LAGENIDIALES 905 



certain cereals and wild grasses; another, Olpidiopsis riccieae (du Plessis, 

 1933), occurs in liverworts. A few species are probably saprophytic or 

 attack only moribund organisms. The majority, however, are true 

 parasites, many apparently restricted to a few hosts. Members of the 

 order are alike in being endobiotic, having walls which generally give a 

 cellulose reaction, forming zoospores with two oppositely directed 

 flagella, and possessing a type of sexuality involving the fusion of the 

 contents of two thalloid bodies, with the consequent production in one 

 of a thick-walled resting spore. In the Olpidiopsidaceae and certain 

 Lagenidiaceae the thallus is always one-celled. In other species it is 

 one-celled or multicellular and monophagous, and in the most highly 

 developed species it is multicellular, somewhat filamentous and hypha- 

 like, and may extend through many cells of the host. 



Development and Morphology 



the thallus 



Infection is accomplished by means of the free-swimming zoospore, 

 which comes to rest on the surface of the substratum and retracts its 

 flagella. In some forms it may move amoeboidly before surrounding 

 itself with a rigid wall and producing an infection tube (Fig. 73 A-E, 

 p. 906). This needle-like structure penetrates the host wall and conveys 

 the contents of the cyst through the wall into the interior of the sub- 

 stratum (Fig. 73 F-G). The development and type of thallus subsequent- 

 ly formed varies with the group. 



In the Olpidiopsidaceae, the members of which are primarily par- 

 asitic on filamentous water molds, the thallus, unlike that of the 

 Lagenidiaceae and possibly of the Sirolpidiaceae, is ordinarily never 

 attached to the tip of the penetration tube, but is, as Scherffel (1925a) 

 pointed out, almost wholly monad-like and undergoes its development 

 free in the host. Differences in the success of infection by the zoospores 

 have been noted by Butler (1907), who concludes that they are due 

 possibly to variations in the infective powers of the zoospores, but more 

 likely to the age of the hyphae attacked. The younger parts of the 

 vegetative system of the host are most readily penetrated, owing, 

 perhaps, to the unmodified condition of the walls. Since the young 



