6 ZYGNEMATACEAE 



are common but are rarely found producing spores. These species 

 cannot be identified because the modern species keys are neces- 

 sarily built around spore characteristics. Akinetes are common 

 in collections from these short-season regions, and by means of 

 them survival and propagation of many species take place. There 

 is no good reason to assume that all these species would produce 

 spores in other environments. It is far more likely that some of 

 these species are wholly vegetative, or akinetic. 



There are other species in several of the genera that also repro- 

 duce by aplanospores. These are slightly more specialized than 

 akinetes. Their forerunners are vegetative cells, the protoplasts 

 of which have contracted more or less, and in which new walls 

 of cellulose, with or without chitinous deposits, have developed. 

 The spore walls may be colorless and smooth, or variously colored, 

 sculptured, and ornamented. The criteria used to distinguish 

 aplanospores in Zygnematales are precisely the same as those used 

 to designate aplanospores among the Ulotrichales (PI. I, Figs. 4-7). 



Of the species here described, thirty-seven reproduce regularly 

 by aplanospores, and zygospores have been found in the same 

 filaments only rarely. Scattered aplanospores also occur along 

 with zygospores in seventy-six additional species which belong to 



7 genera. Moreover, only a few algologists have been interested 

 in looking for aplanospores or the list probably would be larger. 

 When found, aplanospores have often been written off as algal 

 errors, since most authors state definitely that none of the species 

 of Zygnemataceae "forms asexual reproductive bodies" (Smith 

 1933), or suspect that they are the result of lateral conjugation 

 (Czurda 1932). The "lateral conjugation" theory rests on state- 

 ments by Petit (1880), W. & G. S. West (1902), with a figure of a 

 disintegrating chromatophore, and figures by Czurda (1931), 

 without nuclei and also with disintegrating chromatophores. The 

 latter series of figures seems to prove only that this unnamed 

 species of Mougcotia may have under experimental conditions 

 lateral conjugation, as have 6 other species in nature. It has little 

 or no significance in proving that the numerous aplanospores 

 occurring in nature which have been carefully studied during 

 their entire development are zygospores. 



Zygospores result from the union of gametes that are more or 

 less similar in appearance, but physiologically different. The 

 maturation and union of gametes always take place within the 



