HOW TO KNOW THE FRESH-WATER ALGAE 



Figure 142 



201b Chloroplasts diiferent in shape, or if star-shaped, larger than 

 above, and always 2; conjugating cells not becoming filled with 

 pectic substances; zygospores globose, compressed globose, or 

 oval 202 



202a Chloroplasts 2, definitely star-shaped, each containing a large 

 central pyrenoid; aquatic. Fig. 142 ZYGNEMA 



Fig. 142. Zygnema pectina- 

 tion (Vauch.) C. A. Agardh, 

 vegetative cells showing 

 star-shaped chloroplast. 



There are numerous spe- 

 cies of Zygnema, differenti- 

 ated on the basis of t he 

 zygospore morphology. The 

 paired, star-shaped chloro- 

 plasts in each cell make identification of the genus reasonably cer- 

 tain. Frequently the cells ere so densely packed with starch grains 

 and cytoplasmic granules that the shape of the chloroplast is difficult 

 of determination. Application of an iodine solution often facilitates 

 observation, or if one examines several lengths of filaments under low 

 magnification the stellate form of chloroplasts will become apparent. 

 A few species have a conspicuous gelatinous sheath. 



202b Chloroplasts axial as above but with radiating processes much 

 reduced, sometimes bridged so as to form a dumb-bell shaped 

 mass; terrestrial. Fig. 143 ZYGOGONIUM 



Fig. 143. Zygogonium eri- 

 cetorum Kuetz. 



These filaments are 

 somewhat irregular be- 

 cause the cell walls are 

 unevenly thickened and 

 usually are invested by 

 a layer of mucilaginous 

 substance. The cells have 

 the habit of putting out frequent rhizoidal protrusions which may 

 branch. Sometimes the conjugation tubes (when they fail to meet 

 other tubes) will continue to grow as rhizoidal processes. The plant 

 is usually found in aerial or subaerial habitats (occasionally in the 

 water on submerged stumps, etc.). 



Figure 143 



203a (199) Cell sap purplish 204 



203b Cell sap not purplish 205 



93 



