The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
TABLE 6/86. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant A in pot 3, Table 6/85, of the last or second generation. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: From Self-fertilised Plant, crossed by a Crossed Plant. Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation. Pot 1 : 100 2/8 : 98. Pot 1 : 91 : 79. Pot 2 : 110 2/8 : 59 1/8. Pot 2 : 100 4/8 : 66 6/8. Pot 3 : 104 : 79 6/8. Pot 4 : 84 2/8 : 110 4/8. Pot 4 : 76 4/8 : 64 1/8. Total : 666.75 : 557.25. As I wished to ascertain, firstly, whether those self-fertilised plants of the last generation, which greatly exceeded in height their crossed opponents, would transmit the same tendency to their offspring, and secondly, whether they possessed the same sexual constitution, I selected for experiment the two self-fertilised plants marked A and B in Pot 3 in Table 6/85, as these two were of nearly equal height, and were greatly superior to their crossed opponents. Four flowers on each plant were fertilised with their own pollen, and four others on the same plants were crossed with pollen from one of the crossed plants growing in another pot. This plan differs from that before followed, in which seedlings from crossed plants again crossed, have been compared with seedlings from self-fertilised plants again self-fertilised. The seeds from the crossed and self-fertilised capsules of the above two plants were placed in separate watch-glasses and compared, but were not weighed; and in both cases those from the crossed capsules seemed to be rather less numerous than those from the self-fertilised capsules. These seeds were planted in the usual manner, and the heights of the crossed and self-fertilised seedlings, when fully grown, are given in Tables 6/86 and 6/87.
The seven crossed plants in the first of these two tables average 95.25, and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In half the pots a crossed plant, and in the other half a self-fertilised plant flowered first. We now come to the seedlings raised from the other parent-plant B. TABLE 6/87. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant B in pot 3, Table 6/85, of the last or second generation. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: From Self-fertilised Plant, crossed by a Crossed Plant. Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation. Pot 1 : 87 2/8 : 72 4/8. Pot 1 : 49 : 14 2/8. Pot 2 : 98 4/8 : 73. Pot 2 : 0 : 110 4/8. Pot 3 : 99 : 106 4/8. Pot 3 : 15 2/8 : 73 6/8. Pot 4 : 97 6/8 : 48 6/8. Pot 5 : 48 6/8 : 81 2/8. Pot 5 : 0 : 61 2/8. Total : 495.50 : 641.75. The seven crossed plants (for two of them died) here average 70.78 inches, and the nine self-fertilised plants 71.3 inches in height; or as 100 to barely 101. In four out of these five pots, a self-fertilised plant flowered before any one of the crossed plants. So that, differently from the last case, the self-fertilised plants are in some respects slightly superior to the crossed.
If we now consider the crossed and self-fertilised plants of the three generations, we find an extraordinary diversity in their relative heights. In the first generation, the crossed plants were inferior to the self-fertilised as 100 to 178; and the flowers on the original parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than the self-fertilised flowers, in the proportion of 100 to 150. But it is a strange fact that the self-fertilised plants, which were subjected to very severe competition with the crossed, had on two occasions no advantage over them. The inferiority of the crossed plants of this first generation cannot be attributed to the immaturity of the seeds, for I carefully examined them; nor to the seeds being diseased or in any way injured in some one capsule, for the contents of the ten crossed capsules were mingled together and a few taken by chance for sowing. In the second generation the crossed and self-fertilised plants were nearly equal in height.
In the third generation, crossed and self-fertilised seeds were obtained from two plants of the previous generation, and the seedlings raised from them differed remarkably in constitution; the crossed in the one case exceeded the self-fertilised in height in the ratio of 100 to 83, and in the other case were almost equal. This difference between the two lots, raised at the same time from two plants growing in the same pot, and treated in every respect alike, as well as the extraordinary superiority of the self-fertilised over the crossed plants in the first generation, considered together, make me believe that some individuals of the present species differ to a certain extent from others in their sexual affinities (to use the term employed by Gartner), like closely allied species of the same genus. Consequently if two plants which thus differ are crossed, the seedlings suffer and are beaten by those from the self-fertilised flowers, in which the sexual elements are of the same nature. It is known that with our domestic animals certain individuals are sexually incompatible, and will not produce offspring, although fertile with other individuals.
(6/3. I have given evidence on this head in my ‘Variation of Animals and Plants under Domestication’ chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a case which bears more closely on our present one, as it shows that in the genus Nicotiana the varieties differ in their sexual affinities. (6/4. ‘Das Geschlecht der Pflanzen, Zweite Fortsetzung’ 1764 pages 55-60.) He experimented on five varieties of the common tobacco, and proved that they were varieties by showing that they were perfectly fertile when reciprocally crossed; but one of these varieties, if used either as the father or the mother, was more fertile than any of the others when crossed with a widely distinct species, N. glutinosa. As the different varieties thus differ in their sexual affinities, there is nothing surprising in the individuals of the same variety differing in a like manner to a slight degree. Taking the plants of the three generations altogether, the crossed show no superiority over the self-fertilised, and I can account for this fact only by supposing that with this species, which is perfectly self-fertile without insect aid, most of the individuals are in the same condition, as those of the same variety of the common pea and of a few other exotic plants, which have been self-fertilised for many generations. In such cases a cross between two individuals does no good; nor does it in any case, unless the individuals differ in general constitution, either from so-called spontaneous variation, or from their progenitors having been subjected to different conditions. I believe that this is the true explanation in the present instance, because, as we shall immediately see, the offspring of plants, which did not profit at all by being crossed with a plant of the same stock, profited to an extraordinary degree by a cross with a slightly different sub-variety.
THE EFFECTS OF A CROSS WITH A FRESH STOCK. I procured some seed of N. tabacum from Kew and raised some plants, which formed a slightly different sub-variety from my former plants; as the flowers were a shade pinker, the leaves a little more pointed, and the plants not quite so tall. Therefore the advantage in height which the seedlings gained by this cross cannot be attributed to direct inheritance. Two of the plants of the third self-fertilised generation, growing in Pots 2 and 5 in Table 6/87, which exceeded in height their crossed opponents (as did their parents in a still higher degree) were fertilised with pollen from the Kew plants, that is, by a fresh stock. The seedlings thus raised may be called the Kew-crossed. Some other flowers on the same two plants were fertilised with their own pollen, and the seedlings thus raised from the fourth self-fertilised generation. The crossed capsules produced by the plant in Pot 2, Table 6/87, were plainly less fine than the self-fertilised capsules on the same plant. In Pot 5 the one finest capsule was also a self-fertilised one; but the seeds produced by the two crossed capsules together exceeded in number those produced by the two self-fertilised capsules on the same plant. Therefore as far as the flowers on the parent-plants are concerned, a cross with pollen from a fresh stock did little or no good; and I did not expect that the offspring would have received any benefit, but in this I was completely mistaken.
The crossed and self-fertilised seeds from the two plants were placed on bare sand, and very many of the crossed seeds of both sets germinated before the self-fertilised seeds, and protruded their radicles at a quicker rate. Hence many of the crossed seeds had to be rejected, before pairs in an equal state of germination were obtained for planting on the opposite sides of sixteen large pots. The two series of seedlings raised from the parent-plants in the two Pots 2 and 5 were kept separate, and when fully grown were measured to the tips of their highest leaves, as shown in Table 6/88. But as there was no uniform difference in height between the crossed and self-fertilised seedlings raised from the two plants, their heights have been added together in calculating the averages. I should state that by the accidental fall of a large bush in the greenhouse, several plants in both the series were much injured. These were at once measured together with their opponents and afterwards thrown away. The others were left to grow to their full height, and were measured when in flower. This accident accounts for the small height of some of the pairs; but as all the pairs, whether only partly or fully grown, were measured at the same time, the measurements are fair.
The average height of the twenty-six crossed plants in the sixteen pots of the two series is 63.29, and that of the twenty-six self-fertilised plants is 41.67 inches; or as 100 to 66. The superiority of the crossed plants was shown in another way, for in every one of the sixteen pots a crossed plant flowered before a self-fertilised one, with the exception of Pot 6 of the second series, in which the plants on the two sides flowered simultaneously. TABLE 6/88. Nicotiana tabacum. Plants raised from two plants of the third self-fertilised generation in Pots 2 and 5, in Table 6/87. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Kew-crossed Plants, pot 2, Table 6/87. Column 3: Plants of the fourth Self-fertilised generation, pot 2, Table 6/87. Column 4: Kew-crossed Plants, pot 5, Table 6/87. Column 5: Plants of the fourth Self-fertilised generation, pot 5, Table 6/87. Pot 1 : 84 6/8 : 68 4/8 : 77 6/8 : 56. Pot 1 : 31 : 5 : 7 2/8 : 5 3/8. Pot 2 : 78 4/8 : 51 4/8 : 55 4/8 : 27 6/8. Pot 2 : 48 : 70 : 18 : 7. Pot 3 : 77 3/8 : 12 6/8 : 76 2/8 : 60 6/8. Pot 3 : 77 1/8 : 6 6/8.
Pot 4 : 49 2/8 : 29 4/8 : 90 4/8 : 11 6/8. Pot 4 : 15 6/8 : 32 : 22 2/8 : 4 1/8. Pot 5 : 89 : 85 : 94 2/8 : 28 4/8. Pot 5 : 17 : 5 3/8. Pot 6 : 90 : 80 : 78 : 78 6/8. Pot 7 : 84 4/8 : 48 6/8 : 85 4/8 : 61 4/8. Pot 7 : 76 4/8 : 56 4/8. Pot 8 : 83 4/8 : 84 4/8 : 65 5/8 : 78 3/8. Pot 8 : : : 72 2/8 : 27 4/8. Total : 902.63 : 636.13 : 743.13 : 447.38. Some of the remaining seeds of both series, whether or not in a state of germination, were thickly sown on the opposite sides of two very large pots; and the six highest plants on each side of each pot were measured after they had grown to nearly their full height. But their heights were much less than in the former trials, owing to their extremely crowded condition. Even whilst quite young, the crossed seedlings manifestly had much broader and finer leaves than the self-fertilised seedlings. TABLE 6/89. Nicotiana tabacum. Plants of the same parentage as those in Table 6/88, but grown extremely crowded in two large pots. Heights of plants measured in inches.
Column 1: Kew-crossed Plants, from pot 2, Table 6/87. Column 2: Plants of the fourth Self-fertilised generation, from pot 2, Table 6/87. Column 3: Kew-crossed Plants, from pot 5, Table 6/87. Column 4: Plants of the fourth Self-fertilised generation, from pot 5, Table 6/87. 42 4/8 : 22 4/8 : 44 6/8 : 22 4/8. 34 : 19 2/8 : 42 4/8 : 21. 30 4/8 : 14 2/8 : 27 4/8 : 18. 23 4/8 : 16 : 31 2/8 : 15 2/8. 26 6/8 : 13 4/8 : 32 : 13 5/8. 18 3/8 : 16 : 24 6/8 : 14 6/8. 175.63 : 101.50 : 202.75 : 105.13. The twelve tallest crossed plants in the two pots belonging to the two series average here 31.53, and the twelve tallest self-fertilised plants 17.21 inches in height; or as 100 to 54. The plants on both sides, when fully grown, some time after they had been measured, were cut down close to the ground and weighed. The twelve crossed plants weighed 21.25 ounces; and the twelve self-fertilised plants only 7.83 ounces; or in weight as 100 to 37. The rest of the crossed and self-fertilised seeds from the two parent-plants (the same as in the last experiment) was sown on the 1st of July in four long parallel and separate rows in good soil in the open ground; so that the seedlings were not subjected to any mutual competition. The summer was wet and unfavourable for their growth. Whilst the seedlings were very small the two crossed rows had a clear advantage over the two self-fertilised rows. When fully grown the twenty tallest crossed plants and the twenty tallest self-fertilised plants were selected and measured on the 11th of November to the extremities of their leaves, as shown in Table 6/90. Of the twenty crossed plants, twelve had flowered; whilst of the twenty self-fertilised plants one alone had flowered.
TABLE 6/90. Nicotiana tabacum. Plants raised from the same seeds as in the last two experiments, but sown separately in the open ground, so as not to compete together. Heights of plants measured in inches. Column 1: Kew-crossed Plants, from pot 2, Table 6/87. Column 2: Plants of the fourth Self-fertilised generation, from pot 2, Table 6/87. Column 3: Kew-crossed Plants, from pot 5, Table 6/87. Column 4: Plants of the fourth Self-fertilised generation, from pot 5, Table 6/87. 42 2/8 : 22 6/8 : 54 4/8 : 34 4/8. 54 5/8 : 37 4/8 : 51 4/8 : 38 5/8. 39 3/8 : 34 4/8 : 45 : 40 6/8. 53 2/8 : 30 : 43 : 43 2/8. 49 3/8 : 28 6/8 : 43 : 40. 50 3/8 : 31 2/8 : 48 6/8 : 38 2/8. 47 1/8 : 25 4/8 : 44 : 35 6/8. 57 3/8 : 26 2/8 : 48 2/8 : 39 6/8. 37 : 22 3/8 : 55 1/8 : 47 6/8. 48 : 28 : 63 : 58 5/8. 478.75 : 286.86 : 496.13 : 417.25 The twenty tallest crossed plants here average 48.74, and the twenty tallest self-fertilised 35.2 inches in height; or as 100 to 72. These plants after being measured were cut down close to the ground, and the twenty crossed plants weighed 195.75 ounces, and the twenty self-fertilised plants 123.25 ounces; or as 100 to 63.
In Tables 6/88, 6/89 and 6/90, we have the measurements of fifty-six plants derived from two plants of the third self-fertilised generation crossed with pollen from a fresh stock, and of fifty-six plants of the fourth self-fertilised generation derived from the same two plants. These crossed and self-fertilised plants were treated in three different ways, having been put, firstly, into moderately close competition with one another in pots; secondly, having been subjected to unfavourable conditions and to very severe competition from being greatly crowded in two large pots; and thirdly, having been sown separately in open and good ground, so as not to suffer from any mutual competition. In all these cases the crossed plants in each lot were greatly superior to the self-fertilised. This was shown in several ways,--by the earlier germination of the crossed seeds, by the more rapid growth of the seedlings whilst quite young, by the earlier flowering of the mature plants, as well as by the greater height which they ultimately attained. The superiority of the crossed plants was shown still more plainly when the two lots were weighed; the weight of the crossed plants to that of the self-fertilised in the two crowded pots being as 100 to 37. Better evidence could hardly be desired of the immense advantage derived from a cross with a fresh stock.
26. PRIMULACEAE.--Cyclamen persicum. (6/5. Cyclamen repandum according to Lecoq ‘Geographie Botanique de l’Europe’ tome 8 1858 page 150, is proterandrous, and this I believe to be the case with Cyclamen persicum.) Ten flowers crossed with pollen from plants known to be distinct seedlings, yielded nine capsules, containing on an average 34.2 seeds, with a maximum of seventy-seven in one. Ten flowers self-fertilised yielded eight capsules, containing on an average only 13.1 seeds, with a maximum of twenty-five in one. This gives a ratio of 100 to 38 for the average number of seeds per capsule for the crossed and self-fertilised flowers. The flowers hang downwards, and as the stigmas stand close beneath the anthers, it might have been expected that pollen would have fallen on them, and that they would have been spontaneously self-fertilised; but these covered-up plants did not produce a single capsule. On some other occasions uncovered plants in the same greenhouse produced plenty of capsules, and I suppose that the flowers had been visited by bees, which could hardly fail to carry pollen from plant to plant.
The seeds obtained in the manner just described were placed on sand, and after germinating were planted in pairs,--three crossed and three self-fertilised plants on the opposite sides of four pots. When the leaves were 2 or 3 inches in length, including the foot-stalks, the seedlings on both sides were equal. In the course of a month or two the crossed plants began to show a slight superiority over the self-fertilised, which steadily increased; and the crossed flowered in all four pots some weeks before, and much more profusely than the self-fertilised. The two tallest flower-stems on the crossed plants in each pot were now measured, and the average height of the eight stems was 9.49 inches. After a considerable interval of time the self-fertilised plants flowered, and several of their flower-stems (but I forgot to record how many) were roughly measured, and their average height was a little under 7.5 inches; so that the flower-stems on the crossed plants to those on the self-fertilised were at least as 100 to 79. The reason why I did not make more careful measurements of the self-fertilised plants was, that they looked such poor specimens that I determined to there them re-potted in larger pots and in the following year to measure them carefully; but we shall see that this was partly frustrated by so few flower-stems being then produced.
These plants were left uncovered in the greenhouse; and the twelve crossed plants produced forty capsules, whilst the twelve self-fertilised plants produced only five; or as 100 to 12. But this difference does not give a just idea of the relative fertility of the two lots. I counted the seeds in one of the finest capsules on the crossed plants, and it contained seventy-three; whilst the finest of the five capsules produced by the self-fertilised plants contained only thirty-five good seeds. In the other four capsules most of the seeds were barely half as large as those in the crossed capsules. TABLE 6/91. Cyclamen persicum: 0 implies that no flower-stem was produced. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 10 : 0. Pot 1 : 9 2/8 : 0. Pot 1 : 10 2/8 : 0. Pot 2 : 9 2/8 : 0. Pot 2 : 10 : 0. Pot 2 : 10 2/8 : 0. Pot 3 : 9 1/8 : 8. Pot 3 : 9 5/8 : 6 7/8. Pot 3 : 9 5/8 : 6 6/8. Pot 4 : 11 1/8 : 0. Pot 4 : 10 5/8 : 7 7/8. Pot 4 : 10 6/8 : 0. Total : 119.88 : 29.50.
In the following year the crossed plants again bore many flowers before the self-fertilised bore a single one. The three tallest flower-stems on the crossed plants in each of the pots were measured, as shown in Table 6/91. In Pots 1 and 2 the self-fertilised plants did not produce a single flower-stem; in Pot 4 only one; and in Pot 3 six, of which the three tallest were measured. The average height of the twelve flower-stems on the crossed plants is 9.99, and that of the four flower-stems on the self-fertilised plants 7.37 inches; or as 100 to 74. The self-fertilised plants were miserable specimens, whilst the crossed ones looked very vigorous.
Text from Project Gutenberg, public domain in the United States.