The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
This variety presents a strikingly analogous case to that of the plant called the Hero, which appeared in the sixth self-fertilised generation of Ipomoea. If the seeds produced by Hero had been as greatly in excess of those produced by the other plants, as was the case with Mimulus, and if all the seeds had been mingled together, the offspring of Hero would have increased to the entire exclusion of the ordinary plants in the later self-fertilised generations, and from naturally growing taller would have exceeded the crossed plants in height in each succeeding generation. Some of the self-fertilised plants of the sixth generation were intercrossed, as were some in the eighth generation; and the seedlings from these crosses were grown in competition with self-fertilised plants of the two corresponding generations. In the first trial the intercrossed plants were less fertile than the self-fertilised, and less tall in the ratio of 100 to 110. In the second trial, the intercrossed plants were more fertile than the self-fertilised in the ratio of 100 to 73, and taller in the ratio of 100 to 92. Notwithstanding that the self-fertilised plants in the second trial were the product of two additional generations of self-fertilisation, I cannot understand this discordance in the results of the two analogous experiments.
The most important of all the experiments on Mimulus are those in which flowers on plants of the eighth self-fertilised generation were again self-fertilised; other flowers on distinct plants of the same lot were intercrossed; and others were crossed with a new stock of plants from Chelsea. The Chelsea-crossed seedlings were to the intercrossed in height as 100 to 56, and in fertility as 100 to 4; and they were to the self-fertilised plants, in height as 100 to 52, and in fertility as 100 to 3. These Chelsea-crossed plants were also much more hardy than the plants of the other two lots; so that altogether the gain from the cross with a fresh stock was wonderfully great. Lastly, seedlings raised from a cross between flowers on the same plant were not superior to those from flowers fertilised with their own pollen; but this result cannot be absolutely trusted, owing to some previous observations, which, however, were made under very unfavourable circumstances. [Digitalis purpurea.
The flowers of the common Foxglove are proterandrous; that is, the pollen is mature and mostly shed before the stigma of the same flower is ready for fertilisation. This is effected by the larger humble-bees, which, whilst in search of nectar, carry pollen from flower to flower. The two upper and longer stamens shed their pollen before the two lower and shorter ones. The meaning of this fact probably is, as Dr. Ogle remarks, that the anthers of the longer stamens stand near to the stigma, so that they would be the most likely to fertilise it (3/3. ‘Popular Science Review’ January 1870 page 50.); and as it is an advantage to avoid self-fertilisation, they shed their pollen first, thus lessening the chance. There is, however, but little danger of self-fertilisation until the bifid stigma opens; for Hildebrand found that pollen placed on the stigma before it had opened produced no effect. (3/4. ‘Geschlechter-Vertheilung bei den Pflanzen’ 1867 page 20.) The anthers, which are large, stand at first transversely with respect to the tubular corolla, and if they were to dehisce in this position they would, as Dr. Ogle also remarks, smear with pollen the whole back and sides of an entering humble-bee in a useless manner; but the anthers twist round and place themselves longitudinally before they dehisce.
The lower and inner side of the mouth of the corolla is thickly clothed with hairs, and these collect so much of the fallen pollen that I have seen the under surface of a humble-bee thickly dusted with it; but this can never be applied to the stigma, as the bees in retreating do not turn their under surfaces upwards. I was therefore puzzled whether these hairs were of any use; but Mr. Belt has, I think, explained their use: the smaller kinds of bees are not fitted to fertilise the flowers, and if they were allowed to enter easily they would steal much nectar, and fewer large bees would haunt the flowers. Humble-bees can crawl into the dependent flowers with the greatest ease, using the “hairs as footholds while sucking the honey; but the smaller bees are impeded by them, and when, having at length struggled through them, they reach the slippery precipice above, they are completely baffled.” Mr. Belt says that he watched many flowers during a whole season in North Wales, and “only once saw a small bee reach the nectary, though many were seen trying in vain to do so.” (3/5. ‘The Naturalist in Nicaragua’ 1874 page 132. But it appears from H. Muller ‘Die Befruchtung der Blumen’ 1873 page 285, that small insects sometimes succeed in entering the flowers.)
I covered a plant growing in its native soil in North Wales with a net, and fertilised six flowers each with its own pollen, and six others with pollen from a distinct plant growing within the distance of a few feet. The covered plant was occasionally shaken with violence, so as to imitate the effects of a gale of wind, and thus to facilitate as far as possible self-fertilisation. It bore ninety-two flowers (besides the dozen artificially fertilised), and of these only twenty-four produced capsules; whereas almost all the flowers on the surrounding uncovered plants were fruitful. Of the twenty-four spontaneously self-fertilised capsules, only two contained their full complement of seed; six contained a moderate supply; and the remaining sixteen extremely few seeds. A little pollen adhering to the anthers after they had dehisced, and accidentally falling on the stigma when mature, must have been the means by which the above twenty-four flowers were partially self-fertilised; for the margins of the corolla in withering do not curl inwards, nor do the flowers in dropping off turn round on their axes, so as to bring the pollen-covered hairs, with which the lower surface is clothed, into contact with the stigma--by either of which means self-fertilisation might be effected.
Seeds from the above crossed and self-fertilised capsules, after germinating on bare sand, were planted in pairs on the opposite sides of five moderately-sized pots, which were kept in the greenhouse. The plants after a time appeared starved, and were therefore, without being disturbed, turned out of their pots, and planted in the open ground in two close parallel rows. They were thus subjected to tolerably severe competition with one another; but not nearly so severe as if they had been left in the pots. At the time when they were turned out, their leaves were between 5 and 8 inches in length, and the longest leaf on the finest plant on each side of each pot was measured, with the result that the leaves of the crossed plants exceeded, on an average, those of the self-fertilised plants by .4 of an inch. In the following summer the tallest flower-stem on each plant, when fully grown, was measured. There were seventeen crossed plants; but one did not produce a flower-stem. There were also, originally, seventeen self-fertilised plants, but these had such poor constitutions that no less than nine died in the course of the winter and spring, leaving only eight to be measured, as in Table 3/23. TABLE 3/23. Digitalis purpurea. The tallest Flower-stem on each Plant measured in inches: 0 means that the Plant died before a Flower-stem was produced.
Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 53 6/8 : 27 4/8. Pot 1 : 57 4/8 : 55 6/8. Pot 1 : 57 6/8 : 0. Pot 1 : 65 : 0. Pot 2 : 34 4/8 : 39. Pot 2 : 52 4/8 : 32. Pot 2 : 63 6/8 : 21. Pot 3 : 57 4/8 : 53 4/8. Pot 3 : 53 4/8 : 0. Pot 3 : 50 6/8 : 0. Pot 3 : 37 2/8 : 0. Pot 4 : 64 4/8 : 34 4/8. Pot 4 : 37 4/8 : 23 6/8. Pot 4 : -- : 0. Pot 5 : 53 : 0. Pot 5 : 47 6/8 : 0. Pot 5 : 34 6/8 : 0. Total : 821.25 : 287.00. The average height of the flower-stems of the sixteen crossed plants is here 51.33 inches; and that of the eight self-fertilised plants, 35.87; or as 100 to 70. But this difference in height does not give at all a fair idea of the vast superiority of the crossed plants. These latter produced altogether sixty-four flower-stems, each plant producing, on an average, exactly four flower-stems, whereas the eight self-fertilised plants produced only fifteen flower-stems, each producing an average only of 1.87 stems, and these had a less luxuriant appearance. We may put the result in another way: the number of flower-stems on the crossed plants was to those on an equal number of self-fertilised plants as 100 to 48.
Three crossed seeds in a state of germination were also planted in three separate pots; and three self-fertilised seeds in the same state in three other pots. These plants were therefore at first exposed to no competition with one another, and when turned out of their pots into the open ground they were planted at a moderate distance apart, so that they were exposed to much less severe competition than in the last case. The longest leaves on the three crossed plants, when turned out, exceeded those on the self-fertilised plants by a mere trifle, namely, on an average by .17 of an inch. When fully grown the three crossed plants produced twenty-six flower-stems; the two tallest of which on each plant were on an average 54.04 inches in height. The three self-fertilised plants produced twenty-three flower-stems, the two tallest of which on each plant had an average height of 46.18 inches. So that the difference between these two lots, which hardly competed together, is much less than in the last case when there was moderately severe competition, namely, as 100 to 85, instead of as 100 to 70. THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
A fine plant growing in my garden (one of the foregoing seedlings) was covered with a net, and six flowers were crossed with pollen from another flower on the same plant, and six others were fertilised with their own pollen. All produced good capsules. The seeds from each were placed in separate watch-glasses, and no difference could be perceived by the eye between the two lots of seeds; and when they were weighed there was no difference of any significance, as the seeds from the self-fertilised capsules weighed 7.65 grains, whilst those from the crossed capsules weighed 7.7 grains. Therefore the sterility of the present species, when insects are excluded, is not due to the impotence of pollen on the stigma of the same flower. Both lots of seeds and seedlings were treated in exactly the same manner as in Table 3/23, excepting that after the pairs of germinating seeds had been planted on the opposite sides of eight pots, all the remaining seeds were thickly sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young plants during the following spring were turned out of their pots, without being disturbed, and planted in the open ground in two rows, not very close together, so that they were subjected to only moderately severe competition with one another. Very differently to what occurred in the first experiment, when the plants were subjected to somewhat severe mutual competition, an equal number on each side either died or did not produce flower-stems. The tallest flower-stems on the surviving plants were measured, as shown in Table 3/24.
TABLE 3/24. Digitalis purpurea. The tallest Flower-stem on each Plant measured in inches: 0 signifies that the Plant died, or did not produce a Flower-stem. 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 : 49 4/8 : 45 5/8. Pot 1 : 46 7/8 : 52. Pot 1 : 43 6/8 : 0. Pot 2 : 38 4/8 : 54 4/8. Pot 2 : 47 4/8 : 47 4/8. Pot 2 : 0 : 32 5/8. Pot 3 : 54 7/8 : 46 5/8. Pot 4 : 32 1/8 : 41 3/8. Pot 4 : 0 : 29 7/8. Pot 4 : 43 7/8 : 37 1/8. Pot 5 : 46 6/8 : 42 1/8. Pot 5 : 40 4/8 : 42 1/8. Pot 5 : 43 : 0. Pot 6 : 48 2/8 : 47 7/8. Pot 6 : 46 2/8 : 48 3/8. Pot 7 : 48 5/8 : 25. Pot 7 : 42 : 40 5/8. Pot 8 : 46 7/8 : 39 1/8. Pot 9 : 49 : 30 3/8. Pot 9 : 50 3/8 : 15. Pot 9 : 46 3/8 : 36 7/8. Pot 9 : 47 6/8 : 44 1/8. Pot 9 : 0 : 31 6/8. Crowded Plants.
Pot 10 : 46 4/8 : 47 7/8. Pot 10 : 35 2/8 : 0. Pot 10 : 24 5/8 : 34 7/8. Pot 10 : 41 4/8 : 40 7/8. Pot 10 : 17 3/8 : 41 1/8. Crowded Plants. Total : 1078.00 : 995.38. The average height of the flower-stems on the twenty-five crossed plants in all the pots taken together is 43.12 inches, and that of the twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to test this result, the plants planted in pairs in Pots 1 and 8 were considered by themselves, and the average height of the sixteen crossed plants is here 44.9, and that of the sixteen self-fertilised plants 42.03, or as 100 to 94. Again, the plants raised from the thickly sown seed in Pots 9 and 10, which were subjected to very severe mutual competition, were taken by themselves, and the average height of the nine crossed plants is 39.86, and that of the nine self-fertilised plants 35.88, or as 100 to 90. The plants in these two latter pots (9 and 10), after being measured, were cut down close to the ground and weighed: the nine crossed plants weighed 57.66 ounces, and the nine self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we may conclude, especially from the evidence of weight, that seedlings from a cross between flowers on the same plant have a decided, though not great, advantage over those from flowers fertilised with their own pollen, more especially in the case of the plants subjected to severe mutual competition. But the advantage is much less than that exhibited by the crossed offspring of distinct plants, for these exceeded the self-fertilised plants in height as 100 to 70, and in the number of flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and almost certainly from Mimulus, as with these two species a cross between flowers on the same plant did no good.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.