HOW PLANTS MARRY. 85 



makes sex and marriage desirable, makes pilose 

 intermarriage of blood relations undesirable. 

 " Marrying in and in," as it is called, tends to 

 produce weak and feeble offspring, while "an in- 

 fusion of fresh blood " tends to make bofh plants 

 and animals stronger and more vigorous. Hence, 

 if any habit chanced to arise in plants which fa- 

 voured or rendered easier such cross-fertilisation, 

 it would result in stronger and more vigorous 

 young, and would therefore be fixed by natural 

 selection. The actual consequence is that in 

 the world of plants, as we see it to-day, every 

 great dominant or successful race has invented 

 some means of cross-fertilisation, either by the 

 agency of wind or of insects, while only the mis- 

 erable riff-raff and outcasts of plant-life still ad- 

 here to the old and bad method of fertilisation by 

 means of the pollen of their own flowers. 



We are now in a position -to understand jthe 

 main principles which govern the marriage cus- 

 toms of plants ; we will proceed in the next chap- 

 ter to consider in detail how these principles "work 

 out in particular instances. But first we must sum 

 up what we have learnt in this chapter. 



Plants marry and are given in marriage. The 

 very lowest plants, indeed, are sexless, but in the 

 higher there are well-marked distinctions of ^ale 

 and female. An intermediate stage exists in <:ev- 

 tain thread-like pond-weeds, where marriage or 

 intermixture takes place between two adjacent 

 cells, neither of which is male or female. The 

 higher plants, however, are really communities or 

 colonies, of which the leaves are the Workers, and 

 the various parts of the flower the males and 

 females. The central part of the flower, known 



