HOW TO KNOW THE FRESH-WATER ALGAE 



133b Without a notch in the ends of the cell 134 



134a Cells crescent-shaped, with an axial chloroplast bearing ridges in 

 each horn. See Fig. 86 CLOSTERIUM 



134b Cells not crescent-shaped, or with other types of chloroplasts. . 135 



135a Cells constricted in the midregion to form 2 'semicells' which are 

 mirror images of one another 136 



135b Cells not constricted in the midregion to form 'semicells' 141 



136a Cells furnished with whorls of protuberances which bear one or 

 two spines; poles of the cell forked. Fig. 100 TR1PLOCERAS 



Figure 100 



Fig. 100. Tiiploceras gracile Bailey. 



There are apparently only two species of this desmid genus in the 

 United States, easily identified by the whorls of spine-bearing protuber- 

 ances along the walls. Like a number of other desmid genera this one 

 seems to be confined to acid water, especially in Sphagnum bogs. 



136b Cells not furnished with whorls of spiny protuberances 137 



137a Cells 10 or more times longer than broad, usually cylindrical or 

 nearly so, with margins smooth or undulate 138 



137b Cells less than 10 times their diameter in length; cylindric, fusi- 

 form, or tumid, usually straight but sometimes slightly curved. . 139 



70 



