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

by Charles Darwin

THE COWSLIP.

Most of the species in this genus are heterostyled or dimorphic; that is, they present two forms,—one long-styled with short stamens, and the other short-styled with long stamens. (6/6. See my paper ‘On the Two Forms or Dimorphic Condition in the Species of Primula’ in ‘Journal of the Proceedings of the Linnean Society’ volume 6 1862 page 77. A second paper, to which I presently refer ‘On the Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and Trimorphic Plants’ was published in volume 10 1867 page 393 of the same journal.) For complete fertilisation it is necessary that pollen from the one form should be applied to the stigma of the other form; and this is effected under nature by insects. Such unions, and the seedlings raised from them, I have called legitimate. If one form is fertilised with pollen from the same form, the full complement of seed is not produced; and in the case of some heterostyled genera no seed at all is produced. Such unions, and the seedlings raised from them, I have called illegitimate. These seedlings are often dwarfed and more or less sterile, like hybrids. I possessed some long-styled plants of Primula veris, which during four successive generations had been produced from illegitimate unions between long-styled plants; they were, moreover, in some degree inter-related, and had been subjected all the time to similar conditions in pots in the greenhouse. As long as they were cultivated in this manner, they grew well and were healthy and fertile. Their fertility even increased in the later generations, as if they were becoming habituated to illegitimate fertilisation. Plants of the first illegitimate generation when taken from the greenhouse and planted in moderately good soil out of doors grew well and were healthy; but when those of the two last illegitimate generations were thus treated they became excessively sterile and dwarfed, and remained so during the following year, by which time they ought to have become accustomed to growing out of doors, so that they must have possessed a weak constitution.

Under these circumstances, it seemed advisable to ascertain what would be the effect of legitimately crossing long-styled plants of the fourth illegitimate generation with pollen taken from non-related short-styled plants, growing under different conditions. Accordingly several flowers on plants of the fourth illegitimate generation (i.e., great-great-grandchildren of plants which had been legitimately fertilised), growing vigorously in pots in the greenhouse, were legitimately fertilised with pollen from an almost wild short-styled cowslip, and these flowers yielded some fine capsules. Thirty other flowers on the same illegitimate plants were fertilised with their own pollen, and these yielded seventeen capsules, containing on an average thirty-two seeds. This is a high degree of fertility; higher, I believe, than that which generally obtains with illegitimately fertilised long-styled plants growing out of doors, and higher than that of the previous illegitimate generations, although their flowers were fertilised with pollen taken from a distinct plant of the same form.

These two lots of seeds were sown (for they will not germinate well when placed on bare sand) on the opposite sides of four pots, and the seedlings were thinned, so that an equal number were left on the two sides. For some time there was no marked difference in height between the two lots; and in Pot 3, Table 6/93, the self-fertilised plants were rather the tallest. But by the time that they had thrown up young flower-stems, the legitimately crossed plants revealed much the finest, and had greener and larger leaves. The breadth of the largest leaf on each plant was measured, and those on the crossed plants were on an average a quarter of an inch (exactly .28 of an inch) broader than those on the self-fertilised plants. The plants, from being too much crowded, produced poor and short flower-stems. The two finest on each side were measured; the eight on the legitimately crossed plants averaged 4.08, and the eight on the illegitimately self-fertilised plants averaged 2.93 inches in height; or as 100 to 72.

These plants after they had flowered were turned out of their pots, and planted in fairly good soil in the open ground. In the following year (1870), when in full flower, the two tallest flower-stems on each side were again measured, as shown in Table 6/93, which likewise gives the number of flower-stems produced on both sides of all the pots.

TABLE 6/93. Primula veris.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Height: Legitimately crossed Plants.

Column 3: Number of Flower-stems produced: Legitimately crossed Plants.

Column 4: Height: Illegitimately crossed Plants.

Column 5: Number of Flower-stems produced: Illegitimately crossed Plants.

Pot 1 : 9 : 16 : 2 1/8 : 3. Pot 1 : 8 : : 3 4/8.

Pot 2 : 7 : 16 : 6 : 3. Pot 2 : 6 4/8 : : 5 4/8.

Pot 3 : 6 : 16 : 3 : 4. Pot 3 : 6 2/8 : : 0 4/8.

Pot 4 : 7 3/8 : 14 : 2 5/8 : 5. Pot 4 : 6 1/8 : : 2 4/8.

Total : 56.26 : 62 : 25.75 : 15.

The average height of the eight tallest flower-stems on the crossed plants is here 7.03 inches, and that of the eight tallest flower-stems on the self-fertilised plants 3.21 inches; or as 100 to 46. We see, also, that the crossed plants bore sixty-two flower-stems; that is, above four times as many as those (namely fifteen) borne by the self-fertilised plants. The flowers were left exposed to the visits of insects, and as many plants of both forms grew close by, they must have been legitimately and naturally fertilised. Under these circumstances the crossed plants produced 324 capsules, whilst the self-fertilised produced only 16; and these were all produced by a single plant in Pot 2, which was much finer than any other self-fertilised plant. Judging by the number of capsules produced, the fertility of an equal number of crossed and self-fertilised plants was as 100 to 5.

In the succeeding year (1871) I did not count all the flower-stems on these plants, but only those which produced capsules containing good seeds. The season was unfavourable, and the crossed plants produced only forty such flower-stems, bearing 168 good capsules, whilst the self-fertilised plants produced only two such flower-stems, bearing only 6 capsules, half of which were very poor ones. So that the fertility of the two lots, judging by the number of capsules, was as 100 to 3.5.

In considering the great difference in height and the wonderful difference in fertility between the two sets of plants, we should bear in mind that this is the result of two distinct agencies. The self-fertilised plants were the product of illegitimate fertilisation during five successive generations, in all of which, excepting the last, the plants had been fertilised with pollen taken from a distinct individual belonging to the same form, but which was more or less closely related. The plants had also been subjected in each generation to closely similar conditions. This treatment alone, as I know from other observations, would have greatly reduced the size and fertility of the offspring. On the other hand, the crossed plants were the offspring of long-styled plants of the fourth illegitimate generation legitimately crossed with pollen from a short-styled plant, which, as well as its progenitors, had been exposed to very different conditions; and this latter circumstance alone would have given great vigour to the offspring, as we may infer from the several analogous cases already given. How much proportional weight ought to be attributed to these two agencies,—the one tending to injure the self-fertilised offspring, and the other to benefit the crossed offspring,—cannot be determined. But we shall immediately see that the greater part of the benefit, as far as increased fertility is concerned, must be attributed to the cross having been made with a fresh stock.

Primula veris.