THE GREEN ALGAE OF NORTH AMERICA 157 



The two species noted below live in various lower fresh water 

 animals ; they are so intimately associated with the hosts that 

 they were long supposed to be integral parts of the same ; in 

 many cases the alga cell divides at the same time as the host 

 cell. Beyerinck, 1890, has proposed a genus Chlorella, includ- 

 ing these species and some free species, which latter he thinks 

 have been included under Chlorococcnm ; it is difficult to see 

 why Zoochlorella should be superseded, unless the ground is that 

 it was described by a zoologist, and not by a botanist. 



KEY TO THE SPECIES OF ZOOCHLORELLA. 



i. Cells 3-6 n diarn. i. Z. conductrix. 



i. Cells 1.5-3 ,u diam. 2. Z.parasitica. 



1. Z. CONDUCTRIX Brandt, 1882, p. 140. Endozoic ; cells 

 3-6 p. diam., in tissues of Hydra and allied fresh water organ- 

 isms. Fig. 41. Mass. Europe. 



2. Z. PARASITICA Brandt, 1882, p. 140; P. B.-A., No. 1515. 

 Endozoic; cells 1.5-3 M diam.; in tissues of Spongilla, Ophry- 

 dium and other fresh water organisms. Mass. ' Europe. 



2. RHAPHIDIUM Kiitzing, 1845, P- H4- 



Cells elongate, acicular or fusiform, straight or variously 

 curved, w r ith pointed or rounded ends ; chromatophore nearly 

 covering the cell wall, usually without pyrenoid ; cell dividing 

 by oblique cross walls into 2-32 daughter cells, which separate 

 soon after attaining their full shape and size ; less frequently 

 remaining attached in bundles, or to the wall of the mother 

 cell. Widely distributed fresh water plankton algae. 



KEY TO THE SPECIES OF RHAPHIDIUM. 



i. With a long seta at each end. 4. R. setigerum. 



i. Without setae. 2. 



2. Cells 12-25 diam. long, ends very acute. i. R. fa leaf urn, 



2. Cells 3-12 diam. long, ends obtuse or apiculate. 3. 



3. Cells variously twisted and curved. 2. R. convolutum. 



3. Cells straight or nearly so, relatively stout. 3. R. Braunii. 



i. R. FALCATUM (Corda) Cooke, 1882, p. 19, PI. VIII, fig. 4 ; 

 R. polymorphum Wolle 1887, p. 197; P. B.-A., No. 1179. Cells 

 bright or yellowish green, slender, fusiform, at the middle 

 sometimes swollen, sometimes constricted ; ends very acute ; 

 straight or variously curved ; 1.5-3.5 P- diam., 15-25 diam. long, 

 up to 90 /u, ; usually united 2-32 in a bundle. Greenland to Fla. 

 and westward, Porto Rico. ]turope. 



A common and variable species ; Micrasterias falcatus is the 

 first name used for any of the forms now included in this species, 



