THALLOPIIYTES 



65 



fungus that has retained swimming sperms. These sperms are uniciliate, as are 

 the zoospores, a fact which suggested the generic name. 



Peronosporales. — These are the downy mildews, and they include 

 many destructive parasites that live within the tissues of the host, the 

 hyphae branching through the intercellular spaces, crowding between 

 the cells, and sending haustoria into them. This internal mycelium sends 

 sporophores to the surface of the host, and spores are formed by round- 

 ing off the tips of the sporophores or their branches. This process of 

 cutting off spores is called dbstriclion, and such spores are called conidia. 

 Oogonia and antheridia are formed upon the internal mycelium, and 

 fertilization is effected through a 

 fertilizing tube. 



This is the one group of Oomy- 

 cetes with distinctly aerial habit, 

 as in the Zygomycetes ; but the 

 forms are heterogamous, and in 

 the life history of many of them 

 zoospores appear. The promi- 

 nent genera are as follows : 



Albugo.-— A. Candida is the 

 white rust which attacks mem- 

 bers of the mustard family, 

 causing distortions, especially in 

 the flower clusters. The myce- 

 lium traverses the intercellular 

 spaces of the host, the haustoria 

 sent into the host cells being 

 slender branches which enlarge 

 at the ends into little knobs. 

 The sporophores arise in clusters 

 and press up the epidermis, which 

 then appears like a white blister 

 (fig. 158). Finally the epidermis 

 is broken and the sporophores are 

 exposed, each ending in a chain 

 of spores (conidia), which have 

 been formed by successive ab- 

 strictions of the sporophore. The 

 conidia are multinucleate, and 



Fig. 158. — Albugo Candida (white rust): 

 showing mycelium crowding among host cells 

 and sending into them small button-like 

 haustoria, sporophores arising under epidermis 

 and .ihstn'i ting multinucleate conidia (spores), 

 and the detached and dying epidermis. — 



After CUAMBERLAIN. 



