CH. XVI] CROSS-POLLINATION 155 



But even in cases where the stamens and pistil are in 

 the same flower, close examination shows that it is the 

 exception, and not the rule, that the pollen is sown on 

 the stigma of that flower. Numerous adaptations exist 

 to either prevent this " self-pollination " entirely, or to at 

 least delay it, and so admit of pollen from another flower 

 reaching the stigma before that of the same flower can 

 do so. 



It is obvious that some general purpose may lie at 

 the bottom of this phenomenon, and we owe especially 

 to Darwin the establishment of the proof that plants 

 derive positive advantages from cross-pollination : that 

 they set more seeds, and produce more numerous and 

 vigorous seedlings as the result of crossing than when 

 continually self-pollinated. This truth lies at the founda- 

 tion of the multifarious and often very complex mechanisms 

 by which (1) flowers are prevented from self-pollination, 

 and (2) pollen is carried from one flower to another. 

 Self-pollination is of course impossible in unisexual 

 (diclinous) flowers, but several degrees may exist in 

 reference to the distance apart of the staminate and 

 pistillate flowers. 



Where the male flowers are on one plant and the 

 female on another, as in the Willow, Hop, Briony, 

 Mercurialis, we have the extreme case, termed Dicecism. 



In the Pines, Oak, Beech, Nettle, Euphorbia, Carex, 

 Arum, the male and female flowers are on the same 

 plant (monoecious), either on different twigs or in the 

 same inflorescence, and showing all degrees of relative 

 distance in space and in time of maturing. In some 

 Elms and Nettles and in Pellitory a number of herma- 

 phrodite flowers occur amongst the unisexual ones, and 

 they are termed polygamous. 



On investigating the means by which the pollen is 



