HOW TO KNOW THE FRESH-WATER ALGAE 



379b Cells not bead-like; heterocysts absent 



380 



380a Trichomes with a sheath. Fig. 272 LYNGBYA 



Fig. 272. a, Lyngbya contorta 

 Lemm; b, L. Birgei G. M. Smith. 



Most species are straight and 

 rigid, but a few are coiled; some 

 are planktonic. The definite, rath- 

 er firm sheath ex*ending beyond 

 the end of the trichome (the row 

 of cells) is characteristic and 

 helps to separate this genus from 

 Oscillatoria (Fig. 277). One species, L. Birgei, is so characteristically 

 a planktonic species in hard water that it can be used as an index 

 organism. 



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;£:-;:H : ; 



? v oK 



m 



w.: 



H^m 



S P 



Figure 272 



380b Trichomes without a sheath. Fig. 273 ARTHROSPIRA 



Figure 273 



Fig. 273. Arthiospiia Jenneri (Kuetz.) Stiz. 



The plants in this genus are multicellular but at times the cross 

 partitions are difficult to discern and short sections may be mistaken 

 for Spirulina (Fig. 270). Species are differentiated mostly by size and 

 form of coiling of the filaments; occur on damp soil of beaches, or in 

 the tychoplankton. 



381a (377) Trichomes with cells all alike in shape and size (except 

 trichome) may be tapered, or the apical cell capitate (swollen) . 382 



381b Trichomes with certain cells enlarged and appearing empty or 

 with homogeneous contents (heterocysts); in some, certain cells 

 become enlarged and develop thick walls, forming spores (aki- 

 netes) 397 



382a Trichomes tapering from base to apex 383 



382b Trichomes not tapering from base to apex, the same diameter 

 throughout, or tapered only in the extreme apical region 385 



159 



