CHAPTER THREE 



THE GENUS ZYGNEMOPSIS (SKUJA) TRANSEAU 1934 



The vegetative filaments of the 24 species here classified are 

 usually indistinguishable from those of Zygnema. They may have 

 a thin, or a thick, pectic sheath and they have been found floating 

 in ponds, ditches, and slow flowing streams, not infrequently 

 associated with other species of Zygnemataceae. 



At the beginning of the reproductive phase, however, they 

 may be distinguished by the partial replacement of the usual cell 

 contents by a transparent refractive gel. Whether the cells become 

 gametangia, or aplanosporangia, some of them lengthen and suc- 

 cessive layers of cellulose are deposited as the protoplast contracts 

 to the middle of the cell. At maturity the zygospores have four, 

 and the aplanospores two, lamellate solid appendages attached to 

 them. As seen in the illustrations, these may be short and stubby, 

 or may be very much longer than the original vegetative cell. 



All the 19 zygosporic species have isogamous gametes and 

 scalariform conjugation. After the papillae unite in sexual repro- 

 duction the tube is exceedingly narrow, but soon broadens in the 

 plane of the filaments. The zygospore resulting from fusion of 

 the gametes is a compressed, more or less quadrangular pillow- 

 shaped, body which may subsequently become lenticular or irreg- 

 ular in outline. Both the tube development and spore forms 

 resemble those of the quadrangular-spored species of Mougeotia 

 more than those of any species of Zygnema. The outer or first- 

 formed wall bounds the colloidal gel and is composed of cellulose. 

 The median wall is chitinous and in at least one species consists 

 of two layers. Not infrequently yellow or brown granules are 

 deposited between the outer and median walls and may obscure 

 the surface features of the median wall. The innermost wall is 

 transparent and thin and can be seen only when the spore is 

 crushed. Parthenospores are not infrequent in some collections. 

 They have walls similar to those of the zygospores, but are ovoid, 

 smaller, and laterally placed in the gametangia. Aplanospores are 

 common in 5 of the species. They vary in form from ovoid to 

 ellipsoid and have walls similar to those of the zygospores. 



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