TAXONOMIC CHARACTERISTICS 9 



gametic segregates. In the few instances in which such progeny 

 have been studied, individual filaments inherit the factor for cell 

 diameters, forms of receptive gametangia, zygospore size, form, 

 and wall markings independently. They are expressed through 

 the cytoplasm of the female gametes. So long as these segregates 

 reproduce by cell division, by akinetes, by aplanospores, and by 

 conjugation between cells of the same haploid filament, the gene 

 complex of the progeny is identical and the filaments are uniform. 

 Doubtless many of the "species" that are collected and named are 

 taxonomic units, or clones, that have originated in this way. Other 

 species seem to have originated by polyploidy and by mutation, 

 but the evidence for this statement is purely circumstantial. 



The bases for the separation of the genera are shown in 

 the following key. Three genera (Hallasia, Pletirodiscus, and 

 Entransia) are tentatively and purposely defined to emphasize 

 the need for further study of the development and reproduction 

 of these species. All the forms described in this key are designated 

 as "species." Many variations have been seen in the collections 

 studied but until more is known about hybrid segregates, isolated 

 clones, and the effects of the various environmental factors, it 

 seems unprofitable to assign "variety" and "form" names to every 

 variant. Many alleged "varieties" are not even closely related to 

 the species to which they have been assigned. 



Key to the Genera of ZYGNEMATACEAE 



I. Vegetative cells usually with 2 axile chromatophores, which are 

 round, radially branched, pillow-shaped, or disclike, with nucleus 

 contained in the cytoplasmic bridge between them 



I. Chromatophores usually a pair of axile stellate or globose radi- 

 ally symmetric bodies, each with a central pyrenoid 



a) Zygospores compressed-globose or ovoid in the tubes or in 

 one of the gametangia; aplanospores cylindric-ovoid occupy- 

 ing all or most of the cell; gametangia not filled with a 

 dense refractive colloid after conjugation, and without cyto- 

 plasmic residues i. Zygnema 



b) Zygospores quadrangular-ovoid, or round pillow-shaped, 

 formed in the extremely wide but shallow conjugating tubes; 

 aplanospores ellipsoid or asymmetrically ovoid; successive 

 layers of cellulose and pectic colloids added inside the gam- 

 etangial walls during conjugation; similar changes occur in 



