The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom

by Charles Darwin

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.

From the foregoing data it is easy to calculate that:

Ten Westerham-crossed capsules would have contained 6.26 grains weight of seed.

Ten intercrossed capsules would have contained 4.14 grains weight of seed.

Ten self-fertilised capsules would have contained 3.00 grains weight of seed.

We thus get the following ratios:—

Seeds from the Westerham-crossed capsules to those from the capsules of the fifth self-fertilised generation, in weight as 100 to 48.

Seeds from the Westerham-crossed capsules to those from the capsules of the fifth intercrossed generation, in weight as 100 to 66.

Seeds from the intercrossed capsules to those from the self-fertilised capsules, in weight as 100 to 72.

So that a cross with pollen from a fresh stock greatly increased the productiveness of the flowers on plants which had been self-fertilised for the four previous generations, in comparison not only with the flowers on the same plants self-fertilised for the fifth time, but with the flowers on the crossed plants crossed with pollen from another plant of the same old stock for the fifth time.

These three lots of seeds were placed on sand, and were planted in an equal state of germination in seven pots, each made tripartite by three superficial partitions. Some of the remaining seeds, whether or not in a state of germination, were thickly sown in an eighth pot. The pots were kept in the greenhouse, and the plants trained up sticks. They were first measured to the tops of their stems when coming into flower; and the twenty-two Westerham-crossed plants then averaged 25.51 inches; the twenty-three intercrossed plants 30.38; and the twenty-three self-fertilised plants 23.40 inches in height. We thus get the following ratios:—

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.

These three lots of plants were cut down close to the ground and weighed. The twenty-one Westerham-crossed plants weighed 32 ounces; the twenty-two intercrossed plants, 34 ounces, and the twenty-one self-fertilised plants 7 1/4 ounces. The following ratios are calculated for an equal number of plants of each kind. But as the self-fertilised plants were just beginning to wither, their relative weight is here slightly too small; and as the Westerham-crossed were still growing vigorously, their relative weight with time allowed would no doubt have greatly increased.

The Westerham-crossed plants in weight to the self-fertilised as 100 to 22.

The Westerham-crossed plants in weight to the intercrossed as 100 to 101.

The intercrossed plants in weight to the self-fertilised as 100 to 22.3.

We here see, judging by weight instead of as before by height, that the Westerham-crossed and the intercrossed have an immense advantage over the self-fertilised. The Westerham-crossed are inferior to the intercrossed by a mere trifle; but it is almost certain that if they had been allowed to go on growing for another month, the former would have completely beaten the latter.

As I had an abundance of seeds of the same three lots, from which the foregoing plants had been raised, these were sown in three long parallel and adjoining rows in the open ground, so as to ascertain whether under these circumstances the results would be nearly the same as before. Late in the autumn (November 13) the ten tallest plants were carefully selected out of each row, and their heights measured, with the following result:—

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.