CHAPTER TWO 



THE GENUS ZYGNEMA C. A. AGARDH 1824 



The plants classified as species of Zygnema consist of un- 

 branched filaments of short cylindric cells usually covered by a 

 pectose sheath. To the field collector they are less slippery than 

 the Spirogyras but more slippery than the Mougeotias. There are 

 exceptions, of course, because the sheaths vary in thickness from 

 those which are barely visible under the microscope to those which 

 are thicker than the cells themselves. In regions of low tempera- 

 ture and alkaline water, the sheaths are denser and highly stable. 

 Similar sheaths are found on filaments living on wet shaded soil 

 on pond margins. The thicker sheaths often have visible struc- 

 tural lines at right angles to the filaments. 



Zygnemas live as annuals, and may complete their life cycle 

 in a few weeks and then disappear. They are most abundant and 

 more frequently found reproducing in temporary ponds and 

 ditches. In permanent ponds one may find vegetative filaments 

 throughout the year. This is not to be construed as evidence that 

 individual plants are perennial. It is far more probable that the 

 germination of spores and akinetes occurs throughout the year. 

 In cold temperate regions the most abundant germination starts 

 in autumn and winter, and culminates in early spring. As ponds 

 become shaded by the growth of marginal shrubs and trees, repro- 

 duction decreases and finally ceases entirely, but the Zygnemas 

 are perpetuated for a subsequent period of years by the overwin- 

 tering of akinetes and fragments of filaments. They survive cold 

 and dry periods in the ooze and silt of the pond bottom. During 

 the drought period between 1930 and 1935 a small pond near 

 Columbus, Ohio, was dry for three and a half years. When the 

 rains finally restored the pond, filaments of Zygnema sterile be- 

 came abundant within three weeks. This species reproduces by 

 akinetes only. 



Zygnemas have been collected on all the continents from sea 

 level to alpine summits, and from the torrid to the frigid zones. 



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