LEPTOMITALES 853 



completa) form a single oospore in the oogonium. Except in Apodachlya 

 the contents of the oogonium are differentiated into ooplasm and 

 periplasm, and a coenocentrum is distinguishable in the ooplasm 

 (Kevorkian, 1935). In the Rhipidiaceae a functional antheridium is 

 formed which is applied at a definite region on the oogonial wall, and 

 a well-defined, functional fertilization tube is produced. 



Zoospores of the primary type are formed in Leptomitus and Apo- 

 dachlya brachynema. These encyst, as in Saprolegnia, and give rise to 

 swarmers of the secondary type. In the Rhipidiaceae the primary stage 

 is apparently completely suppressed and only secondary zoospores are 

 produced, a further point of resemblance to the Peronosporales. 



In spite of the fact that the body structure is obviously similar to that 

 typical of the Blastocladiales (strikingly exemplified by a comparison 

 of Blastocladia pringsheimii with Mindeniella spinospora) and in the 

 Monoblepharidales (as illustrated by the segmented hyphae of Gona- 

 podya and Apodachyla), there is abundant evidence to show that there 

 is no close relationship between them. This is indicated primarily in the 

 structure of the zoospore, the aspect of the protoplasm, the composition 

 of the walls, and the radically different methods of sexual reproduction. 

 These dissimilarities are apparent when the diagnoses of the three 

 orders are compared (cf. pp. 633, 713, and 871). Here again it can be 

 pointed out that a resemblance in the body plan of various fungi has 

 given rise to faulty ideas of relationships. 



Development and Morphology 

 the thallus 



The thalli of the two families of the order are unlike in certain 

 features, particularly in the degree of differentiation of the basal cell 

 and the extent of the development of the hyphal branches. 



In the Leptomitaceae there appears to be little or no specialization 

 of the proximal portion of the thallus; further investigation of this 

 feature, however, is desirable. Apparently no specialized system of 

 holdfasts anchors the plant to the substratum, although Dangeard 

 (1890-9 lb: 120) briefly mentions that such structures are formed in 

 Leptomitus. The hyphae of members of this family are well developed 



