THE GREEN ALGAE OF NORTH AMERICA 161 



tion by the division of the contents of a cell into 2-16 similar 

 daughter cells, which remain enclosed for a lojnger or shorter 

 time in the persistent mother cell wall ; chromatophore covering 

 nearly the whole surface of the cell, with one pyrenoid. 



KEY TO THE SPECIES OF NEPHROCYTIUM. 



i. Daughter cells 2-7 /* diam., 3-6 diam. long. i. N. Agardhianum. 

 i. Daughter cells 11-22 /j. diam., 2 diam. long. 2. A^. Naegelii. 



1. N. AGARDHIANUM Nageli, 1848, p. 79, PI. III.C \ 



Wolle, 1887, p. 197, PI. CIvXIII, figs. 12-15, i? ; Wittr. and 

 Nordst., Alg. Exsicc., No. 528. Daughter cells 2-7 /u, diam., 

 3-6 diam. long ; usually 2-4-8 together, somewhat spirally 

 arranged within the mother cell wall which may reach a length 

 of 60 /A. Fig. 46. Me., Mass. J^urope, 



2. N. NAEGELII Grunowin Rabenhorst, 1868, p. 62 ; Wolle, 

 1887, p. 197. Daughter cells 11-22 //-diam., about 2 diam long; 

 usually 1 6 together, irregularly arranged within the mother cell 

 wall. Me. Europe. 



7. TETRAEDRON Kiitzing, 1845, p. 129. 



Cells solitary, free, with parietal chromatophore occupying 

 the whole or nearly the whole of the cell wall, with one pyre- 

 noid ; of very various form, either polyhedric with 4-many 

 angles, or flattened, 3-many angled, and in side view more or 

 less elliptic ; angles from obtuse and rounded to very much pro- 

 longed and often branched, sometimes repeatedly and finely di- 

 vided ; asexual reproduction by aplanospores, formed 3-many in 

 a cell, developing into the typical form of the mature plant 

 either before or after escape from the mother cell. 



A genus containing species differing greatly in form, some of 

 them very much like stages in the development of other algae ; 

 Hydrodictyon and Pediastrum, for instance, produce "polyhe- 

 dric" resting spores, and it is probable that some such forms 

 have been described as species of Tctraedron. Whether forms 

 here included will some time have to be removed for such reason 

 can be decided only by future investigations. It now seems 

 probable that some Tetraedron species, at least, are autonomous 

 forms, not forming part of the life cycle of any other plant. 

 Polyedrium Nageli, is a better known name for this genus than 

 Tetraedron Kiitzing, and more appropriate ; but the latter has a 

 priority of four years. 



KEY TO THE SPECIES OF TETRAEDRON. 



i. Cells flattened, 3 or more sided. 2. 



i. Cells polyhedric, but often quite irregular. 10. 



