14 ZYGNEMATACEAE 



In higher latitudes and altitudes the species are not well known 

 because they rarely have been found with zygospores. 



In warm regions it is not improbable that some species are 

 perennial; that is, some of the filaments form spores and die, 

 while others merely break into segments which continue vegeta- 

 tive growth. In well-aerated running water the Zygnemas are 

 rarely found fruiting, but in the shallow pools adjoining such 

 streams one can usually collect the same species in a fruiting 

 condition. 



The average length of the filaments of Zygnemas is a few 

 inches, much shorter than that of the Spirogyras. Apparently 

 before the filaments become very long the pectic sheath breaks 

 and the cells separate. Cell diameters vary from 8 to 58 /^ but more 

 than half the species have cell diameters between 20 and 30M. 



Usually there are two axial chromatophores in each cell with 

 the nucleus contained in the cytoplasmic bridge connecting them. 

 Each chromatophore consists of a round body, with irregular short 

 branches radiating in all planes, containing a conspicuous central 

 pyrenoid. As a result of food accumulation the chromatophores 

 may become merely two large globose bodies, or may completely 

 fill the lumen of the cell. 



On rare occasions one may find filaments in which each cell 

 has two axial chromatophores on either side of the nucleus. In 

 three collections that I have examined there were four chroma- 

 tophores in the cells of some filaments arranged in the form of 

 an X with the nucleus in the center. 



Sexual reproduction occurs in 82 of the 95 described species. 

 Twelve others reproduce by aplanospores, and i by akinetes only. 

 Of the described species reproducing by zygospores 37 are isog- 

 amous, with spores in the conjugating tubes; and 45 are ani- 

 sogamous, with spores in one of the gametangia. Of the 95 species, 

 70 have scalariform conjugation, 9 both scalariform and lateral 

 conjugation, and 3 lateral conjugation; no conjugation has been 

 observed in the remaining species. Thirty species have blue 

 median spore walls when mature. During development these 

 walls change from colorless to yellow, to brown, to blue. As not 

 all spores mature at the same time, one may find all these stages 

 in a collection. If some of the spores are blue, one may be sure 

 that the other spores in the filament are immature. 



Four chromatophores are usually visible in immature zygo- 



