HOW TO KNOW THE FRESH-WATER ALGAE 



81b Cells straight or only slightly crescent-shaped, loosely entangled 

 about one another (frequently solitary rather than colonial). Fig. 

 68 ANKISTRODESMUS 



Fig. 68a. Ankistrodesmus falcatus (Cor- 

 da) Ralfs; b. A. convolutus Corda; c, 

 A. Braunii (Naeg.) Brunn.; d, A. fractus 

 (W. & G. S. West) Brunn. 



Although there are 5 or 6 species of 

 this genus common in the United States, 

 A. falcatus (Corda) Ralfs is probably the 

 one most frequently collected. It occurs 

 as solitary or loosely clustered needles 

 Figure 68 or slightly fusiform-shaped cells inter- 



mingled with other algae, or sometimes 



forming almost pure growths in artificial pools or in laboratory aquaria. 



One species, A. spiralis (Turn.) Lemm., has needle-shaped cells spirally 



twisted about one another to form bundles. 



82a (80) Cells attached either along their side or end walls to form 

 definite patterns, nets, plates, triangular clusters, or short rows. .83 

 end only in juxtaposition. See Fig. 57 TETRALLANTOS 



82b Cells attached otherwise, if adjoined by lateral walls then not 

 forming definite patterns 98 



83a Cells cylindrical, one cell attached by 2 others at the end walls 

 repeatedly to form a network. Fig. 69 HYDRODICTYON 



Fig. 69. Hydrodictyon reticulatum (L.) Lager- 

 heim. 



This is the familiar "water-net" which often 

 grows in such dense mats in lakes, small 

 ponds and irrigation ditches as to become a 

 troublesome weed. This unique alga is able 

 to reproduce very rapidly because each cell 

 of the net in turn produces a new cylindrical 

 net of small cells within it, which upon es- 

 cape enlarge enormously, each cell again 

 producing a net. The nets are of macroscop- 

 ic size and there is a report of one being 

 found more than 2 feet in length. It is thought that the first written 

 records referring to a specific alga is of Hydrodictyon in ancient 

 Chinese literature. 



83b Cells some other shape, not attached to form a network 



84 



84a Cells arranged to form flat, circular or rectangular plates. (Fig. 70) 

 85 



54 



