64 ZYGNEMATACEAE 



soils, rocks, and bogs. From there they are sometimes carried into 

 streams and ditches by rains, and continue to grow in the water 

 medium. 



In Zygogojiinm encetorum there is a strong tendency toward 

 encystmcnt whenever habitat conditions change; this is true even 

 of the gametes. The gametes may be walled off in the conjugating 

 papillae before the solution of the wall between the papillae. Sub- 

 sequently this wall may be dissolved and the gametes may unite. 

 This was the mode of reproduction first seen and described by 

 de Bary (1858). Subsequently normal reproduction was described 

 by the Wests (1894), t>ut in 1918 Hodgetts found and described 

 the reproduction by encysted gametes and insisted that this is 

 the normal process. In 1933 Transeau published figures show- 

 ing both modes of conjugation in the same pairs of filaments. 

 Since none of the other species exhibits so-called secondary gam- 

 etangia, these must be regarded as a peculiarity of Z. encetorum, 

 and direct conjugation the normal procedure. 



The taxonomic characteristics of the genus Zygogonium may 

 be summarized as follows: 



1. The species are terrestrial or amphibious on acid substrates. 



2. The filaments may become branched, sometimes with horizontal 

 filaments on the soil and erect branches arising from them. 



3. Reproduction by akinetes and aplanospores is common. All the 

 vegetative cells of some filaments may be changed to these structures. 



4. Reproduction by zygospores is apparently infrequent, and the 

 zygospores are enclosed in a sporangium wall with an equatorial suture. 



5. Cytoplasmic residues are present in sporiferous and gametangial 

 cells after spore formation. 



6. Cell walls and cell contents are often colored purple, and in ter- 

 restrial specimens the cell walls may become very thick, opaque, and 

 yellow or brown. 



7. Accumulation of fat globules and starch in terrestrial specimens 

 often obscures both the nucleus and the chromatophores. 



Key to the Species or ZYGOGONIUM 



I. With aplanospores only (zygospores unknown) 7 



1. With zygospores (sometimes aplanospores also) 2 



2. Median spore wall smooth 3 



2. Median spore wall not smoodi, yellow to brown 4 



a- Median spore wall not smooth, blue 13. Z. indicum 



