8 ZYGNEMATACEAE 



between cells, the first step in conjugation had been taken. Adhe- 

 sion without conjugation occurs in several species of Mougeotia. 

 Conjugation is attained when the double wall formed by 

 adhesion is dissolved locally by enzyme action. The two proto- 

 plasts or gametes are then in direct contact and the conjugating 

 apparatus is established. 



Obviously the primitive akinetic or aplanosporic species lacked 

 the most characteristic feature of present-day Zygnemataceae until 

 the regular union of gametes through openings between adjacent 

 gametangia became the most prevalent mode of reproduction. 



From this point of view parthenospores are protoplasts which 

 started up the gametic pathway but had their development ter- 

 minated at the level of aplanospores. Since they contain the same 

 factors for wall color and patterns, the walls of parthenospores 

 resemble those of the aplanospores of the same species. Since 

 zygospores also contain these same factors for wall characteristics, 

 one would hardly expect them to have other wall structures. One 

 would anticipate larger sizes and other forms, since they are the 

 product of two protoplasts. From this point of view, one should 

 say that the walls of zygospores resemble those of the aplanospores 

 rather than the reverse. These surmises concerning the order of 

 development of the several reproductive structures seem far more 

 plausible than the traditional statements and implications that the 

 ancestral form of reproduction was by zygospores, and that the 

 parthenospores and aplanospores have eventuated from the con- 

 jugation apparatus by the loss of factors, enzymes, and hormones. 



Of the 534 species of Zygnemataceae here described i repro- 

 duces only by akinetes, 38 by aplanospores, 494 by zygospores, and 

 I has been found only in a vegetative condition. Of the 494 

 zygosporic species, scalariform conjugation occurs in 400 species; 

 72 species have both lateral and scalariform conjugation; and 22 

 species usually conjugate only laterally. Probably all the species 

 propagate by akinetes, and 77 of the zygosporic species also repro- 

 duce by aplanospores. Most spores are colorless, yellow, or brown. 

 Blue spore walls have been found only in the genera: Zygnema 

 (30 species), Zynemopsis (i species), Zygogonium (2 species), 

 and Mougeotia (8 species). 



In this family only the zygospores may be hybrid in character 

 following conjugation between different species. The filaments 

 that develop from hybrid zygospores are haploid, meiotic, or 



