BLASTOCLADIALES 681 



1949a) have grown Blastocladia pringsheimii and B. ramosa in pure 

 culture on a medium composed of glucose, yeast extract, and certain 

 inorganic salts, and have studied their nutrient and (in B. pringsheimii) 

 vitamin requirements. One of the most interesting results of their ex- 

 periments was the fact that resting spores of B. pringsheimii were pro- 

 duced in culture only when a 99.5 per cent carbon dioxide atmosphere 

 was maintained or when the plants were grown in media containing 

 carbonate. 



The "ramosa" group of Blastocladia approximates Allomyces in the 

 habit of the thallus. No pseudosepta are formed, however, and in plants 

 as found in nature the zoospores are discharged through only a single 

 apical papilla. 



Identification of species is complicated by the diverse shapes which 

 the basal apparatus may assume, and in some instances specific char- 

 acters based on this feature will probably be shown in the future to be of 

 questionable value. In general, however, reliance can be placed on the 

 shapes of the resting spores and the sporangia and, to a lesser degree, 

 on the presence or absence of a distal swelling of the basal cell or its 

 branches. A study of the variations on different substrata of strains 

 originating from single-spore cultures is greatly needed to clarify specific 

 concepts. 



A few studies of the development of the thallus indicate that the 

 holdfast system, which here as in Allomyces is considered the homologue 

 of the rhizoids of the chytrids, is derived from the primary germ tube 

 of the zoospore. The basal cell represents the expanded body of the spore 

 itself, and the reproductive organs originate as papilla-like outgrowths 

 from its surface. Although resembling Rhipidium, Mindeniella, and 

 Araiospora of the Leptomitales in superficial aspect, exhibiting, in fact, 

 evolutionary parallelism with these genera, Blastocladia differs in bear- 

 ing sessile sporangia, resting spores rather than oospores, and posterior- 

 ly uniflagellate zoospores. 



KEY TO THE SPECIES OF BLASTOCLADIA 



Sporangia predominantly cylindrical, at least four times as long as wide; 

 thallus variable in habit 

 Basal cell clavate or globose, with lobes or clavate branches 



