The Peronospore^ of Iowa. 47 



For introductory studies in this group of plants no species is 

 more convenient than P. effusa occurring on Chcnopodium 

 album ("lambs quarters"). It is readily available in early 

 summer, and by its abundance and color (when seen in quantity 

 sordid violet) eas}- to discover. Sections of the leaf affected 

 are easy to make and satisfactory in demonstrating the myce- 

 lium of the fungus. 



The genus Cystopus is distinguished from the other genera 

 here mentioned in that its conidiophores (fertile or fruiting 

 branches of the mycelium) are formed not in the open air but 

 inside the leaf of the host plant, just beneath the epidermis. 

 In consequence, the appearance of the fungus when in fruit is 

 peculiar. In the t3'pical Peronospora the fruit appears as deli- 

 cate patches of frost or down on the lower surface of the leaf 

 (where the stomata abound), whereas in Cvstopus dull whitish 

 somewhat swollen spots often on the upper surface of the leaf 

 are the only signs of fruiting to be seen. It will be observed 

 that this fjenus is limited to but few hosts. That on 

 Portuhica oleracca, purslane, is ver}' common. 



McB. 



As is usual with parasitic fungi the Pcronos-porece produce 

 two kinds of spores, oospores, or resting spores which carry 

 the species through unfavorable conditions, and the conidia 

 which serve for immediate propagation. 



The oospores are formed within the tissues of the host 

 plant. They are usually dark brown, nearl}^ globular cells, 

 with the surface smooth or covered with the ridges or tuber- 

 cles. The number is so great in some cases, as to distort the 

 host. 



The three genera which have been reported from this 

 state are defined by conidial characters as follows: 



Phvtofhthora: conidia ovate, nearly sessile on the simple or 

 sparingly branched conidiophore. In most cases the spores 

 have fallen off when the specimen is examined, but the conidio- 



