English stock; the two stocks having been long subjected to widely different conditions.
If we now turn to the fertility of the three lots of plants we find a very different result. I may premise that in five out of the nine pots the first plant which flowered was one of the English-crossed; in four of the pots it was a self-fertilised plant; and in not one did an intercrossed plant flower first; so that these latter plants were beaten in this respect, as in so many other ways. The three closely adjoining rows of plants growing in the open ground flowered profusely, and the flowers were incessantly visited by bees, and certainly thus intercrossed. The manner in which several plants in the previous experiments continued to be almost sterile as long as they were covered by a net, but set a multitude of capsules immediately that they were uncovered, proves how effectually the bees carry pollen from plant to plant. My gardener gathered, at three successive times, an equal number of ripe capsules from the plants of the three lots, until he had collected forty-five from each lot. It is not possible to judge from external appearance whether or not a capsule contains any good seeds; so that I opened all the capsules. Of the forty-five from the English-crossed plants, four were empty; of those from the intercrossed, five were empty; and of those from the self-fertilised, nine were empty. The seeds were counted in twenty-one capsules taken by chance out of each lot, and the average number of seeds in the capsules from the English-crossed plants was 67; from the intercrossed, 56; and from the self-fertilised, 48.52. It therefore follows that:—
The forty-five capsules (the four empty ones included) from the English-crossed plants contained 2747 seeds.
The forty-five capsules (the five empty ones included) from the intercrossed plants contained 2240 seeds.
The forty-five capsules (the nine empty ones included) from the self-fertilised plants contained 1746.7 seeds.
The reader should remember that these capsules are the product of cross-fertilisation, effected by the bees; and that the difference in the number of the contained seeds must depend on the constitution of the plants;—that is, on whether they were derived from a cross with a distinct stock, or from a cross between plants of the same stock, or from self-fertilisation. From the above facts we obtain the following ratios:—
Number of seeds contained in an equal number of naturally fertilised capsules produced:—
By the English-crossed and self-fertilised plants, as 100 to 63.
By the English-crossed and intercrossed plants, as 100 to 81.
By the intercrossed and self-fertilised plants, as 100 to 78.
But to have ascertained the productiveness of the three lots of plants, it would have been necessary to know how many capsules were produced by the same number of plants. The three long rows, however, were not of quite equal lengths, and the plants were much crowded, so that it would have been extremely difficult to have ascertained how many capsules were produced by them, even if I had been willing to undertake so laborious a task as to collect and count all the capsules. But this was feasible with the plants grown in pots in the greenhouse; and although these were much less fertile than those growing out of doors, their relative fertility appeared, after carefully observing them, to be the same. The nineteen plants of the English-crossed stock in the pots produced altogether 240 capsules; the intercrossed plants (calculated as nineteen) produced 137.22 capsules; and the nineteen self-fertilised plants, 152 capsules. Now, knowing the number of seeds contained in forty-five capsules of each lot, it is easy to calculate the relative numbers of seeds produced by an equal number of the plants of the three lots.
Number of seeds produced by an equal number of naturally-fertilised plants:—
Plants of English-crossed and self-fertilised parentage, as 100 to 40 seeds.
Plants of English-crossed and intercrossed parentage, as 100 to 45 seeds.
Plants of intercrossed and self-fertilised parentage, as 100 to 89 seeds.
The superiority in productiveness of the intercrossed plants (that is, the product of a cross between the grandchildren of the plants which grew in Brazil) over the self-fertilised, small as it is, is wholly due to the larger average number of seeds contained in the capsules; for the intercrossed plants produced fewer capsules in the greenhouse than did the self-fertilised plants. The great superiority in productiveness of the English-crossed over the self-fertilised plants is shown by the larger number of capsules produced, the larger average number of contained seeds, and the smaller number of empty capsules. As the English-crossed and intercrossed plants were the offspring of crosses in every previous generation (as must have been the case from the flowers being sterile with their own pollen), we may conclude that the great superiority in productiveness of the English-crossed over the intercrossed plants is due to the two parents of the former having been long subjected to different conditions.
The English-crossed plants, though so superior in productiveness, were, as we have seen, decidedly inferior in height and weight to the self-fertilised, and only equal to, or hardly superior to, the intercrossed plants. Therefore, the whole advantage of a cross with a distinct stock is here confined to productiveness, and I have met with no similar case.
8. RESEDACEAE.—Reseda lutea.
Seeds collected from wild plants growing in this neighbourhood were sown in the kitchen-garden; and several of the seedlings thus raised were covered with a net. Of these, some were found (as will hereafter be more fully described) to be absolutely sterile when left to fertilise themselves spontaneously, although plenty of pollen fell on their stigmas; and they were equally sterile when artificially and repeatedly fertilised with their own pollen; whilst other plants produced a few spontaneously self-fertilised capsules. The remaining plants were left uncovered, and as pollen was carried from plant to plant by the hive and humble-bees which incessantly visit the flowers, they produced an abundance of capsules. Of the necessity of pollen being carried from one plant to another, I had ample evidence in the case of this species and of R. odorata; for those plants, which set no seeds or very few as long as they were protected from insects, became loaded with capsules immediately that they were uncovered.
Seeds from the flowers spontaneously self-fertilised under the net, and from flowers naturally crossed by the bees, were sown on opposite sides of five large pots. The seedlings were thinned as soon as they appeared above ground, so that an equal number were left on the two sides. After a time the pots were plunged into the open ground. The same number of plants of crossed and self-fertilised parentage were measured up to the summits of their flower-stems, with the result given in Table 4/35. Those which did not produce flower-stems were not measured.
TABLE 4/35. Reseda lutea, in pots.
Heights of plants to the summits of the flower-stems measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 21 : 12 7/8. Pot 1 : 14 2/8 : 16. Pot 1 : 19 1/8 : 11 7/8. Pot 1 : 7 : 15 2/8. Pot 1 : 15 1/8 : 19 1/8.
Pot 2 : 20 4/8 : 12 4/8. Pot 2 : 17 3/8 : 16 2/8. Pot 2 : 23 7/8 : 16 2/8. Pot 2 : 17 1/8 : 13 3/8. Pot 2 : 20 6/8 : 13 5/8.
Pot 3 : 16 1/8 : 14 4/8. Pot 3 : 17 6/8 : 19 4/8. Pot 3 : 16 2/8 : 20 7/8. Pot 3 : 10 : 7 7/8. Pot 3 : 10 : 17 6/8.
Pot 4 : 22 1/8 : 9. Pot 4 : 19 : 11 4/8. Pot 4 : 18 7/8 : 11. Pot 4 : 16 4/8 : 16. Pot 4 : 19 2/8 : 16 3/8.
Pot 5 : 25 2/8 : 14 6/8. Pot 5 : 22 : 16. Pot 5 : 8 6/8 : 14 3/8. Pot 5 : 14 2/8 : 14 2/8.
Total : 412.25 : 350.86.
The average height of the twenty-four crossed plants is here 17.17 inches, and that of the same number of self-fertilised plants 14.61; or as 100 to 85. Of the crossed plants all but five flowered, whilst several of the self-fertilised did not do so. The above pairs, whilst still in flower, but with some capsules already formed, were afterwards cut down and weighed. The crossed weighed 90.5 ounces; and an equal number of the self-fertilised only 19 ounces, or as 100 to 21; and this is an astonishing difference.
Seeds of the same two lots were also sown in two adjoining rows in the open ground. There were twenty crossed plants in the one row and thirty-two self-fertilised plants in the other row, so that the experiment was not quite fair; but not so unfair as it at first appears, for the plants in the same row were not crowded so much as seriously to interfere with each other’s growth, and the ground was bare on the outside of both rows. These plants were better nourished than those in the pots and grew to a greater height. The eight tallest plants in each row were measured in the same manner as before, with the following result:—
TABLE 4/36. Reseda lutea, growing in the open ground.
Heights of plants to the summits of the flower-stems measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
Total : 224.75 : 185.13
The average height of the crossed plants, whilst in full flower, was here 28.09, and that of the self-fertilised 23.14 inches; or as 100 to 82. It is a singular fact that the tallest plant in the two rows, was one of the self-fertilised. The self-fertilised plants had smaller and paler green leaves than the crossed. All the plants in the two rows were afterwards cut down and weighed. The twenty crossed plants weighed 65 ounces, and twenty self-fertilised (by calculation from the actual weight of the thirty-two self-fertilised plants) weighed 26.25 ounces; or as 100 to 40. Therefore the crossed plants did not exceed in weight the self-fertilised plants in nearly so great a degree as those growing in the pots, owing probably to the latter having been subjected to more severe mutual competition. On the other hand, they exceeded the self-fertilised in height in a slightly greater degree.
Reseda odorata.
Plants of the common mignonette were raised from purchased seed, and several of them were placed under separate nets. Of these some became loaded with spontaneously self-fertilised capsules; others produced a few, and others not a single one. It must not be supposed that these latter plants produced no seed because their stigmas did not receive any pollen, for they were repeatedly fertilised with pollen from the same plant with no effect; but they were perfectly fertile with pollen from any other plant. Spontaneously self-fertilised seeds were saved from one of the highly self-fertile plants, and other seeds were collected from the plants growing outside the nets, which had been crossed by the bees. These seeds after germinating on sand were planted in pairs on the opposite sides of five pots. The plants were trained up sticks, and measured to the summits of their leafy stems—the flower-stems not being included. We here have the result:—
TABLE 4/37. Reseda odorata (seedlings from a highly self-fertile plant).
Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 20 7/8 : 22 4/8. Pot 1 : 34 7/8 : 28 5/8. Pot 1 : 26 6/8 : 23 2/8. Pot 1 : 32 6/8 : 30 4/8.
Pot 2 : 34 3/8 : 28 5/8. Pot 2 : 34 5/8 : 30 5/8. Pot 2 : 11 6/8 : 23. Pot 2 : 33 3/8 : 30 1/8.
Pot 3 : 17 7/8 : 4 4/8. Pot 3 : 27 : 25. Pot 3 : 30 1/8 : 26 3/8. Pot 3 : 30 2/8 : 25 1/8.
Pot 4 : 21 5/8 : 22 6/8. Pot 4 : 28 : 25 4/8. Pot 4 : 32 5/8 : 15 1/8. Pot 4 : 32 3/8 : 24 6/8.
Pot 5 : 21 : 11 6/8. Pot 5 : 25 2/8 : 19 7/8. Pot 5 : 26 6/8 : 10 4/8.
Total : 522.25 : 428.50.
The average height of the nineteen crossed plants is here 27.48, and that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All these plants were cut down in the early autumn and weighed: the crossed weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to 67. These two lots having been left freely exposed to the visits of insects, did not present any difference to the eye in the number of seed-capsules which they produced.
The remainder of the same two lots of seeds were sown in two adjoining rows in the open ground; so that the plants were exposed to only moderate competition. The eight tallest on each side were measured, as shown in Table 4/38.
TABLE 4/38. Reseda odorata, growing in the open ground.
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
Total : 206.13 : 216.75
The average height of the eight crossed plants is 25.76, and that of the eight self-fertilised 27.09; or as 100 to 105.
We here have the anomalous result of the self-fertilised plants being a little taller than the crossed; of which fact I can offer no explanation. It is of course possible, but not probable, that the labels may have been interchanged by accident.
Another experiment was now tried: all the self-fertilised capsules, though very few in number, were gathered from one of the semi-self-sterile plants under a net; and as several flowers on this same plant had been fertilised with pollen from a distinct individual, crossed seeds were thus obtained. I expected that the seedlings from this semi-self-sterile plant would have profited in a higher degree from a cross, than did the seedlings from the fully self-fertile plants. But my anticipation was quite wrong, for they profited in a less degree. An analogous result followed in the case of Eschscholtzia, in which the offspring of the plants of Brazilian parentage (which were partially self-sterile) did not profit more from a cross, than did the plants of the far more self-fertile English stock. The above two lots of crossed and self-fertilised seeds from the same plant of Reseda odorata, after germinating on sand, were planted on opposite sides of five pots, and measured as in the last case, with the result in Table 4/39.
TABLE 4/39. Reseda odorata (seedlings from a semi-self-sterile plant).
Heights of plants to the summits of the leafy stems, flower-stems not included, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 33 4/8 : 31. Pot 1 : 30 6/8 : 28. Pot 1 : 29 6/8 : 13 2/8. Pot 1 : 20 : 32.
Pot 2 : 22 : 21 6/8. Pot 2 : 33 4/8 : 26 6/8. Pot 2 : 31 2/8 : 25 2/8. Pot 2 : 32 4/8 : 30 4/8.
Pot 3 : 30 1/8 : 17 2/8. Pot 3 : 32 1/8 : 29 6/8. Pot 3 : 31 4/8 : 24 6/8. Pot 3 : 32 2/8 : 34 2/8.
Pot 4 : 19 1/8 : 20 6/8. Pot 4 : 30 1/8 : 32 6/8. Pot 4 : 24 3/8 : 31 4/8. Pot 4 : 30 6/8 : 36 6/8.
Pot 5 : 34 6/8 : 24 5/8. Pot 5 : 37 1/8 : 34. Pot 5 : 31 2/8 : 22 2/8. Pot 5 : 33 : 37 1/8.
Total : 599.75 : 554.25.
The average height of the twenty crossed plants is here 29.98, and that of the twenty self-fertilised 27.71 inches; or as 100 to 92. These plants were then cut down and weighed; and the crossed in this case exceeded the self-fertilised in weight by a mere trifle, namely, in the ratio of 100 to 99. The two lots, left freely exposed to insects, seemed to be equally fertile.
The remainder of the seed was sown in two adjoining rows in the open ground; and the eight tallest plants in each row were measured, with the result in Table 4/40.
TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant, planted in the open ground).
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
Total : 207.38 : 188.38.
The average height of the eight crossed plants is here 25.92, and that of the eight self-fertilised plants 23.54 inches; or as 100 to 90.
9. VIOLACEAE.—Viola tricolor.
Whilst the flowers of the common cultivated heartsease are young, the anthers shed their pollen into a little semi-cylindrical passage, formed by the basal portion of the lower petal, and surrounded by papillae. The pollen thus collected lies close beneath the stigma, but can seldom gain access into its cavity, except by the aid of insects, which pass their proboscides down this passage into the nectary. (4/5. The flowers of this plant have been fully described by Sprengel, Hildebrand, Delpino, and H. Muller. The latter author sums up all the previous observations in his ‘Befruchtung der Blumen’ and in ‘Nature’ November 20, 1873 page 44. See also Mr. A.W. Bennett in ‘Nature’ May 15, 1873 page 50 and some remarks by Mr. Kitchener ibid page 143. The facts which follow on the effects of covering up a plant of V. tricolor have been quoted by Sir J. Lubbock in his ‘British Wild Flowers’ etc. page 62.) Consequently when I covered up a large plant of a cultivated variety, it set only eighteen capsules, and most of these contained very few good seeds—several from only one to three; whereas an equally fine uncovered plant of the same variety, growing close by, produced 105 fine capsules. The few flowers which produce capsules when insects are excluded, are perhaps fertilised by the curling inwards of the petals as their wither, for by this means pollen-grains adhering to the papillae might be inserted into the cavity of the stigma. But it is more probable that their fertilisation is effected, as Mr. Bennett suggests, by Thrips and certain minute beetles which haunt the flowers, and which cannot be excluded by any net. Humble-bees are the usual fertilisers; but I have more than once seen flies (Rhingia rostrata) at work, with the under sides of their bodies, heads and legs dusted with pollen; and having marked the flowers which they visited, I found them after a few days fertilised. (4/6. I should add that this fly apparently did not suck the nectar, but was attracted by the papillae which surround the stigma. Hermann Muller also saw a small bee, an Andrena, which could not reach the nectar, repeatedly inserting its proboscis beneath the stigma, where the papillae are situated; so that these papillae must be in some way attractive to insects. A writer asserts ‘Zoologist’ volume 3-4 page 1225, that a moth (Plusia) frequently visits the flowers of the pansy. Hive-bees do not ordinarily visit them, but a case has been recorded ‘Gardeners’ Chronicle’ 1844 page 374, of these bees doing so. Hermann Muller has also seen the hive-bee at work, but only on the wild small-flowered form. He gives a list ‘Nature’ 1873 page 45, of all the insects which he has seen visiting both the large and small-flowered forms. From his account, I suspect that the flowers of plants in a state of nature are visited more frequently by insects than those of the cultivated varieties. He has seen several butterflies sucking the flowers of wild plants, and this I have never observed in gardens, though I have watched the flowers during many years.) It is curious for how long a time the flowers of the heartsease and of some other plants may be watched without an insect being seen to visit them. During the summer of 1841, I observed many times daily for more than a fortnight some large clumps of heartsease growing in my garden, before I saw a single humble-bee at work. During another summer I did the same, but at last saw some dark-coloured humble-bees visiting on three successive days almost every flower in several clumps; and almost all these flowers quickly withered and produced fine capsules. I presume that a certain state of the atmosphere is necessary for the secretion of nectar, and that as soon as this occurs the insects discover the fact by the odour emitted, and immediately frequent the flowers.
As the flowers require the aid of insects for their complete fertilisation, and as they are not visited by insects nearly so often as most other nectar-secreting flowers, we can understand the remarkable fact discovered by H. Muller and described by him in ‘Nature,’ namely, that this species exists under two forms. One of these bears conspicuous flowers, which, as we have seen, require the aid of insects, and are adapted to be cross-fertilised by them; whilst the other form has much smaller and less conspicuously coloured flowers, which are constructed on a slightly different plan, favouring self-fertilisation, and are thus adapted to ensure the propagation of the species. The self-fertile form, however, is occasionally visited, and may be crossed by insects, though this is rather doubtful.
In my first experiments on Viola tricolor I was unsuccessful in raising seedlings, and obtained only one full-grown crossed and self-fertilised plant. The former was 12 1/2 inches and the latter 8 inches in height. On the following year several flowers on a fresh plant were crossed with pollen from another plant, which was known to be a distinct seedling; and to this point it is important to attend. Several other flowers on the same plant were fertilised with their own pollen. The average number of seeds in the ten crossed capsules was 18.7, and in the twelve self-fertilised capsules 12.83; or as 100 to 69. These seeds, after germinating on bare sand, were planted in pairs on the opposite sides of five pots. They were first measured when about a third of their full size, and the crossed plants then averaged 3.87 inches, and the self-fertilised only 2.00 inches in height; or as 100 to 52. They were kept in the greenhouse, and did not grow vigorously. Whilst in flower they were again measured to the summits of their stems (see Table 4/41), with the following result:—
TABLE 4/41. Viola tricolor.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 8 2/8 : 0 2/8. Pot 1 : 7 4/8 : 2 4/8. Pot 1 : 5 : 1 2/8.
Pot 2 : 5 : 6. Pot 2 : 4 : 4. Pot 2 : 4 4/8 : 3 1/8.
Pot 3 : 9 4/8 : 3 1/8. Pot 3 : 3 3/8 : 1 7/8. Pot 3 : 8 4/8 : 0 5/8.
Pot 4 : 4 7/8 : 2 1/8. Pot 4 : 4 2/8 : 1 6/8. Pot 4 : 4 : 2 1/8.
Pot 5 : 6 : 3. Pot 5 : 3 3/8 : 1 4/8.
Total : 78.13 : 33.25.
The average height of the fourteen crossed plants is here 5.58 inches, and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four out of the five pots, a crossed plant flowered before any one of the self-fertilised; as likewise occurred with the pair raised during the previous year. These plants without being disturbed were now turned out of their pots and planted in the open ground, so as to form five separate clumps. Early in the following summer (1869) they flowered profusely, and being visited by humble-bees set many capsules, which were carefully collected from all the plants on both sides. The crossed plants produced 167 capsules, and the self-fertilised only 17; or as 100 to 10. So that the crossed plants were more than twice the height of the self-fertilised, generally flowered first, and produced ten times as many naturally fertilised capsules.
By the early part of the summer of 1870 the crossed plants in all the five clumps had grown and spread so much more than the self-fertilised, that any comparison between them was superfluous. The crossed plants were covered with a sheet of bloom, whilst only a single self-fertilised plant, which was much finer than any of its brethren, flowered. The crossed and self-fertilised plants had now grown all matted together on the respective sides of the superficial partitions still separating them; and in the clump which included the finest self-fertilised plant, I estimated that the surface covered by the crossed plants was about nine times as large as that covered by the self-fertilised plants. The extraordinary superiority of the crossed over the self-fertilised plants in all five clumps, was no doubt due to the crossed plants at first having had a decided advantage over the self-fertilised, and then robbing them more and more of their food during the succeeding seasons. But we should remember that the same result would follow in a state of nature even to a greater degree; for my plants grew in ground kept clear of weeds, so that the self-fertilised had to compete only with the crossed plants; whereas the whole surface of the ground is naturally covered with various kinds of plants, all of which have to struggle together for existence.
The ensuing winter was very severe, and in the following spring (1871) the plants were again examined. All the self-fertilised were now dead, with the exception of a single branch on one plant, which bore on its summit a minute rosette of leaves about as large as a pea. On the other hand, all the crossed plants without exception were growing vigorously. So that the self-fertilised plants, besides their inferiority in other respects, were more tender.
Another experiment was now tried for the sake of ascertaining how far the superiority of the crossed plants, or to speak more correctly, the inferiority of the self-fertilised plants, would be transmitted to their offspring. The one crossed and one self-fertilised plant, which were first raised, had been turned out of their pot and planted in the open ground. Both produced an abundance of very fine capsules, from which fact we may safely conclude that they had been cross-fertilised by insects. Seeds from both, after germinating on sand, were planted in pairs on the opposite sides of three pots. The naturally crossed seedlings derived from the crossed plants flowered in all three pots before the naturally crossed seedlings derived from the self-fertilised plants. When both lots were in full flower, the two tallest plants on each side of each pot were measured, and the result is shown in Table 4/42.
TABLE 4/42. Viola tricolor: seedlings from crossed and self-fertilised plants, the parents of both sets having been left to be naturally fertilised.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Naturally Crossed Plants from artificially crossed Plants.
Column 3: Naturally Crossed Plants from Self-fertilised Plants.
Pot 1 : 12 1/8 : 9 6/8. Pot 1 : 11 6/8 : 8 3/8.
Pot 2 : 13 2/8 : 9 6/8. Pot 2 : 10 : 11 4/8.
Pot 3 : 14 4/8 : 11 1/8. Pot 3 : 13 6/8 : 11 3/8.
Total : 75.38 : 61.88.
The average height of the six tallest plants derived from the crossed plants is 12.56 inches; and that of the six tallest plants derived from the self-fertilised plants is 10.31 inches; or as 100 to 82. We here see a considerable difference in height between the two sets, though very far from equalling that in the previous trials between the offspring from crossed and self-fertilised flowers. This difference must be attributed to the latter set of plants having inherited a weak constitution from their parents, the offspring of self-fertilised flowers; notwithstanding that the parents themselves had been freely intercrossed with other plants by the aid of insects.
10. RANUNCULACEAE.—Adonis aestivalis.
The results of my experiments on this plant are hardly worth giving, as I remark in my notes made at the time, “seedlings, from some unknown cause, all miserably unhealthy.” Nor did they ever become healthy; yet I feel bound to give the present case, as it is opposed to the general results at which I have arrived. Fifteen flowers were crossed and all produced fruit, containing on an average 32.5 seeds; nineteen flowers were fertilised with their own pollen, and they likewise all yielded fruit, containing a rather larger average of 34.5 seeds; or as 100 to 106. Seedlings were raised from these seeds. In one of the pots all the self-fertilised plants died whilst quite young; in the two others, the measurements were as follows:
TABLE 4/43. Adonis aestivalis.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 14 : 13 4/8. Pot 1 : 13 4/8 : 13 4/8.
Pot 2 : 16 2/8 : 15 2/8. Pot 2 : 13 2/8 : 15.
Total : 57.00 : 57.25.
The average height of the four crossed plants is 14.25, and that of the four self-fertilised plants 14.31; or as 100 to 100.4; so that they were in fact of equal height. According to Professor H. Hoffman, this plant is proterandrous (4/7. ‘Zur Speciesfrage’ 1875 page 11.); nevertheless it yields plenty of seeds when protected from insects.
Delphinium consolida.
It has been said in the case of this plant, as of so many others, that the flowers are fertilised in the bud, and that distinct plants or varieties can never naturally intercross. (4/8. Decaisne ‘Comptes-Rendus’ July 1863 page 5.) But this is an error, as we may infer, firstly from the flowers being proterandrous,—the mature stamens bending up, one after the other, into the passage which leads to the nectary, and afterwards the mature pistils bending in the same direction; secondly, from the number of humble-bees which visit the flowers (4/9. Their structure is described by H. Muller ‘Befruchtung’ etc., page 122.); and thirdly, from the greater fertility of the flowers when crossed with pollen from a distinct plant than when spontaneously self-fertilised. In the year 1863 I enclosed a large branch in a net, and crossed five flowers with pollen from a distinct plant; these yielded capsules containing on an average 35.2 very fine seeds, with a maximum of forty-two in one capsule. Thirty-two other flowers on the same branch produced twenty-eight spontaneously self-fertilised capsules, containing on an average 17.2 seeds, with a maximum in one of thirty-six seeds. But six of these capsules were very poor, yielding only from one to five seeds; if these are excluded, the remaining twenty-two capsules give an average of 20.9 seeds, though many of these seeds were small. The fairest ratio, therefore, for the number of seeds produced by a cross and by spontaneous self-fertilisation is as 100 to 59. These seeds were not sown, as I had too many other experiments in progress.
In the summer of 1867, which was a very unfavourable one, I again crossed several flowers under a net with pollen from a distinct plant, and fertilised other flowers on the same plant with their own pollen. The former yielded a much larger proportion of capsules than the latter; and many of the seeds in the self-fertilised capsules, though numerous, were so poor that an equal number of seeds from the crossed and self-fertilised capsules were in weight as 100 to 45. The two lots were allowed to germinate on sand, and pairs were planted on the opposite sides of four pots. When nearly two-thirds grown they were measured, as shown in Table 4/44.
TABLE 4/44. Delphinium consolida.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 11 : 11.
Pot 2 : 19 : 16 2/8. Pot 2 : 16 2/8 : 11 4/8.
Pot 3 : 26 : 22.
Pot 4 : 9 4/8 : 8 2/8. Pot 4 : 8 : 6 4/8.
Total : 89.75 : 75.50.
The six crossed plants here average 14.95, and the six self-fertilised 12.50 inches in height; or as 100 to 84. When fully grown they were again measured, but from want of time only a single plant on each side was measured; so that I have thought it best to give the earlier measurements. At the later period the three tallest crossed plants still exceeded considerably in height the three tallest self-fertilised, but not in quite so great a degree as before. The pots were left uncovered in the greenhouse, but whether the flowers were intercrossed by bees or self-fertilised I do not know. The six crossed plants produced 282 mature and immature capsules, whilst the six self-fertilised plants produced only 159; or as 100 to 56. So that the crossed plants were very much more productive than the self-fertilised.
11. CARYOPHYLLACEAE.—Viscaria oculata.
Twelve flowers were crossed with pollen from another plant, and yielded ten capsules, containing by weight 5.77 grains of seeds. Eighteen flowers were fertilised with their own pollen and yielded twelve capsules, containing by weight 2.63 grains. Therefore the seeds from an equal number of crossed and self-fertilised flowers would have been in weight as 100 to 38. I had previously selected a medium-sized capsule from each lot, and counted the seeds in both; the crossed one contained 284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds were sown on opposite sides of three pots, and several seedlings raised; but only the tallest flower-stem of one plant on each side was measured. The three on the crossed side averaged 32.5 inches, and the three on the self-fertilised side 34 inches in height; or as 100 to 104. But this trial was on much too small a scale to be trusted; the plants also grew so unequally that one of the three flower-stems on the crossed plants was very nearly twice as tall as that on one of the others; and one of the three flower-stems on the self-fertilised plants exceeded in an equal degree one of the others.
In the following year the experiment was repeated on a larger scale: ten flowers were crossed on a new set of plants and yielded ten capsules containing by weight 6.54 grains of seed. Eighteen spontaneously self-fertilised capsules were gathered, of which two contained no seed; the other sixteen contained by weight 6.07 grains of seed. Therefore the weight of seed from an equal number of crossed and spontaneously self-fertilised flowers (instead of artificially fertilised as in the previous case) was as 100 to 58.
The seeds after germinating on sand were planted in pairs on the opposite sides of four pots, with all the remaining seeds sown crowded in the opposite sides of a fifth pot; in this latter pot only the tallest plant on each side was measured. Until the seedlings had grown about 5 inches in height no difference could be perceived in the two lots. Both lots flowered at nearly the same time. When they had almost done flowering, the tallest flower-stem on each plant was measured, as shown in Table 4/45.
TABLE 4/45. Viscaria oculata.
Tallest flower-stem on each plant measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 19 : 32 3/8. Pot 1 : 33 : 38. Pot 1 : 41 : 38. Pot 1 : 41 : 28 7/8.
Pot 2 : 37 4/8 : 36. Pot 2 : 36 4/8 : 32 3/8. Pot 2 : 38 : 35 6/8.
Pot 3 : 44 4/8 : 36. Pot 3 : 39 4/8 : 20 7/8. Pot 3 : 39 : 30 5/8.
Pot 4 : 30 2/8 : 36. Pot 4 : 31 : 39. Pot 4 : 33 1/8 : 29. Pot 4 : 24 : 38 4/8.
Pot 5 : 30 2/8 : 32. Crowded.
Total : 517.63 : 503.36.
Tallest flower-stem on each plant measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 19 : 32 3/8. Pot 1 : 33 : 38. Pot 1 : 41 : 38. Pot 1 : 41 : 28 7/8.
Pot 2 : 37 4/8 : 36. Pot 2 : 36 4/8 : 32 3/8. Pot 2 : 38 : 35 6/8.
Pot 3 : 44 4/8 : 36. Pot 3 : 39 4/8 : 20 7/8. Pot 3 : 39 : 30 5/8.
Pot 4 : 30 2/8 : 36. Pot 4 : 31 : 39. Pot 4 : 33 1/8 : 29. Pot 4 : 24 : 38 4/8.
Pot 5 : 30 2/8 : 32. Crowded.
Total : 517.63 : 503.36.
The fifteen crossed plants here average 34.5, and the fifteen self-fertilised 33.55 inches in height; or as 100 to 97. So that the excess of height of the crossed plants is quite insignificant. In productiveness, however, the difference was much more plainly marked. All the capsules were gathered from both lots of plants (except from the crowded and unproductive ones in Pot 5), and at the close of the season the few remaining flowers were added in. The fourteen crossed plants produced 381, whilst the fourteen self-fertilised plants produced only 293 capsules and flowers; or as 100 to 77.
Dianthus caryophyllus.
The common carnation is strongly proterandrous, and therefore depends to a large extent upon insects for fertilisation. I have seen only humble-bees visiting the flowers, but I dare say other insects likewise do so. It is notorious that if pure seed is desired, the greatest care is necessary to prevent the varieties which grow in the same garden from intercrossing. (4/10. ‘Gardeners’ Chronicle’ 1847 page 268.) The pollen is generally shed and lost before the two stigmas in the same flower diverge and are ready to be fertilised. I was therefore often forced to use for self-fertilisation pollen from the same plant instead of from the same flower. But on two occasions, when I attended to this point, I was not able to detect any marked difference in the number of seeds produced by these two forms of self-fertilisation.
Several single-flowered carnations were planted in good soil, and were all covered with a net. Eight flowers were crossed with pollen from a distinct plant and yielded six capsules, containing on an average 88.6 seeds, with a maximum in one of 112 seeds. Eight other flowers were self-fertilised in the manner above described, and yielded seven capsules containing on an average 82 seeds, with a maximum in one of 112 seeds. So that there was very little difference in the number of seeds produced by cross-fertilisation and self-fertilisation, namely, as 100 to 92. As these plants were covered by a net, they produced spontaneously only a few capsules containing any seeds, and these few may perhaps be attributed to the action of Thrips and other minute insects which haunt the flowers. A large majority of the spontaneously self-fertilised capsules produced by several plants contained no seeds, or only a single one. Excluding these latter capsules, I counted the seeds in eighteen of the finest ones, and these contained on an average 18 seeds. One of the plants was spontaneously self-fertile in a higher degree than any of the others. On another occasion a single covered-up plant produced spontaneously eighteen capsules, but only two of these contained any seed, namely 10 and 15.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION.
The many seeds obtained from the above crossed and artificially self-fertilised flowers were sown out of doors, and two large beds of seedlings, closely adjoining one another, thus raised. This was the first plant on which I experimented, and I had not then formed any regular scheme of operation. When the two lots were in full flower, I measured roughly a large number of plants but record only that the crossed were on an average fully 4 inches taller than the self-fertilised. Judging from subsequent measurements, we may assume that the crossed plants were about 28 inches, and the self-fertilised about 24 inches in height; and this will give us a ratio of 100 to 86. Out of a large number of plants, four of the crossed ones flowered before any one of the self-fertilised plants.
Thirty flowers on these crossed plants of the first generation were again crossed with pollen from a distinct plant of the same lot, and yielded twenty-nine capsules, containing on an average 55.62 seeds, with a maximum in one of 110 seeds.
Thirty flowers on the self-fertilised plants were again self-fertilised; eight of them with pollen from the same flower, and the remainder with pollen from another flower on the same plant; and these produced twenty-two capsules, containing on an average 35.95 seeds, with a maximum in one of sixty-one seeds. We thus see, judging by the number of seeds per capsule, that the crossed plants again crossed were more productive than the self-fertilised again self-fertilised, in the ratio of 100 to 65. Both the crossed and self-fertilised plants, from having grown much crowded in the two beds, produced less fine capsules and fewer seeds than did their parents.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
The crossed and self-fertilised seeds from the crossed and self-fertilised plants of the last generation were sown on opposite sides of two pots; but the seedlings were not thinned enough, so that both lots grew very irregularly, and most of the self-fertilised plants after a time died from being smothered. My measurements were, therefore, very incomplete. From the first the crossed seedlings appeared the finest, and when they were on an average, by estimation, 5 inches high, the self-fertilised plants were only 4 inches. In both pots the crossed plants flowered first. The two tallest flower-stems on the crossed plants in the two pots were 17 and 16 1/2 inches in height; and the two tallest flower-stems on the self-fertilised plants 10 1/2 and 9 inches; so that their heights were as 100 to 58. But this ratio, deduced from only two pairs, obviously is not in the least trustworthy, and would not have been given had it not been otherwise supported. I state in my notes that the crossed plants were very much more luxuriant than their opponents, and seemed to be twice as bulky. This latter estimate may be believed from the ascertained weights of the two lots in the next generation. Some flowers on these crossed plants were again crossed with pollen from another plant of the same lot, and some flowers on the self-fertilised plants again self-fertilised; and from the seeds thus obtained the plants of the next generation were raised.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
The seeds just alluded to were allowed to germinate on bare sand, and were planted in pairs on the opposite sides of four pots. When the seedlings were in full flower, the tallest stem on each plant was measured to the base of the calyx. The measurements are given in Table 4/46. In Pot 1 the crossed and self-fertilised plants flowered at the same time; but in the other three pots the crossed flowered first. These latter plants also continued flowering much later in the autumn than the self-fertilised.
TABLE 4/46. Dianthus caryophyllus (third generation).
Tallest flower-stem on each plant measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 28 6/8 : 30. Pot 1 : 27 3/8 : 26.
Pot 2 : 29 : 30 7/8. Pot 2 : 29 4/8 : 27 4/8.
Pot 3 : 28 4/8 : 31 6/8. Pot 3 : 23 4/8 : 24 5/8.
Pot 4 : 27 : 30. Pot 4 : 33 4/8 : 25.
Total : 227.13 : 225.75.
Tallest flower-stem on each plant measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 28 6/8 : 30. Pot 1 : 27 3/8 : 26.
Pot 2 : 29 : 30 7/8. Pot 2 : 29 4/8 : 27 4/8.
Pot 3 : 28 4/8 : 31 6/8. Pot 3 : 23 4/8 : 24 5/8.
Pot 4 : 27 : 30. Pot 4 : 33 4/8 : 25.
Total : 227.13 : 225.75.
The average height of the eight crossed plants is here 28.39 inches, and of the eight self-fertilised 28.21; or as 100 to 99. So that there was no difference in height worth speaking of; but in general vigour and luxuriance there was an astonishing difference, as shown by their weights. After the seed-capsules had been gathered, the eight crossed and the eight self-fertilised plants were cut down and weighed; the former weighed 43 ounces, and the latter only 21 ounces; or as 100 to 49.
These plants were all kept under a net, so that the capsules which they produced must have been all spontaneously self-fertilised. The eight crossed plants produced twenty-one such capsules, of which only twelve contained any seed, averaging 8.5 per capsule. On the other hand, the eight self-fertilised plants produced no less than thirty-six capsules, of which I examined twenty-five, and, with the exception of three, all contained seeds, averaging 10.63 seeds per capsule. Thus the proportional number of seeds per capsule produced by the plants of crossed origin to those produced by the plants of self-fertilised origin (both lots being spontaneously self-fertilised) was as 100 to 125. This anomalous result is probably due to some of the self-fertilised plants having varied so as to mature their pollen and stigmas more nearly at the same time than is proper to the species; and we have already seen that some plants in the first experiment differed from the others in being slightly more self-fertile.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
Twenty flowers on the self-fertilised plants of the last or third generation, in Table 4/46, were fertilised with their own pollen, but taken from other flowers on the same plants. These produced fifteen capsules, which contained (omitting two with only three and six seeds) on an average 47.23 seeds, with a maximum of seventy in one. The self-fertilised capsules from the self-fertilised plants of the first generation yielded the much lower average of 35.95 seeds; but as these latter plants grew extremely crowded, nothing can be inferred with respect to this difference in their self-fertility. The seedlings raised from the above seeds constitute the plants of the fourth self-fertilised generation in Table 4/47.
Twelve flowers on the same plants of the third self-fertilised generation, in Table 4/46, were crossed with pollen from the crossed plants in the same table. These crossed plants had been intercrossed for the three previous generations; and many of them, no doubt, were more or less closely inter-related, but not so closely as in some of the experiments with other species; for several carnation plants had been raised and crossed in the earlier generations. They were not related, or only in a distant degree, to the self-fertilised plants. The parents of both the self-fertilised and crossed plants had been subjected to as nearly as possible the same conditions during the three previous generations. The above twelve flowers produced ten capsules, containing on an average 48.66 seeds, with a maximum in one of seventy-two seeds. The plants raised from these seeds may be called the INTERCROSSED.
Lastly, twelve flowers on the same self-fertilised plants of the third generation were crossed with pollen from plants which had been raised from seeds purchased in London. It is almost certain that the plants which produced these seeds had grown under very different conditions to those to which my self-fertilised and crossed plants had been subjected; and they were in no degree related. The above twelve flowers thus crossed all produced capsules, but these contained the low average of 37.41 seeds per capsule, with a maximum in one of sixty-four seeds. It is surprising that this cross with a fresh stock did not give a much higher average number of seeds; for, as we shall immediately see, the plants raised from these seeds, which may be called the LONDON-CROSSED, benefited greatly by the cross, both in growth and fertility.
The above three lots of seeds were allowed to germinate on bare sand. Many of the London-crossed germinated before the others, and were rejected; and many of the intercrossed later than those of the other two lots. The seeds after thus germinating were planted in ten pots, made tripartite by superficial divisions; but when only two kinds of seeds germinated at the same time, they were planted on the opposite sides of other pots; and this is indicated by blank spaces in one of the three columns in Table 4/47. A 0 in the table signifies that the seedling died before it was measured; and a + signifies that the plant did not produce a flower-stem, and therefore was not measured. It deserves notice that no less than eight out of the eighteen self-fertilised plants either died or did not flower; whereas only three out of the eighteen intercrossed, and four out of the twenty London-crossed plants, were in this predicament. The self-fertilised plants had a decidedly less vigorous appearance than the plants of the other two lots, their leaves being smaller and narrower. In only one pot did a self-fertilised plant flower before one of the two kinds of crossed plants, between which there was no marked difference in the period of flowering. The plants were measured to the base of the calyx, after they had completed their growth, late in the autumn.
TABLE 4/47. Dianthus caryophyllus.
Heights of plants to the base of the calyx, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: London-Crossed Plants.
Column 3: Intercrossed Plants.
Column 4: Self-fertilised Plants.
Pot 1 : 39 5/8 : 25 1/8 : 29 2/8. Pot 1 : 30 7/8 : 21 6/8 : +.
Pot 2 : 36 2/8 : : 22 3/8. Pot 2 : 0 : : +.
Pot 3 : 28 5/8 : 30 2/8 : . Pot 3 : + : 23 1/8 : .
Pot 4 : 33 4/8 : 35 5/8 : 30. Pot 4 : 28 7/8 : 32 : 24 4/8.
Pot 5 : 28 : 34 4/8 : +. Pot 5 : 0 : 24 2/8 : +.
Pot 6 : 32 5/8 : 24 7/8 : 30 3/8. Pot 6 : 31 : 26 : 24 4/8.
Pot 7 : 41 7/8 : 29 7/8 : 27 7/8. Pot 7 : 34 7/8 : 26 4/8 : 27.
Pot 8 : 34 5/8 : 29 : 26 6/8. Pot 8 : 28 5/8 : 0 : +.
Pot 9 : 25 5/8 : 28 5/8 : +. Pot 9 : 0 : + : 0.
Pot 10 : 38 : 28 4/8 : 22 7/8. Pot 10 : 32 1/8 : + : 0.
Total : 525.13 : 420.00 : 265.50.
Heights of plants to the base of the calyx, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: London-Crossed Plants.
Column 3: Intercrossed Plants.
Column 4: Self-fertilised Plants.
Pot 1 : 39 5/8 : 25 1/8 : 29 2/8. Pot 1 : 30 7/8 : 21 6/8 : +.
Pot 2 : 36 2/8 : : 22 3/8. Pot 2 : 0 : : +.
Pot 3 : 28 5/8 : 30 2/8 : . Pot 3 : + : 23 1/8 : .
Pot 4 : 33 4/8 : 35 5/8 : 30. Pot 4 : 28 7/8 : 32 : 24 4/8.
Pot 5 : 28 : 34 4/8 : +. Pot 5 : 0 : 24 2/8 : +.
Pot 6 : 32 5/8 : 24 7/8 : 30 3/8. Pot 6 : 31 : 26 : 24 4/8.
Pot 7 : 41 7/8 : 29 7/8 : 27 7/8. Pot 7 : 34 7/8 : 26 4/8 : 27.
Pot 8 : 34 5/8 : 29 : 26 6/8. Pot 8 : 28 5/8 : 0 : +.
Pot 9 : 25 5/8 : 28 5/8 : +. Pot 9 : 0 : + : 0.
Pot 10 : 38 : 28 4/8 : 22 7/8. Pot 10 : 32 1/8 : + : 0.
Total : 525.13 : 420.00 : 265.50.
The average height of the sixteen London-crossed plants in Table 4/47 is 32.82 inches; that of the fifteen intercrossed plants, 28 inches; and that of the ten self-fertilised plants, 26.55.
So that in height we have the following ratios:—
The London-crossed to the self-fertilised as 100 to 81.
The London-crossed to the intercrossed as 100 to 85.
The intercrossed to the self-fertilised as 100 to 95.
The London-crossed to the self-fertilised as 100 to 81.
The London-crossed to the intercrossed as 100 to 85.
The intercrossed to the self-fertilised as 100 to 95.
These three lots of plants, which it should be remembered were all derived on the mother-side from plants of the third self-fertilised generation, fertilised in three different ways, were left exposed to the visits of insects, and their flowers were freely crossed by them. As the capsules of each lot became ripe they were gathered and kept separate, the empty or bad ones being thrown away. But towards the middle of October, when the capsules could no longer ripen, all were gathered and were counted, whether good or bad. The capsules were then crushed, and the seed cleaned by sieves and weighed. For the sake of uniformity the results are given from calculation, as if there had been twenty plants in each lot.
The sixteen London-crossed plants actually produced 286 capsules; therefore twenty such plants would have produced 357.5 capsules; and from the actual weight of the seeds, the twenty plants would have yielded 462 grains weight of seeds.
The fifteen intercrossed plants actually produced 157 capsules; therefore twenty of them would have produced 209.3 capsules and the seeds would have weighed 208.48 grains.
The ten self-fertilised plants actually produced 70 capsules, therefore twenty of them would have produced 140 capsules; and the seeds would have weighed 153.2 grains.
From these data we get the following ratios:—
NUMBER OF CAPSULES PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE LOTS.
NUMBER OF CAPSULES:
The London-crossed to the self-fertilised as 100 to 39.
The London-crossed to the intercrossed as 100 to 45.
The intercrossed to the self-fertilised as 100 to 67.
The London-crossed to the intercrossed as 100 to 45.
The intercrossed to the self-fertilised as 100 to 67.
WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE LOTS.
WEIGHT OF SEED:
The London-crossed to the self-fertilised as 100 to 33.
The London-crossed to the intercrossed as 100 to 45.
The intercrossed to the self-fertilised as 100 to 73.
The London-crossed to the intercrossed as 100 to 45.
The intercrossed to the self-fertilised as 100 to 73.
We thus see how greatly the offspring from the self-fertilised plants of the third generation crossed by a fresh stock, had their fertility increased, whether tested by the number of capsules produced or by the weight of the contained seeds; this latter being the more trustworthy method. Even the offspring from the self-fertilised plants crossed by one of the crossed plants of the same stock, notwithstanding that both lots had been long subjected to the same conditions, had their fertility considerably increased, as tested by the same two methods.
In conclusion it may be well to repeat in reference to the fertility of these three lots of plants, that their flowers were left freely exposed to the visits of insects and were undoubtedly crossed by them, as may be inferred from the large number of good capsules produced. These plants were all the offspring of the same mother-plants, and the strongly marked difference in their fertility must be attributed to the nature of the pollen employed in fertilising their parents; and the difference in the nature of the pollen must be attributed to the different treatment to which the pollen-bearing parents had been subjected during several previous generations.
COLOUR OF THE FLOWERS.
The flowers produced by the self-fertilised plants of the last or fourth generation were as uniform in tint as those of a wild species, being of a pale pink or rose colour. Analogous cases with Mimulus and Ipomoea, after several generations of self-fertilisation, have been already given. The flowers of the intercrossed plants of the fourth generation were likewise nearly uniform in colour. On the other hand, the flowers of the London-crossed plants, or those raised from a cross with the fresh stock which bore dark crimson flowers, varied extremely in colour, as might have been expected, and as is the general rule with seedling carnations. It deserves notice that only two or three of the London-crossed plants produced dark crimson flowers like those of their fathers, and only a very few of a pale pink like those of their mothers. The great majority had their petals longitudinally and variously striped with the two colours,—the groundwork tint being, however, in some cases darker than that of the mother-plants.
12. MALVACEAE.—Hibiscus africanus.
Many flowers on this Hibiscus were crossed with pollen from a distinct plant, and many others were self-fertilised. A rather larger proportional number of the crossed than of the self-fertilised flowers yielded capsules, and the crossed capsules contained rather more seeds. The self-fertilised seeds were a little heavier than an equal number of the crossed seeds, but they germinated badly, and I raised only four plants of each lot. In three out of the four pots, the crossed plants flowered first.
TABLE 4/48. Hibiscus africanus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 13 4/8 : 16 2/8.
Pot 2 : 14 : 14.
Pot 3 : 8 : 7.
Pot 4 : 17 4/8 : 20 4/8.
Total : 53.00 : 57.75.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 13 4/8 : 16 2/8.
Pot 2 : 14 : 14.
Pot 3 : 8 : 7.
Pot 4 : 17 4/8 : 20 4/8.
Total : 53.00 : 57.75.
The four crossed plants average 13.25, and the four self-fertilised 14.43 inches in height; or as 100 to 109. Here we have the unusual case of self-fertilised plants exceeding the crossed in height; but only four pairs were measured, and these did not grow well or equally. I did not compare the fertility of the two lots.
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