124 ZYGNEMATACEAE 



insists that cell lengths are of no taxonomic importance since 

 they may be mocJiiied in some species by environmental condi- 

 tions. Nevertheless there are some species in which the cells are 

 relatively short (one-half to two diameters), and others in which 

 they are relatively long (eight to twenty-five diameters). In these 

 species at least, the cell lengths may be contributory evidence to 

 their identification. Cells 6oom long have been seen in at least 

 5 species, the diameters of which are between 201^ and 45/^ 



The chromatophores are ribbonlike or troughlike, with or 

 without a median ridge, arranged in a left-handed, or counter- 

 clockwise spiral in the parietal cytoplasm. The number of spirals 

 in a cell may vary from one to sixteen. In some species the number 

 of chromatophores is uniform. In others there are occasional cells 

 in filaments with one more than the usual number — particularly 

 in the one-spiraled species. How these arise is not known. In 

 still other species the number regularly varies within certain limits. 



In determining the number of chromatophores in a given 

 specimen, counts should be made only in filaments attached to 

 the sporangia or gametangia being studied, as there may be vege- 

 tative filaments of nearly the same dimensions but of another 

 species present in any collection. In most species the number of 

 spirals is readily determined by focusing just below an upper-half 

 turn of the spiral, counting this turn as one and adding to it the 

 number of optical intersections made by the spirals on the oppo- 

 site side of the cell. In very large species and in those with tightly 

 coiled chromatophores this may be impossible. In such species 

 and in those with nearly straight chromatophores, the numbers 

 are best determined by counting the ends of the spirals near the 

 cross walls. 



Each chromatophore has from several to many disc-shaped 

 pyrenoids spaced at regular intervals, and in certain species inter- 

 rupting the median ridge. The margins of the chromatophores 

 may be nearly smooth, or variously crenulate. Just after cell 

 division the spiral pattern is continuous from one cell to the next, 

 interrupted only by the thin partition wall. Chromatophores are 

 examples of direct cytoplasmic inheritance. Through aplanospores 

 they are derived directly from the chromatophores of the sporog- 

 enous cells. Through zygospores they are derived from the chro- 

 matophores of the receptiue gametangia. 



The cells of a filament are enclosed in a pcctic sheath im to 17M 



