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Show 398 MEANS OF CROSS-FERTILISATION. CHAP. X. comparativ"e growth of the two lots of seedlings; but we have similar evidence in many cases frmn the much greater fertility of the non-castrated flowers on the n1other-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 ncar together, and on the effects of cross-fertilising flowers which are self· fertile and have not been castrat d, 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 cov red with innumerable hermaphrodite flowers seems at :first sight strongly opposed to the belief in the frequency of intercrosses between distinct individuals. Th flowers which grow on the opposite sides of such a tr e will have been exposed to somewhat different conditions, and a cross betvi'een 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 frequ nt 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 flow rs, for instance, on a horsechestnut or lime-tree, an incomparably larger number CHAP. X. . PREPOTENT POLLEN. 399 of flowers must be fertilised by pollen brought from other flowers on the same tree, than from flowers on . distinct tree. But we should bear in mind that wit~ 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 pedunc:e, the latter often drop off; and it is probable that this would occur with many of the self-fertilised flowers on a la:·~e tree, if other and adjoining flowers were cross-fertilise~. .of the flowers annually produced by a great tree, It Is al~.o_st 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 a~ least one seedling should hereafter struggl ' to matunty, then a large proportion of the seedlings woul~ necessarily be derived from self-fertilised seeds. Bu~ 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 ~he p~oduct of a cross between distinct individuals. n this manner the production of a vast n urn ber of flowers , b esi· d es serv·i ng to enti·c e numerous insects ancl 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 bloo1n * 'V· . t' una lOU under Domestication,' ch. xvii. 2nd edit. vol. ii. p. 120. |