The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
MEANS OF CROSS-FERTILISATION.
We have hitherto considered only the prepotent fertilising power of pollen from a distinct variety over a plants’ own pollen,--both kinds of pollen being placed on the same stigma. It is a much more remarkable fact that pollen from another individual of the same variety is prepotent over a plant’s own pollen, as shown by the superiority of the seedlings raised from a cross of this kind over seedlings from self-fertilised flowers. Thus in Tables 7/A, B, and C, there are at least fifteen species which are self-fertile when insects are excluded; and this implies that their stigmas must receive their own pollen; nevertheless, most of the seedlings which were raised by fertilising the non-castrated flowers of these fifteen species with pollen from another plant were greatly superior, in height, weight, and fertility, to the self-fertilised offspring. (10/40. These fifteen species consist of Brassica oleracea, Reseda odorata and lutea, Limnanthes douglasii, Papaver vagum, Viscaria oculata, Beta vulgaris, Lupinus luteus, Ipomoea purpurea, Mimulus luteus, Calceolaria, Verbascum thapsus, Vandellia nummularifolia, Lactuca sativa, and Zea mays.) For instance, with Ipomoea purpurea every single intercrossed plant exceeded in height its self-fertilised opponent until the sixth generation; and so it was with Mimulus luteus until the fourth generation. Out of six pairs of crossed and self-fertilised cabbages, every one of the former was much heavier than the latter. With Papaver vagum, out of fifteen pairs, all but two of the crossed plants were taller than their self-fertilised opponents. Of eight pairs of Lupinus luteus, all but two of the crossed were taller; of eight pairs of Beta vulgaris all but one; and of fifteen pairs of Zea mays all but two were taller. Of fifteen pairs of Limnanthes douglasii, and of seven pairs of Lactuca sativa, every single crossed plant was taller than its self-fertilised opponent. It should also be observed that in these experiments no particular care was taken to cross-fertilise the flowers immediately after their expansion; it is therefore almost certain that in many of these cases some pollen from the same flower will have already fallen on and acted on the stigma.
There can hardly be a doubt that several other species of which the crossed seedlings are more vigorous than the self-fertilised, as shown in Tables 7/A, 7/B and 7/C, besides the above fifteen, must have received their own pollen and that from another plant at nearly the same time; and if so, the same remarks as those just given are applicable to them. Scarcely any result from my experiments has surprised me so much as this of the prepotency of pollen from a distinct individual over each plant’s own pollen, as proved by the greater constitutional vigour of the crossed seedlings. The evidence of prepotency is here deduced from the comparative growth of the two lots of seedlings; but we have similar evidence in many cases from the much greater fertility of the non-castrated flowers on the mother-plant, when these received at the same time their own pollen and that from a distinct plant, in comparison with the flowers which received only their own pollen. From the various facts now given on the spontaneous intercrossing of varieties growing near together, and on the effects of cross-fertilising flowers which are self-fertile and have not been castrated, we may conclude that pollen brought by insects or by the wind from a distinct plant will generally prevent the action of pollen from the same flower, even though it may have been applied some time before; and thus the intercrossing of plants in a state of nature will be greatly favoured or ensured.
The case of a great tree covered with innumerable hermaphrodite flowers seems at first sight strongly opposed to the belief in the frequency of intercrosses between distinct individuals. The flowers which grow on the opposite sides of such a tree will have been exposed to somewhat different conditions, and a cross between them may perhaps be in some degree beneficial; but it is not probable that it would be nearly so beneficial as a cross between flowers on distinct trees, as we may infer from the inefficiency of pollen taken from plants which have been propagated from the same stock, though growing on separate roots. The number of bees which frequent certain kinds of trees when in full flower is very great, and they may be seen flying from tree to tree more frequently than might have been expected. Nevertheless, if we consider how numerous are the flowers, for instance, on a horse-chestnut or lime-tree, an incomparably larger number of flowers must be fertilised by pollen brought from other flowers on the same tree, than from flowers on a distinct tree.
But we should bear in mind that with the horse-chestnut, for instance, only one or two of the several flowers on the same peduncle produce a seed; and that this seed is the product of only one out of several ovules within the same ovarium. Now we know from the experiments of Herbert and others that if one flower is fertilised with pollen which is more efficient than that applied to the other flowers on the same peduncle, the latter often drop off (10/41. ‘Variation under Domestication’ chapter 17 2nd edition volume 2 page 120.); and it is probable that this would occur with many of the self-fertilised flowers on a large tree, if other and adjoining flowers were cross-fertilised. Of the flowers annually produced by a great tree, it is almost certain that a large number would be self-fertilised; and if we assume that the tree produced only 500 flowers, and that this number of seeds were requisite to keep up the stock, so that at least one seedling should hereafter struggle to maturity, then a large proportion of the seedlings would necessarily be derived from self-fertilised seeds. But if the tree annually produced 50,000 flowers, of which the self-fertilised dropped off without yielding seeds, then the cross-fertilised flowers might yield seeds in sufficient number to keep up the stock, and most of the seedlings would be vigorous from being the product of a cross between distinct individuals. In this manner the production of a vast number of flowers, besides serving to entice numerous insects and to compensate for the accidental destruction of many flowers by spring-frosts or otherwise, would be a very great advantage to the species; and when we behold our orchard-trees covered with a white sheet of bloom in the spring, we should not falsely accuse nature of wasteful expenditure, though comparatively little fruit is produced in the autumn.
Text from Project Gutenberg, public domain in the United States.