to be mixed with other filamentous algae. The unusually long, 

 slender cells and the long ovate spores are distinctive features. , 



Entangled among other algae in many lakes and swamps; common. 

 Mich., Wis. 



ZYGNEMA C. A. Agardh 1824, p. 77 



Unbranched filaments of short or long cylindrical cells with plane 

 end walls, inclosed by a soft (usually) mucilaginous sheath, rarely 

 with irregular rhizoidal outgrowths of a few cells when in contact 

 with the substrate; chloroplasts 2 (rarely 4) axial, stellate masses, 

 each containing a large central pyrenoid with a conspicuous starch 

 sheath. Conjugation mostly scalariform, with the zygospores formed 

 in the connecting tube, or rarely in one of the gametangia, but not 

 cut off by membranes from the gametangial cells. Zygospores spheri- 

 cal, ovate, or ellipsoid, with a thick, 3-layered wall, the outer and 

 middle layers variously sculptured with scrobiculations and puncta- 

 tions. Asexual reproduction by aplanospores and akinetes. 



The genus Zygnema contains about 85 known species, and many 

 more of these certainly occur in our region than can be listed now. 

 Numerous collections of Zygema were made in the sterile condition 

 and are, therefore, unidentifiable. One species was invariably found 

 in soft water lakes attached about the culms of Scirpus spp., where 

 it formed pale green, cottony masses. Although this species was 

 collected repeatedly at several different seasons no fruiting speci- 

 mens were found. This is interpreted as having a distinct ecological 

 significance, and several possible explanations present themselves. 

 One of these has to do with water chemistry. Species of both 

 Zygnemataceae and Oedogoniaceae were found to flourish vegeta- 

 tively in soft water lakes but were scarcely ever collected in a 

 fruiting condition. In hard water lakes or in shallow ponds where 

 there was a high concentration of organic matter and organic acids, 

 these plants are more luxuriant and fruit abundantly. 



Key to the Species 



1. Zygospores not formed; conjugation unknown; reproduction by 



aplanospores; vegetative cells 44-54/^ in diameter Z. sterile 



1. Zygospores formed by conjugation; aplanospores present 



or absent; vegetative cells smaller _ 2 



2. Spore wall blue when mature 3 



2. Spore wall yellow, yellow-brown, or colorless when mature 6 



3. Spores formed in one of the gametangia ^ Z. chalybeospermum 



3. Spores formed in the conjugation tube, extending 



into the gametangia or not 4 



4. Median spore wall smooth Z. cyanosporum 



4. Median spore wall punctate 5 



[323 ] 



