ZYGNEMA 15 



spores, and in parthenospores and aplanospores only two. In 

 general the zygospores are compressed-globose or ovoid. There is 

 an equatorial suture often marked by a slight ridge or keel 

 (carinate), and in some species there are two lesser ridges on 

 either side of the suture and parallel with it. These may not be 

 visible in fresh fully distended spores but they become evident 

 in dried or plasmolyzed specimens. Usually, the polar, or flatter, 

 sides of the spores lie in the plane of the tubes, but there are 

 6 species in which the plane of compression is at right angles to 

 the axis of the tubes. 



The most abundant and generally distributed of the brown- 

 spored isogamous species is Z. pectinatum, of the blue-spored 

 isogamous species, Z. synadelphum. Of the anisogamous brown- 

 spored species Z. stellintim is both abundant and widely distrib- 

 uted; Z. peliosporum is its counterpart among the blue-spored 

 species. As might be expected, all these species are highly variable 

 in dimensions, and local varieties and forms are apparently com- 

 mon. It is not improbable that they have been the mutating fore- 

 runners of nearly related species found locally wherever the 

 Zygnemas have been studied intensively. 



There is no good evidence that the taxonomic characteristics 

 of species are changed materially by environmental conditions. 

 There are alterations in chromatophores, accumulated foods, and 

 thickness of walls due to exposure on soil, restricted photosyn- 

 thesis, low temperatures, and mineral deficiencies. Reproductive 

 capacity may be decreased or increased by external conditions. 

 The mode of reproduction by zygospores or aplanospores, and the 

 placement of the zygospores in the tubes, or in one of the gam- 

 etangia, are the results of hereditary rather than environmental 

 factors. In any one species the position of the spores, whether in 

 the tubes or in the gametangia, does not change from season to 

 season (see Fritsch and Rich, New Phytologist, 26 [1927]). 

 There are several species in which the spores occur in either the 

 gametangia or the tubes — even in the same pair of conjugating 

 filaments. 



The identification of a species depends partly upon the dimen- 

 sions of the vegetative cells. Vegetative cell diameters should be 

 measured at the partition walls. Most important are the relative 

 dimensions of the spores, their form, and the color and ornamen- 

 tation of the several layers of the spore walls. Obviously only 



