THE GRE*EN ALGAE OP NORTH AMERICA 101 



5. Cells uiulti nucleate, chromatophore net-shaped, or of numerous 

 small disks in a cell. 5. SIPHOXOCI.ADIALES. 



Order I. CONJVGAI.KS. 



Grass-green algae, starch forming, with cell wall of cellulose, 

 whose cells divide only in one direction, and are either isolated 

 or in filaments ; not incrusted with lime. Zygospores formed 

 by the union of the protoplasm of two similar or only slightly 

 different cells ' ' aplanogametes ' ' ; after a longer or shorter resting 

 period the outer membrane of the spore breaks, and a new vege- 

 tative development sets in. Thick-walled resting cells "akin- 

 etes " and asexual spores " aplaiiospores " sometimes formed; 

 no motile spores. 



The Conjugales are distributed all over the world, and seem 

 more independent of geographic limitations than other algae. Of 

 the 47 species of Z3'gnemaceaehere described, all but three occur 

 in Europe. While there is no such knowlege of these plants in 

 Asia and Africa as there is in Europe and America, what 

 species have been observed are very largely the familiar forms 

 of the better explored regions. In view of this cosmopolitanism, 

 any localities given must be considered as indicating where 

 specimens have been found, not as limiting the range of distrib- 

 ution. All are plants of fresh water, only rarely extending 

 into brackish water. 



KEY TO THE FAMILIES OF CONJUGATES. 



i. Cells usually divided by a constriction into symmetrical halves; 



solitary or in filaments ; the cell arising from the germinating 



zygospore either taking the normal form or producing 2-8 such 



forms. (DESMIDIACEAE.) 



i. Cells cylindrical, without constrictions, always united in filaments; 



the new filament always formed directly from the zygospore. 2. 

 2. The entire protoplasmic contents of the conjugating cells uniting 



to form the spore. i. XYGXEMACEAE. 



2. Only a part of the contents of the conjugating cells used for the 

 spore. 2. MESOCARPACEAE. 



The family Desmidiaceae is not included in this work. 

 Rich in genera and species, it constitutes a special field apart 

 from other algae, and for its proper presentation a special 

 treatment would be needed. The two other families are closely 

 related and distinctly marked off from all other green algae by 

 the character of the fructification. 



