SAPROLEGNIALES 793 



inhabitants of the water? The general structure of the thallus and the 

 fact that the free-swimming zoospore is ordinarily the nonsexual 

 propagative unit, however, argues for their being primarily aquatic. 



Development and Morphology 



the thallus 



The thallus in members of the Saprolegniaceae is a richly branched 

 coenocytic aggregation of hyphae, usually with unlimited capacity for 

 growth. This aggregation may be pustular or form a dense turf; it is 

 easily visible macroscopically on the substratum. In the other two fam- 

 ilies, the Ectrogellaceae and Thraustochytriaceae, however, the body is 

 of very simple structure, of limited growth, saccate, and distinctly 

 chytrid-like. 



Characteristically, the thallus walls in all members of the order give 

 a pronounced cellulose reaction with chloriodide of zinc. In the hypha- 

 forming species there does not appear to be, as in the Leptomitales, a 

 strong differentiation of the lower part of the thallus into a basal cell 

 and a holdfast system. The multinucleate contents are not highly 

 refractive except in actively growing regions. Frequently they are 

 somewhat thin and watery in appearance, with the granules arranged in 

 a loose longitudinally disposed reticulum around the periphery of a 

 central vacuole. Cross walls are laid down in normal material of eucar- 

 pic species only to delimit the reproductive organs and "gemmae" 

 (chlamydospores). The latter are simply distended parts of the hyphae, 

 of somewhat variable shape and often formed in catenulate series, 

 within which the protoplasm accumulates. They may eventually dis- 

 articulate and, upon the advent of favorable conditions, germinate to 

 produce either hyphae or short-stalked zoosporangia. 



REPRODUCTION 



Nonsexual Reproduction 

 Nonsexual reproduction is ordinarily accomplished by means of 

 zoospores borne in zoosporangia. The zoospores, after escape, swarm, 

 but eventually come to rest and germinate to form a new mycelium. 



