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

by Charles Darwin

Thirty-one intercrossed plants raised from a cross between flowers on the same plants were grown in ten pots in competition with the same number of self-fertilised plants, and the former were to the latter in height as 100 to 105. So that the self-fertilised plants were a little taller than the intercrossed; and in eight out of the ten pots a self-fertilised plant flowered before any one of the crossed plants in the same pots. The plants which were not greatly crowded in nine of the pots (and these offer the fairest standard of comparison) were cut down and weighed; and the weight of the twenty-seven intercrossed plants was to that of the twenty-seven self-fertilised as 100 to 124; so that by this test the superiority of the self-fertilised was strongly marked. To this subject of the superiority of the self-fertilised plants in certain cases, I shall have to recur in a future chapter. If we now turn to the offspring from a cross between distinct plants when put into competition with self-fertilised plants, we find that the mean height of seventy-three such crossed plants, in the course of ten generations, was to that of the same number of self-fertilised plants as 100 to 77; and in the case of the plants of the tenth generation in weight as 100 to 44. Thus the contrast between the effects of crossing flowers on the same plant, and of crossing flowers on distinct plants, is wonderfully great.

3. Mimulus luteus.

Twenty-two plants raised by crossing flowers on the same plant were grown in competition with the same number of self-fertilised plants; and the former were to the latter in height as 100 to 105, and in weight as 100 to 103. Moreover, in seven out of the eight pots a self-fertilised plant flowered before any of the intercrossed plants. So that here again the self-fertilised exhibit a slight superiority over the intercrossed plants. For the sake of comparison, I may add that seedlings raised during three generations from a cross between distinct plants were to the self-fertilised plants in height as 100 to 65.

4. Pelargonium zonale.

Two plants growing in separate pots, which had been propagated by cuttings from the same plant, and therefore formed in fact parts of the same individual, were intercrossed, and other flowers on one of these plants were self-fertilised; but the seedlings obtained by the two processes did not differ in height. When, on the other hand, flowers on one of the above plants were crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised, the crossed offspring thus obtained were to the self-fertilised in height as 100 to 74.

5. Origanum vulgare.

A plant which had been long cultivated in my kitchen garden, had spread by stolons so as to form a large bed or clump. Seedlings raised by intercrossing flowers on these plants, which strictly consisted of the same plant, and other seedlings raised from self-fertilised flowers, were carefully compared from their earliest youth to maturity; and they did not differ at all in height or in constitutional vigour. Some flowers on these seedlings were then crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised; two fresh lots of seedlings being thus raised, which were the grandchildren of the plant that had spread by stolons and formed a large clump in my garden. These differed much in height, the crossed plants being to the self-fertilised as 100 to 86. They differed, also, to a wonderful degree in constitutional vigour. The crossed plants flowered first, and produced exactly twice as many flower-stems; and they afterwards increased by stolons to such an extent as almost to overwhelm the self-fertilised plants.

Reviewing these five cases, we see that in four of them, the effect of a cross between flowers on the same plant (even on offsets of the same plant growing on separate roots, as with the Pelargonium and Origanum) does not differ from that of the strictest self-fertilisation. Indeed, in two of the cases the self-fertilised plants were superior to such intercrossed plants. With Digitalis a cross between the flowers on the same plant certainly did do some good, yet very slight compared with that from a cross between distinct plants. On the whole the results here arrived at, if we bear in mind that the flower-buds are to a certain extent distinct individuals and occasionally vary independently of one another, agree well with our general conclusion, that the advantages of a cross depend on the progenitors of the crossed plants possessing somewhat different constitutions, either from having been exposed to different conditions, or to their having varied from unknown causes in a manner which we in our ignorance are forced to speak of as spontaneous. Hereafter I shall have to recur to this subject of the inefficiency of a cross between the flowers on the same plant, when we consider the part which insects play in the cross-fertilisation of flowers.

ON THE TRANSMISSION OF THE GOOD EFFECTS FROM A CROSS AND OF THE EVIL EFFECTS FROM SELF-FERTILISATION.

We have seen that seedlings from a cross between distinct plants almost always exceed their self-fertilised opponents in height, weight, and constitutional vigour, and, as will hereafter be shown, often in fertility. To ascertain whether this superiority would be transmitted beyond the first generation, seedlings were raised on three occasions from crossed and self-fertilised plants, both sets being fertilised in the same manner, and therefore not as in the many cases given in Tables 7/A, 7/B, 7/C, in which the crossed plants were again crossed and the self-fertilised again self-fertilised.