84 BOTANY. [CHAP. n. 



an external covering ; the whole cluster eventually bursts 

 through the epidermis of the leaf, the external covering 

 is ruptured at the top into a number of teeth that curl 

 backwards, the dense mass of golden-yellow spores being 

 now dry and powdery, and in this state are readily blown 

 away by the wind. This stage of the parasite is popu- 

 larly known as " cluster-cups," on account of the mem- 

 brane covering the spores more or less resembling a cup 

 with a toothed margin after bursting open, and in being 

 produced in clusters. Finally, the spores of the cluster- 

 cup stage must be carried by some means on to the 

 surface of a grass leaf or stem, where they at once 

 germinate, the germ-tube entering the leaf through a 

 stoma, where a mycelium is formed that gives origin to 

 the rust- coloured streaks consisting of the one-celled 

 spores that the description commenced with. Until 

 recently the three stages described above were considered 

 as totally distinct individuals. The term Heteroecism is 

 used to express the fact that a parasite passes different 

 phases of its life-cycle on different host-plants. The 

 above is not an isolated or exceptional example ; we have 

 in Britain alone numerous hetercecismal species of fungi, 

 the cluster-cup or Mddium stage being amongst the 

 most beautiful of microscopic objects. 



Up to the present true parasites have been dealt with, 

 and in such instances everything is in favour of the 

 parasite, the host being in no instance benefited by the 

 presence of its unwelcome guest, but in some instances 

 we find that parasite and host have become so thoroughly 

 adjusted that both benefit mutually by the combination. 

 This condition of things is called mutualism or commen- 

 salism, and in the vegetable world is well shown in the 



