SURVIVAL OF ORGANS 293 



Nummularia, the flowers of which hardly ever 

 produce seeds. How is it that in such species 

 flowers are still produced ? The probable ex- 

 planation of this anomaly is, that for the disappear- 

 ance of flowers there would have to be produced 

 individuals with this advantageous variation. It 

 is the case, however, that the Mcaria and the 

 Lysimachia reproduce most actively by asexual 

 methods, and variations are extremely rare in 

 cases of these modes of reproduction. The result 

 is that these species having begun to form sterile 

 flowers continue to produce them through simple 

 lack of variation. 



An analogous case is presented by Elodea 

 Canadensis. This unisexual plant is represented 

 in Europe by only female plants. These plants 

 have multiplied asexually so luxuriantly that in 

 Holland they began to choke up the canals, and 

 it became necessary to make provision in the 

 budget of that country for the extermination of 

 the pest. The plants are, of course, able to multiply 

 only asexually, as the female flowers cannot be 

 fertilized, and these useless flowers have been 

 maintained simply from the absence of variations. 



Straiiotes alo'ides, a plant belonging to the 

 same family as Elodea, is practically only repre- 

 sented by male individuals. Females are extremely 

 rare, and none the less the male flowers are pro- 

 duced, although in the vast majority of cases they 

 must be useless. 



