Heteroecism. 



57 



* is affixed to those species which I have personally investigated, 

 t to those of which I have repeated the cultures, but am not able to confirm 

 the above statements. 



The question naturally presents itself to us, Why are 

 some species hetercecious and others not .'* One reason is 

 pretty obvious, namely, that those Puccinige and Uromyces 

 which are hetercecious occUr upon host-plants whose cuticle 

 is, if not silicons, at least very hard and difficult for the germ- 

 tube of the promycelial spore to pierce — namely, on grasses, 

 Carices and Junci. This, however, can hardly be the only 

 reason, since Schroter has produced the yEcidium on 

 Euphorbia cyparissias from Uromyces pisi ; in this case both 

 the host-plants have soft epidermal cells. The Coleosporia 

 and Melampsora afford similar instances. Whatever may 

 have been the cause or causes in bygone ages, the fact is 

 that at the present time so completely have these parasites 

 become heteroecismal in habit, that the most profuse appli- 

 cation of their promycelial spores to the graminaceous host 

 is always without result. 



