The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
TABLE 6/78. Petunia violacea (third generation; plants very young). Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 1 4/8 : 5 6/8. Pot 1 : 1 : 4 4/8. Pot 2 : 5 7/8 : 8 3/8. Pot 2 : 5 6/8 : 6 7/8. Pot 3 : 4 : 5 5/8. Pot 4 : 1 4/8 : 5 3/8. Total : 19.63 : 36.50. The above seeds were placed on sand, and after germinating were planted in pairs on the opposite sides of four pots; and all the remaining seeds were thickly sown on the two sides of a fifth large pot. The result was surprising, for the self-fertilised seedlings very early in life beat the crossed, and at one time were nearly double their height. At first the case appeared like that of Mimulus, in which after the third generation a tall and highly self-fertile variety appeared. But as in the two succeeding generations the crossed plants resumed their former superiority over the self-fertilised, the case must be looked at as an anomaly. The sole conjecture which I can form is that the crossed seeds had not been sufficiently ripened, and thus produced weakly plants, as occurred with Iberis. When the crossed plants were between 3 and 4 inches in height, the six finest in four of the pots were measured to the summits of their stems, and at the same time the six finest of the self-fertilised plants. The measurements are given in Table 6/78, and it may be here seen that all the self-fertilised plants exceed their opponents in height, whereas when subsequently measured the excess of the self-fertilised depended chiefly on the unusual tallness of two of the plants in Pot 2. The crossed plants here average 3.27, and the self-fertilised 6.08 inches in height; or as 100 to 186.
When fully grown they were again measured, as follows:-- TABLE 6/79. Petunia violacea (third generation; plants fully grown). Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 41 4/8 : 40 6/8. Pot 1 : 48 : 39. Pot 1 : 36 : 48. Pot 2 : 36 : 47. Pot 2 : 21 : 80 2/8. Pot 2 : 36 2/8 : 86 2/8. Pot 3 : 52 : 46. Pot 4 : 57 : 43 6/8. Total : 327.75 : 431.00. The eight crossed plants now averaged 40.96, and the eight self-fertilised plants 53.87 inches in height, or as 100 to 131; and this excess chiefly depended, as already stated, on the unusual tallness of two of the self-fertilised plants in Pot 2. The self-fertilised had therefore lost some of their former great superiority over the crossed plants. In three of the pots the self-fertilised plants flowered first; but in Pot 3 at the same time with the crossed.
The case is rendered the more strange, because the crossed plants in the fifth pot (not included in the two last tables), in which all the remaining seeds had been thickly sown, were from the first finer plants than the self-fertilised, and had larger leaves. At the period when the two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high, the two tallest self-fertilised were only 4 inches. When the two crossed plants were 12 and 10 inches high, the two self-fertilised were only 8 inches. These latter plants, as well as many others on the same side of this pot never grew any higher, whereas several of the crossed plants grew to the height of two feet! On account of this great superiority of the crossed plants, the plants on neither side of this pot have been included in the two last tables. Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were again crossed, and produced seventeen capsules. Thirty flowers on the self-fertilised plants in the same two pots were again self-fertilised, but produced only seven capsules. The contents of each capsule of both lots were placed in separate watch-glasses, and the seeds from the crossed appeared to the eye to be at least double the number of those from the self-fertilised capsules.
In order to ascertain whether the fertility of the self-fertilised plants had been lessened by the plants having been self-fertilised for the three previous generations, thirty flowers on the crossed plants were fertilised with their own pollen. These yielded only five capsules, and their seeds being placed in separate watch-glasses did not seem more numerous than those from the capsules on the self-fertilised plants self-fertilised for the fourth time. So that as far as can be judged from so few capsules, the self-fertility of the self-fertilised plants had not decreased in comparison with that of the plants which had been intercrossed during the three previous generations. It should, however, be remembered that both lots of plants had been subjected in each generation to almost exactly similar conditions. Seeds from the crossed plants again crossed, and from the self-fertilised again self-fertilised, produced by the plants in Pot 1 (Table 6/79), in which the three self-fertilised plants were on an average only a little taller than the crossed, were used in the following experiment. They were kept separate from two similar lots of seeds produced by the two plants in Pot 4 in the same table, in which the crossed plant was much taller than its self-fertilised opponent. CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 1, TABLE 6/79).
Crossed and self-fertilised seeds from plants of the last generation in Pot 1 in Table 6/79, were placed on sand, and after germinating, were planted in pairs on the opposite sides of four pots. The seedlings when in full flower were measured to the base of the calyx. The remaining seeds were sown crowded on the two sides of Pot 5; and the four tallest plants on each side of this pot were measured in the same manner. TABLE 6/80. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 1, table 6/79). Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants. Column 3: Self-fertilised Plants. Pot 1 : 29 2/8 : 30 2/8. Pot 1 : 36 2/8 : 34 6/8. Pot 1 : 49 : 31 3/8. Pot 2 : 33 3/8 : 31 5/8. Pot 2 : 37 3/8 : 38 2/8. Pot 2 : 56 4/8 : 38 4/8. Pot 3 : 46 : 45 1/8. Pot 3 : 67 2/8 : 45. Pot 3 : 54 3/8 : 23 2/8. Pot 4 : 51 6/8 : 34. Pot 4 : 51 7/8 : 0. Pot 5 : 49 4/8 : 22 3/8. Pot 5 : 46 3/8 : 24 2/8. Pot 5 : 40 : 24 6/8. Pot 5 : 53 : 30. Crowded plants.
Total : 701.88 : 453.50. The fifteen crossed plants average 46.79, and the fourteen (one having died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So that the crossed plants in this generation had recovered their wonted superiority over the self-fertilised plants; though the parents of the latter in Pot 1, Table 6/79, were a little taller than their crossed opponents. CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 4, TABLE 6/79). Two similar lots of seeds, obtained from the plants in Pot 4 in Table 6/79, in which the single crossed plant was at first shorter, but ultimately much taller than its self-fertilised opponent, were treated in every way like their brethren of the same generation in the last experiment. We have in Table 6/81 the measurements of the present plants. Although the crossed plants greatly exceeded in height the self-fertilised; yet in three out of the five pots a self-fertilised plant flowered before any one of the crossed; in a fourth pot simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered first. TABLE 6/81. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 4, Table 6/79). Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Crossed Plants.
Column 3: Self-fertilised Plants. Pot 1 : 46 : 30 2/8. Pot 1 : 46 : 28. Pot 2 : 50 6/8 : 25. Pot 2 : 40 2/8 : 31 3/8. Pot 2 : 37 3/8 : 22 4/8. Pot 3 : 54 2/8 : 22 5/8. Pot 3 : 61 1/8 : 26 6/8. Pot 3 : 45 : 32. Pot 4 : 30 : 24 4/8. Pot 4 : 29 1/8 : 26. Pot 5 : 37 4/8 : 40 2/8. Pot 5 : 63 : 18 5/8. Pot 5 : 41 2/8 : 17 4/8. Crowded plants. Total : 581.63 : 349.36. The thirteen crossed plants here average 44.74, and the thirteen self-fertilised plants 26.87 inches in height; or as 100 to 60. The crossed parents of these were much taller, relatively to the self-fertilised parents, than in the last case; and apparently they transmitted some of this superiority to their crossed offspring. It is unfortunate that I did not turn these plants out of doors, so as to observe their relative fertility, for I compared the pollen from some of the crossed and self-fertilised plants in Pot 1, Table 6/81, and there was a marked difference in its state; that of the crossed plants contained hardly any bad and empty grains, whilst such abounded in the pollen of the self-fertilised plants.
THE EFFECTS OF A CROSS WITH A FRESH STOCK. I procured from a garden in Westerham, whence my plants originally came, a fresh plant differing in no respect from mine except in the colour of the flowers, which was a fine purple. But this plant must have been exposed during at least four generations to very different conditions from those to which my plants had been subjected, as these had been grown in pots in the greenhouse. Eight flowers on the self-fertilised plants in Table 6/81, of the last or fourth self-fertilised generation, were fertilised with pollen from this fresh stock; all eight produced capsules containing together by weight 5.01 grains of seeds. The plants raised from these seeds may be called the Westerham-crossed. Eight flowers on the crossed plants of the last or fourth generation in Table 6/81 were again crossed with pollen from one of the other crossed plants, and produced five capsules, containing by weight 2.07 grains of seeds. The plants raised from these seeds may be called the INTERCROSSED; and these form the fifth intercrossed generation. Eight flowers on the self-fertilised plants of the same generation in Table 6/81 were again self-fertilised, and produced seven capsules, containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants raised from these seeds form the fifth self-fertilised generation. These latter plants and the intercrossed are comparable in all respects with the crossed and self-fertilised plants of the four previous generations.
The Westerham-crossed plants in height to the self-fertilised as 100 to 91. The Westerham-crossed plants in height to the intercrossed as 100 to 119. The intercrossed plants in height to the self-fertilised as 100 to 77. These plants were again measured when their growth appeared on a casual inspection to be complete. But in this I was mistaken, for after cutting them down, I found that the summits of the stems of the Westerham-crossed plants were still growing vigorously; whilst the intercrossed had almost, and the self-fertilised had quite completed their growth. Therefore I do not doubt, if the three lots had been left to grow for another month, that the ratios would have been somewhat different from those deduced from the measurements in Table 6/82. TABLE 6/82. Petunia violacea. Heights of plants measured in inches. Column 1: Number (Name) of Pot. Column 2: Westerham-Crossed Plants (from self-fertilised Plants of fourth generation crossed by a fresh stock). Column 3: Intercrossed Plants (Plants of one and the same stock intercrossed for five generations). Column 4: Self-fertilised Plants (self-fertilised for five generations). Pot 1 : 64 5/8 : 57 2/8 : 43 6/8. Pot 1 : 24 : 64 : 56 3/8. Pot 1 : 51 4/8 : 58 6/8 : 31 5/8. Pot 2 : 48 7/8 : 59 7/8 : 41 5/8. Pot 2 : 54 4/8 : 58 2/8 : 41 2/8. Pot 2 : 58 1/8 : 53 : 18 2/8.
Pot 3 : 62 : 52 2/8 : 46 6/8. Pot 3 : 53 2/8 : 54 6/8 : 45. Pot 3 : 62 7/8 : 61 6/8 : 19 4/8. Pot 4 : 44 4/8 : 58 7/8 : 37 5/8. Pot 4 : 49 2/8 : 65 2/8 : 33 2/8. Pot 4 : .. : 59 6/8 : 32 2/8. Pot 5 : 43 1/8 : 35 6/8 : 41 6/8. Pot 5 : 53 7/8 : 34 6/8 : 26 4/8. Pot 5 : 53 2/8 : 54 6/8 : 0. Pot 6 : 37 4/8 : 56 : 46 4/8. Pot 6 : 61 : 63 5/8 : 29 6/8. Pot 6 : 0 : 57 7/8 : 14 4/8. Pot 7 : 59 6/8 : 51 : 43. Pot 7 : 43 4/8 : 49 6/8 : 12 2/8. Pot 7 : 50 5/8 : 0 : 0. Pot 8 : 37 7/8 : 38 5/8 : 21 6/8. Pot 8 : 37 2/8 : 44 5/8 : 14 5/8. Total : 1051.25 : 1190.50 : 697.88. The twenty-one Westerham-crossed plants now averaged 50.05 inches; the twenty-two intercrossed plants, 54.11 inches; and the twenty-one self-fertilised plants, 33.23 inches in height. We thus get the following ratios:-- The Westerham-crossed plants in height to the self-fertilised as 100 to 66.
The Westerham-crossed plants in height to the intercrossed as 100 to 108. The intercrossed plants in height to the self-fertilised as 100 to 61. We here see that the Westerham-crossed (the offspring of plants self-fertilised for four generations and then crossed with a fresh stock) have gained greatly in height, since they were first measured, relatively to the plants self-fertilised for five generations. They were then as 100 to 91, and now as 100 to 66 in height. The intercrossed plants (i.e., those which had been intercrossed for the last five generations) likewise exceed in height the self-fertilised plants, as occurred in all the previous generations with the exception of the abnormal plants of the third generation. On the other hand, the Westerham-crossed plants are exceeded in height by the intercrossed; and this is a surprising fact, judging from most of the other strictly analogous cases. But as the Westerham-crossed plants were still growing vigorously, while the intercrossed had almost ceased to grow, there can hardly be a doubt that if left to grow for another month they would have beaten the intercrossed in height. That they were gaining on them is clear, as when measured before they were as 100 to 119, and now as only 100 to 108 in height. The Westerham-crossed plants had also leaves of a darker green, and looked altogether more vigorous than the intercrossed; and what is much more important, they produced, as we shall presently see, much heavier seed-capsules. So that in fact the offspring from the self-fertilised plants of the fourth generation crossed by a fresh stock were superior to the intercrossed, as well as to the self-fertilised plants of the fifth generation--of which latter fact there could not be the least doubt.
TABLE 6/83. Petunia violacea (plants growing in the open ground). Heights of plants measured in inches. Column 1: Westerham-Crossed Plants (from self-fertilised Plants of the fourth generation crossed by a fresh stock). Column 2: intercrossed Plants (Plants of one and the same stock intercrossed for five generations). Column 3: self-fertilised Plants (self-fertilised for five generations). 34 2/8 : 38 : 27 3/8. 36 2/8 : 36 2/8 : 23. 35 2/8 : 39 5/8 : 25. 32 4/8 : 37 : 24 1/8. 37 : 36 : 22 4/8. 36 4/8 : 41 3/8 : 23 3/8. 40 7/8 : 37 2/8 : 21 5/8. 37 2/8 : 40 : 23 4/8. 38 2/8 : 41 2/8 : 21 3/8. 38 5/8 : 36 : 21 2/8. 366.76 : 382.76 : 233.13. The ten Westerham-crossed plants here average 36.67 inches in height; the ten intercrossed plants, 38.27 inches; and the ten self-fertilised, 23.31 inches. These three lots of plants were also weighed; the Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41 ounces; and the self-fertilised, 14.75 ounces. We thus get the following ratios:-- The Westerham-crossed plants in height to the self-fertilised as 100 to 63. The Westerham-crossed plants in weight to the self-fertilised as 100 to 53. The Westerham-crossed plants in height to the intercrossed as 100 to 104.
The Westerham-crossed plants in weight to the intercrossed as 100 to 146. The intercrossed plants in height to the self-fertilised as 100 to 61. The intercrossed plants in weight to the self-fertilised as 100 to 36. Here the relative heights of the three lots are nearly the same (within three or four per cent) as with the plants in the pots. In weight there is a much greater difference: the Westerham-crossed exceed the self-fertilised by much less than they did before; but the self-fertilised plants in the pots had become slightly withered, as before stated, and were in consequence unfairly light. The Westerham-crossed plants are here inferior in weight to the intercrossed plants in a much higher degree than in the pots; and this appeared due to their being much less branched, owing to their having germinated in greater numbers and consequently being much crowded. Their leaves were of a brighter green than those of the intercrossed and self-fertilised plants.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.