Chap. VII. POLYGAMOUS PLANTS. 283 



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 sta- 

 mens 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 generations if they 

 failed altogether to perform their proper functions. Ac- 

 cording to Gartner,* if the anthers on a plant are con- 

 tabescent (and when this occurs it is always at a very 

 early period of growth) the female organs are some- 

 times precociously developed. I mention this case as 

 it appears to be one of compensation. 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 worth- 

 less 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 Compositae,f 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 even to the 

 whole plant ; and in this latter case the species becomes 

 dioecious. In those rare instances mentioned in the 



* 'Beitragezur Kenntniss,' &o., chap, xviii. — 8nd edit. vol. ii. 



p. 117 et seq. The whole subject pp. 146-56. 



of the sterility of plants from va- t ' Ueber die Geschlechtsver- 



rious causes has been discussed in haltnisse bet den Compositen,' 



ray ' Variation of Animals and 1869, p. 89. 

 Plants under Domestication,' 



