400 MEANS OF CROSS-FERTILISATION. CHAP. X. 



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 inter- 

 crossing 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 bene- 

 ficial 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 on a great tree, an incom- 

 parably larger number 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 many species only a few flowers on the same 

 peduncle produce a seed ; and that these seeds are 

 often 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 flowei 



* ' Variation under Domestication,' ch. xvii. 2nd edit. vol. ii. p. 120. 



