The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
Seeds from plants of the fifth generation crossed and self-fertilised in the usual manner were sown on opposite sides of several pots. On the self-fertilised side every single plant belonged to the tall white variety. On the crossed side some plants belonged to this variety, but the greater number approached in character to the old and shorter kinds with smaller yellowish flowers blotched with coppery brown. When the plants on both sides were from 2 to 3 inches in height they were equal, but when fully grown the self-fertilised were decidedly the tallest and finest plants, but, from want of time, they were not actually measured. In half the pots the first plant which flowered was a self-fertilised one, and in the other half a crossed one. And now another remarkable change was clearly perceived, namely, that the self-fertilised plants had become more self-fertile than the crossed. The pots were all put under a net to exclude insects, and the crossed plants produced spontaneously only fifty-five capsules, whilst the self-fertilised plants produced eighty-one capsules, or as 100 to 147. The seeds from nine capsules of both lots were placed in separate watch-glasses for comparison, and the self-fertilised appeared rather the more numerous. Besides these spontaneously self-fertilised capsules, twenty flowers on the crossed plants again crossed yielded sixteen capsules; twenty-five flowers on the self-fertilised plants again self-fertilised yielded seventeen capsules, and this is a larger proportional number of capsules than was produced by the self-fertilised flowers on the self-fertilised plants in the previous generations. The contents of ten capsules of both these lots were compared in separate watch-glasses, and the seeds from the self-fertilised appeared decidedly more numerous than those from the crossed plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION. Crossed and self-fertilised seeds from the crossed and self-fertilised plants of the sixth generation were sown in the usual manner on opposite sides of three pots, and the seedlings were well and equally thinned. Every one of the self-fertilised plants (and many were raised) in this, as well as in the eighth and ninth generations, belonged to the tall white variety. Their uniformity of character, in comparison with the seedlings first raised from the purchased seed, was quite remarkable. On the other hand, the crossed plants differed much in the tints of their flowers, but not, I think, to so great a degree as those first raised. I determined this time to measure the plants on both sides carefully. The self-fertilised seedlings came up rather before the crossed, but both lots were for a time of equal height. When first measured, the average height of the six tallest crossed plants in the three pots was 7.02, and that of the six tallest self-fertilised plants 8.97 inches, or as 100 to 128. When fully grown the same plants were again measured, with the result shown in Table 3/18. TABLE 3/18. Mimulus luteus (Seventh Generation). Heights of Plants in inches: Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants.
Pot 1 : 11 2/8 : 19 1/8. Pot 1 : 11 7/8 : 18. Pot 2 : 12 6/8 : 18 2/8. Pot 2 : 11 2/8 : 14 6/8. Pot 3 : 9 6/8 : 12 6/8. Pot 3 : 11 6/8 : 11. Total : 68.63 : 93.88. The average height of the six crossed is here 11.43, and that of the six self-fertilised 15.64, or as 100 to 137. As it is now evident that the tall white variety transmitted its characters faithfully, and as the self-fertilised plants consisted exclusively of this variety, it was manifest that they would always exceed in height the crossed plants which belonged chiefly to the original shorter varieties. This line of experiment was therefore discontinued, and I tried whether intercrossing two self-fertilised plants of the sixth generation, growing in distinct pots, would give their offspring any advantage over the offspring of flowers on one of the same plants fertilised with their own pollen. These latter seedlings formed the seventh generation of self-fertilised plants, like those in the right hand column in Table 3/18; the crossed plants were the product of six previous self-fertilised generations with an intercross in the last generation. The seeds were allowed to germinate on sand, and were planted in pairs on opposite sides of four pots, all the remaining seeds being sown crowded on opposite sides of Pot 5 in Table 3/19; the three tallest on each side in this latter pot being alone measured. All the plants were twice measured--the first time whilst young, and the average height of the crossed plants to that of the self-fertilised was then as 100 to 122. When fully grown they were again measured, as in Table 3/19.
TABLE 3/19. Mimulus luteus. Heights of Plants in inches: Column 1: Number (Name) of Pot. Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth Generation. Column 3: Self-fertilised Plants of the Seventh Generation. Pot 1 : 12 6/8 : 15 2/8. Pot 1 : 10 4/8 : 11 5/8. Pot 1 : 10 : 11. Pot 1 : 14 5/8 : 11. Pot 2 : 10 2/8 : 11 3/8. Pot 2 : 7 6/8 : 11 4/8. Pot 2 : 12 1/8 : 8 5/8. Pot 2 : 7 : 14 3/8. Pot 3 : 13 5/8 : 10 3/8. Pot 3 : 12 2/8 : 11 6/8. Pot 4 : 7 1/8 : 14 6/8. Pot 4 : 8 2/8 : 7. Pot 4 : 7 2/8 : 8. Pot 5 : 8 5/8 : 10 2/8 Pot 5 : 9 : 9 3/8. Pot 5 : 8 2/8 : 9 2/8. Crowded. Total : 159.38 : 175.50. The average height of the sixteen intercrossed plants is here 9.96 inches, and that of the sixteen self-fertilised plants 10.96, or as 100 to 110; so that the intercrossed plants, the progenitors of which had been self-fertilised for the six previous generations, and had been exposed during the whole time to remarkably uniform conditions, were somewhat inferior in height to the plants of the seventh self-fertilised generation. But as we shall presently see that a similar experiment made after two additional generations of self-fertilisation gave a different result, I know not how far to trust the present one. In three of the five pots in Table 3/19 a self-fertilised plant flowered first, and in the other two a crossed plant. These self-fertilised plants were remarkably fertile, for twenty flowers fertilised with their own pollen produced no less than nineteen very fine capsules!
THE EFFECTS OF A CROSS WITH A DISTINCT STOCK. Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were fertilised with their own pollen, and plants of the eighth self-fertilised generation were thus raised, merely to serve as parents in the following experiment. Several flowers on these plants were allowed to fertilise themselves spontaneously (insects being of course excluded), and the plants raised from these seeds formed the ninth self-fertilised generation; they consisted wholly of the tall white variety with crimson blotches. Other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from another plant of the same lot; so that the seedlings thus raised were the offspring of eight previous generations of self-fertilisation with an intercross in the last generation; these I will call the INTERCROSSED PLANTS. Lastly, other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from plants which had been raised from seed procured from a garden at Chelsea. The Chelsea plants bore yellow flowers blotched with red, but differed in no other respect. They had been grown out of doors, whilst mine had been cultivated in pots in the greenhouse for the last eight generations, and in a different kind of soil. The seedlings raised from this cross with a wholly different stock may be called the CHELSEA-CROSSED. The three lots of seeds thus obtained were allowed to germinate on bare sand; and whenever a seed in all three lots, or in only two, germinated at the same time, they were planted in pots superficially divided into three or two compartments. The remaining seeds, whether or not in a state of germination, were thickly sown in three divisions in a large pot, 10, in Table 3/20. When the plants had grown to their full height they were measured, as shown in Table 3/20; but only the three tallest plants in each of the three divisions in Pot 10 were measured.
TABLE 3/20. Mimulus luteus. Heights of Plants in inches: Column 1: Number (Name) of Pot. Column 2: Plants from Self-fertilised Plants of the Eighth Generation crossed by Chelsea Plants. Column 3: Plants from an intercross between the Plants of the Eighth Self-fertilised Generation. Column 4: Self-fertilised Plants of the Ninth Generation from Plants of the Eighth Self-fertilised Generation. Pot 1 : 30 7/8 : 14 : 9 4/8. Pot 1 : 28 3/8 : 13 6/8 : 10 5/8. Pot 1 : -- : 13 7/8 : 10. Pot 2 : 20 6/8 : 11 4/8 : 11 6/8. Pot 2 : 22 2/8 : 12 : 12 3/8. Pot 2 : -- : 9 1/8 : --. Pot 3 : 23 6/8 : 12 2/8 : 8 5/8. Pot 3 : 24 1/8 : -- : 11 4/8. Pot 3 : 25 6/8 : -- : 6 7/8. Pot 4 : 22 5/8 : 9 2/8 : 4. Pot 4 : 22 : 8 1/8 : 13 3/8. Pot 4 : 17 : -- : 11. Pot 5 : 22 3/8 : 9 : 4 4/8. Pot 5 : 19 5/8 : 11 : 13. Pot 5 : 23 4/8 : -- : 13 4/8. Pot 6 : 28 2/8 : 18 6/8 : 12. Pot 6 : 22 : 7 : 16 1/8. Pot 6 : -- : 12 4/8 : --.
Pot 7 : 12 4/8 : 15 : --. Pot 7 : 24 3/8 : 12 3/8 : --. Pot 7 : 20 4/8 : 11 2/8 : --. Pot 7 : 26 4/8 : 15 2/8 : --. Pot 8 : 17 2/8 : 13 3/8 : --. Pot 8 : 22 6/8 : 14 5/8 : --. Pot 8 : 27 : 14 3/8 : --. Pot 9 : 22 6/8 : 11 6/8 : --. Pot 9 : 6 : 17 : --. Pot 9 : 20 2/8 : 14 7/8 : --. Pot 10 : 18 1/8 : 9 2/8 : 10 3/8. Pot 10 : 16 5/8 : 8 2/8 : 8 1/8. Pot 10 : 17 4/8 : 10 : 11 2/8. Crowded plants. Total : 605.38 : 329.50 : 198.50.
In this table the average height of the twenty-eight Chelsea-crossed plants is 21.62 inches; that of the twenty-seven intercrossed plants 12.2; and that of the nineteen self-fertilised 10.44. But with respect to the latter it will be the fairest plan to strike out two dwarfed ones (only 4 inches in height), so as not to exaggerate the inferiority of the self-fertilised plants; and this will raise the average height of the seventeen remaining self-fertilised plants to 11.2 inches. Therefore the Chelsea-crossed are to the intercrossed in height as 100 to 56; the Chelsea-crossed to the self-fertilised as 100 to 52; and the intercrossed to the self-fertilised as 100 to 92. We thus see how immensely superior in height the Chelsea-crossed are to the intercrossed and to the self-fertilised plants. They began to show their superiority when only one inch high. They were also, when fully grown, much more branched with larger leaves and somewhat larger flowers than the plants of the other two lots, so that if they had been weighed, the ratio would certainly have been much higher than that of 100 to 56 and 52. The intercrossed plants are here to the self-fertilised in height as 100 to 92; whereas in the analogous experiment given in Table 3/19 the intercrossed plants from the self-fertilised plants of the sixth generation were inferior in height to the self-fertilised plants in the ratio of 100 to 110. I doubt whether this discordance in the results of the two experiments can be explained by the self-fertilised plants in the present case having been raised from spontaneously self-fertilised seeds, whereas in the former case they were raised from artificially self-fertilised seeds; nor by the present plants having been self-fertilised during two additional generations, though this is a more probable explanation.
With respect to fertility, the twenty-eight Chelsea-crossed plants produced 272 capsules; the twenty-seven intercrossed plants produced 24; and the seventeen self-fertilised plants 17 capsules. All the plants were left uncovered so as to be naturally fertilised, and empty capsules were rejected. Therefore 20 Chelsea-crossed plants would have produced 194.29 capsules. Therefore 20 Intercrossed plants would have produced 17.77 capsules. Therefore 20 Self-fertilised plants would have produced 20.00 capsules. The seeds contained in 8 capsules from the Chelsea-crossed plants weighed 1.1 grains. The seeds contained in 8 capsules from the Intercrossed plants weighed 0.51 grains. The seeds contained in 8 capsules from the Self-fertilised plants weighed 0.33 grains. If we combine the number of capsules produced together with the average weight of contained seeds, we get the following extraordinary ratios: Weight of seed produced by the same number of Chelsea-crossed and intercrossed plants as 100 to 4. Weight of seed produced by the same number of Chelsea-crossed and self-fertilised plants as 100 to 3. Weight of seeds produced by the same number of intercrossed and self-fertilised plants as 100 to 73.
It is also a remarkable fact that the Chelsea-crossed plants exceeded the two other lots in hardiness, as greatly as they did in height, luxuriance, and fertility. In the early autumn most of the pots were bedded out in the open ground; and this always injures plants which have been long kept in a warm greenhouse. All three lots consequently suffered greatly, but the Chelsea-crossed plants much less than the other two lots. On the 3rd of October the Chelsea-crossed plants began to flower again, and continued to do so for some time; whilst not a single flower was produced by the plants of the other two lots, the stems of which were cut almost down to the ground and seemed half dead. Early in December there was a sharp frost, and the stems of Chelsea-crossed were now cut down; but on the 23rd of December they began to shoot up again from the roots, whilst all the plants of the other two lots were quite dead. Although several of the self-fertilised seeds, from which the plants in the right hand column in Table 3/20 were raised, germinated (and were of course rejected) before any of those of the other two lots, yet in only one of the ten pots did a self-fertilised plant flower before the Chelsea-crossed or the intercrossed plants growing in the same pots. The plants of these two latter lots flowered at the same time, though the Chelsea-crossed grew so much taller and more vigorously than the intercrossed.
As already stated, the flowers of the plants originally raised from the Chelsea seeds were yellow; and it deserves notice that every one of the twenty-eight seedlings raised from the tall white variety fertilised, without being castrated, with pollen from the Chelsea plants, produced yellow flowers; and this shows how prepotent this colour, which is the natural one of the species, is over the white colour. THE EFFECTS ON THE OFFSPRING OF INTERCROSSING FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS. In all the foregoing experiments the crossed plants were the product of a cross between distinct plants. I now selected a very vigorous plant in Table 3/20, raised by fertilising a plant of the eighth self-fertilised generation with pollen from the Chelsea stock. Several flowers on this plant were crossed with pollen from other flowers on the same plant, and several other flowers were fertilised with their own pollen. The seed thus produced was allowed to germinate on bare sand; and the seedlings were planted in the usual manner on the opposite sides of six pots. All the remaining seeds, whether or not in a state of germination, were sown thickly in Pot 7; the three tallest plants on each side of this latter pot being alone measured. As I was in a hurry to learn the result, some of these seeds were sown late in the autumn, but the plants grew so irregularly during the winter, that one crossed plant was 28 1/2 inches, and two others only 4, or less than 4 inches in height, as may be seen in Table 3/21. Under such circumstances, as I have observed in many other cases, the result is not in the least trustworthy; nevertheless I feel bound to give the measurements.
TABLE 3/21. Mimulus luteus. Heights of Plants in inches: Column 1: Number (Name) of Pot. Column 2: Plants raised from a Cross between different Flowers on the same Plant. Column 3: Plants raised from Flowers fertilised with their own Pollen. Pot 1 : 17 : 17. Pot 1 : 9 : 3 1/8. Pot 2 : 28 2/8 : 19 1/8. Pot 2 : 16 4/8 : 6. Pot 2 : 13 5/8 : 2. Pot 3 : 4 : 15 6/8. Pot 3 : 2 2/8 : 10. Pot 4 : 23 4/8 : 6 2/8. Pot 4 : 15 4/8 : 7 1/8. Pot 5 : 7 : 13 4/8. Pot 6 : 18 3/8 : 1 4/8. Pot 6 : 11 : 2. Pot 7 : 21 : 15 1/8. Pot 7 : 11 6/8 : 11. Pot 7 : 12 1/8 : 11 2/8. Crowded. Total : 210.88 : 140.75. The fifteen crossed plants here average 14.05, and the fifteen self-fertilised plants 9.38 in height, or as 100 to 67. But if all the plants under ten inches in height are struck out, the ratio of the eleven crossed plants to the eight self-fertilised plants is as 100 to 82. On the following spring, some remaining seeds of the two lots were treated in exactly the same manner; and the measurements of the seedlings are given in Table 3/22.
TABLE 3/22. Mimulus luteus. Heights of Plants in inches: Column 1: Number (Name) of Pot. Column 2: Plants raised from a Cross between different Flowers on the same Plant. Column 3: Plants raised from Flowers fertilised with their own Pollen. Pot 1 : 15 1/8 : 19 1/8. Pot 1 : 12 : 20 5/8. Pot 1 : 10 1/8 : 12 6/8. Pot 2 : 16 2/8 : 11 2/8. Pot 2 : 13 5/8 : 19 3/8. Pot 2 : 20 1/8 : 17 4/8. Pot 3 : 18 7/8 : 12 6/8. Pot 3 : 15 : 15 6/8. Pot 3 : 13 7/8 : 17. Pot 4 : 19 2/8 : 16 2/8. Pot 4 : 19 6/8 : 21 5/8. Pot 5 : 25 3/8 : 22 5/8. Pot 6 : 15 : 19 5/8. Pot 6 : 20 2/8 : 16 2/8. Pot 6 : 27 2/8 : 19 5/8. Pot 7 : 7 6/8 : 7 6/8. Pot 7 : 14 : 8. Pot 7 : 13 4/8 : 7. Pot 8 : 18 2/8 : 20 3/8. Pot 8 : 18 6/8 : 17 6/8. Pot 8 : 18 3/8 : 15 4/8. Pot 8 : 18 3/8 : 15 1/8. Crowded. Total : 370.88 : 353.63.
Here the average height of the twenty-two crossed plants is 16.85, and that of the twenty-two self-fertilised plants 16.07; or as 100 to 95. But if four of the plants in Pot 7, which are much shorter than any of the others, are struck out (and this would be the fairest plan), the twenty-one crossed are to the nineteen self-fertilised plants in height as 100 to 100.6--that is, are equal. All the plants, except the crowded ones in Pot 8, after being measured were cut down, and the eighteen crossed plants weighed 10 ounces, whilst the same number of self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if the dwarfed plants in Pot 7 had been excluded, the self-fertilised would have exceeded the crossed in weight in a higher ratio. In all the previous experiments in which seedlings were raised from a cross between distinct plants, and were put into competition with self-fertilised plants, the former generally flowered first; but in the present case, in seven out of the eight pots a self-fertilised plant flowered before a crossed one on the opposite side. Considering all the evidence with respect to the plants in Table3/ 22, a cross between two flowers on the same plant seems to give no advantage to the offspring thus produced, the self-fertilised plants being in weight superior. But this conclusion cannot be absolutely trusted, owing to the measurements given in Table 3/21, though these latter, from the cause already assigned, are very much less trustworthy than the present ones.]
SUMMARY OF OBSERVATIONS ON Mimulus luteus. In the three first generations of crossed and self-fertilised plants, the tallest plants alone on each side of the several pots were measured; and the average height of the ten crossed to that of the ten self-fertilised plants was as 100 to 64. The crossed were also much more fertile than the self-fertilised, and so much more vigorous that they exceeded them in height, even when sown on the opposite side of the same pot after an interval of four days. The same superiority was likewise shown in a remarkable manner when both kinds of seeds were sown on the opposite sides of a pot with very poor earth full of the roots of another plant. In one instance crossed and self-fertilised seedlings, grown in rich soil and not put into competition with each other, attained to an equal height. When we come to the fourth generation the two tallest crossed plants taken together exceeded by only a little the two tallest self-fertilised plants, and one of the latter beat its crossed opponent,--a circumstance which had not occurred in the previous generations. This victorious self-fertilised plant consisted of a new white-flowered variety, which grew taller than the old yellowish varieties. From the first it seemed to be rather more fertile, when self-fertilised, than the old varieties, and in the succeeding self-fertilised generations became more and more self-fertile. In the sixth generation the self-fertilised plants of this variety compared with the crossed plants produced capsules in the proportion of 147 to 100, both lots being allowed to fertilise themselves spontaneously. In the seventh generation twenty flowers on one of these plants artificially self-fertilised yielded no less than nineteen very fine capsules!
This variety transmitted its characters so faithfully to all the succeeding self-fertilised generations, up to the last or ninth, that all the many plants which were raised presented a complete uniformity of character; thus offering a remarkable contrast with the seedlings raised from the purchased seeds. Yet this variety retained to the last a latent tendency to produce yellow flowers; for when a plant of the eighth self-fertilised generation was crossed with pollen from a yellow-flowered plant of the Chelsea stock, every single seedling bore yellow flowers. A similar variety, at least in the colour of its flowers, also appeared amongst the crossed plants of the third generation. No attention was at first paid to it, and I know not how far it was at first used either for crossing or self-fertilisation. In the fifth generation most of the self-fertilised plants, and in the sixth and all the succeeding generations every single plant consisted of this variety; and this no doubt was partly due to its great and increasing self-fertility. On the other hand, it disappeared from amongst the crossed plants in the later generations; and this was probably due to the continued intercrossing of the several plants. From the tallness of this variety, the self-fertilised plants exceeded the crossed plants in height in all the generations from the fifth to the seventh inclusive; and no doubt would have done so in the later generations, had they been grown in competition with one another. In the fifth generation the crossed plants were in height to the self-fertilised, as 100 to 126; in the sixth, as 100 to 147; and in the seventh generation, as 100 to 137. This excess of height may be attributed not only to this variety naturally growing taller than the other plants, but to its possessing a peculiar constitution, so that it did not suffer from continued self-fertilisation.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.