8o ZYGNEMATACEAE 



occupying only a part of the axis of mature cells. The nucleus is 

 near the center of the cell, placed on one side of the chromato- 

 phore in those species which have a single chromatophore. In 

 species with two chromatophores, such as M. protia, the nucleus 

 lies in the bridge between the chromatophores. Under natural 

 conditions favoring active growth the chromatophores of M. 

 capticina may be either ribbonlike or rodlike, and the cells may 

 have purple cell sap in some filaments instead of the usual color- 

 less solution. The pyrenoids in most species are arranged in a 

 linear row; in a few they are scattered throughout the broad plate- 

 like chromatophores. 



Of the 99 species here described, 92 usually reproduce by means 

 of zygospores, and 7 by aplanospores. Of the 92 zygosporic spe- 

 cies, 25 also occasionally produce aplanospores either in the same 

 filament with zygospores, or in separate aplanosporic filaments. 



Scalariform conjugation occurs in 86 species. Conjugation 

 may be either scalariform or lateral in 4 species, and in only 2 is 

 it regularly lateral. 



Almost all the species are isogamous; only 3 species are strictly 

 anisogamous, and 2 others somewhat variable even in the same 

 paired filaments. The distinction between parthenospores and 

 aplanospores is not always easy to make. In many of the 25 spe- 

 cies reproducing both by zygospores and aplanospores, the form 

 and placement of the spores are somewhat different. In those 

 species having aplanosporic filaments the distinction is more 

 evident. These filaments are quite regularly zigzag with the 

 spores at the angles, the first facing in one direction and the next 

 in another direction. Hassall described such filaments in 1842, 

 and figured an immature one in 1845 {M. notab'iUs). Wittrock 

 (1878) also discussed this feature of aplanosporic filaments when 

 he proposed the genus Gonatonema. Paul Petit (1880), the Wests 

 (1902), and Czurda (1931) have suggested that these spores may 

 result from internal division of a vegetative cell followed by 

 lateral conjugation. This speculation still awaits cytological evi- 

 dence. In the hundreds of developing aplanospores studied by 

 me and my associates, not a single example of preliminary division 

 of either the protoplast or the nucleus has been found. There is 

 good reason to believe that rare instances of lateral conjugation 

 may be found among the usually scalariform species of Mougeotia 

 just as in Zygnema and Spirogyra. Such instances, however, can- 



