CHAP. VII. POLYGAMOUS PLANTS. 283 



Plants under cultivation or changed conditions of 

 life frequently become sterile ; and the male organs 

 are much oftener affected than the female, though the 

 latter alone are sometimes affected. The sterility of 

 the stamens is generally accompanied by a reduction 

 in their size ; and we may feel sure, from a wide-spread 

 analogy, that both the male and female organs would 

 become rudimentary in the course of many genera- 

 tions if they failed altogether to perform their proper 

 functions. According to Gartner,* if the anthers on 

 a plant are contabescent (and when this occurs it is 

 always at a very early period of growth) the female 

 organs are sometimes precociously developed. I 

 mention this case as it appears to be one of com- 

 pensation. So again is the well-known fact, that 

 plants which increase largely by stolons or other such 

 means are often utterly barren, with a large proportion 

 of their pollen-grains in a worthless condition. 



Hildebrand has shown that with hermaphrodite 

 plants which are strongly proterandrous, the stamens 

 in the flowers which open first sometimes abort ; and 

 this seems to follow from their being useless, as no 

 pistils are then ready to be fertilised. Conversely 

 the pistils in the flowers which open last sometimes 

 abort ; as when they are ready for fertilisation all the 

 pollen has been shed. He further shows by means of 

 a series of gradations amongst the Composits6,t that 

 a tendency from the causes just specified to produce 

 either male or female florets, sometimes spreads 

 to all the florets on the same head, and sometimes 



* 'Beitrage zur Kenntniss,' &c. chap, xviii. 2nd edit. vol. ii. 



p. 1 17 et seq. The whole subject pp. H6-56. 



of the sterility of plants from f ' Ueber die Geschlechtaver- 



various causes has been discussed haltnisse bei dun Couipositeu, 



in my ' Variation of Animals 1869, p. 89. 

 and Plants under Domestication,' 



