TAXONOMIC CHARACTERISTICS 5 



the opposite side. The bulges may be rounded, or cylindric with 

 shoulders at both ends, or spindle-shaped. In any event the 

 resulting forms of the expanding cell walls are remarkably uni- 

 form in a given species. In other species many of the nonrepro- 

 ducing cells enlarge and become distended or even bullate. These 

 changes in cell form are definitely the result of hereditary factors. 



In some environments one may also find vegetative filaments 

 composed of variously rounded or distorted cells, even with rhi- 

 zoidlike outgrowths, in extreme instances. These collections, in 

 my experience, have always been gathered from ponds with warm 

 water and low oxygen pressure. Since these effects were noted on 

 several of the species present, the causes are probably environ- 

 mental. 



The following key to the genera is based primarily on the 

 characteristics of the chromatophores, and secondarily on the 

 changes that take place in the reproductive cells during spore 

 formation. 



In the subsequent keys to the species grouped under each genus 

 stress is laid on the characteristics of the spores. Consequently one 

 must be quite clear about the nature of these several kinds of 

 reproductive structures. 



The simplest, most universally present, and probably the most 

 primitive type of propagating cell is the akjnete (PI. I, Figs. 1-3). 

 This is usually just a vegetative cell, the walls of which have been 

 thickened by additional layers of cellulose, or cellulose and pectose. 

 These cells survive long periods of drought, high and low tem- 

 peratures, as well as the effects of these factors on the concen- 

 tration of solutes in their immediate environments. Akinetes 

 germinate readily and initiate new filaments. Only one species 

 (Zygiiema sterile) has been found to reproduce by this method 

 alone. There are probably many others but they are not so easy 

 to identify as this one, which happens to be the largest species of 

 Zygnema in the Ohio valley. 



There is a prevalent notion that akinetes are formed when 

 conditions are unfavorable. It has been our experience that they 

 develop both when conditions seem most favorable and when 

 least favorable. In cultures, they appear both at high temperatures 

 and low temperatures. The causes for the initiation of akinetes 

 are best stated as unknown. 



At high altitudes and latitudes the species of Zygnemataceae 



