140 MICROSCOPIC FUNGI. 



upon the creeping mycelium or spawn. All the 

 members of this genus with which we are acquainted 

 are parasitic on living plants^ inducing in them 

 speedy decay^ but preceding that decay of which 

 they are themselves the cause. Hence we have 

 deemed it the more advantageous course both for 

 writer and reader to associate together the different 

 species of this particular genus of parasitic moulds 

 in one chapter, rather than bring together the 

 different kinds of fungi, belonging perhaps to 

 widely separated genera, but all associated with, 

 or parasitic upon, the same plant. The botanical 

 student will thank us for following this plan, and 

 the general reader will labour under no disad- 

 vantage, in this instance at least, from the similarity 

 of the diseases produced in the plants infested. 



It has been recently proposed to associate the 

 genera Peronospora and Cystopus together in one 

 group, under the name of Peronosporei ; but with 

 the discussion of this subject we have nothing 

 to do in this volume. Having announced this fact, 

 we shall continue to notice them in the several 

 positions heretofore accorded to them. 



The mycelium (root-like fibres) in this genus 

 greatly resembles that described for the '^ white 

 rusts,^^ though neither so thick, nor is the mem- 

 brane so gelatinous as in that genus. In some 

 instances the mycelium is confined to the inter- 

 cellulary passages; but in most cases they also 

 perforate the cells of the plant which nourishes 



