MOULDS. 141 



them. The sucker-hke bodies already described in 

 Oystopus are often found produced on the mycelium 

 of Peronospora, but occasionally they appear to be 

 absent, especially in the mould causing the potato- 

 disease. 



From the mycelium erect threads are produced, 

 upon which one form of fruit, which may be 

 termed " acrospores,^' is borne. These filaments are 

 sometimes single, and sometimes in small tufts or 

 fascicles. In some instances they are considerably 

 branched, so as to present a dendroidal or tree-like 

 appearance ; in others they are nearly simple, being 

 only surmounted by short spicules ; or, in one 

 instance, quite simple, and only surmounted by a 

 single acrospore. The branching habit is by far 

 the most common. 



Each ultimate branch in the ramification of the 

 fertile filaments engenders a single acrospore. Its 

 extremity, at first thin and pointed, swells in the 

 form of a globular vesicle, which soon takes the 

 elliptical or ovate shape of the perfected acrospore^ 

 and at length separates itself from the branch that 

 supports it. 



In all instances the acrospores have a similar 

 structure, but with minute difi'erences in form, &c., 

 which have their importance in the determination 

 of species. In most cases the apex of the acrospore 

 is obtuse, and the entire body has a violet tint, 

 more or less deep ; in some it is completely colour- 

 less. These acrospores, when placed in favourable 



