THE GREEN ALGAE OF NORTH AMERICA 91 



schemes; those interested will find them in Bohlin, 1901 a; 

 Blackman and Tansley, 1902; West, 1904; Oltmanns, 1905. 

 The general statement of the classification used here is this : - 

 One small class, the Heterokontae, including algae whose motile 

 cells have cilia of unequal length ; whose vegetative cells con- 

 tain chromatophores colored yellowish with xanthophyll, with- 

 out pyrenoid, and whose reserve materials are in the form of oil 

 rather than starch. The other class, the Chlorophyceae, in- 

 cludes all algae with true chlorophyll-green chromatophores, 

 starch reserves, pyrenoids usually present, and whose motile 

 cells have cilia of equal length. The Heterokontae contain but 

 one order, Confervales ; the Chlorophyceae six ; Volvocales 

 with the motile stage more prominent than the non-motile ; 

 Conjugates, with no motile stage ; and four others with repro- 

 duction by various sexual or non-sexual motile spores or by 

 aplanospores, evidently a modification of the zoospores ; in 

 these orders there is so much variety in this respect, other char- 

 acters often remaining unchanged, that a classification based on 

 the reproductive bodies would seem highly artificial, a distinc- 

 tion based on other characters more natural. So we have the 

 uninucleate Protococcales, the cells solitary or in loose colonies ; 

 the membranaceous or filamentous Ulotrichales, also with uni- 

 nucleate cells ; Siphonocladiales with multinucleate cells ; and 

 Siphonales with no distinction of cells, the many nuclei distrib- 

 uted all through the interior of the plant. In each of these 

 orders there is great diversity of characters, both as to com- 

 plexity of structure and as to reproductive characters ; ex- 

 tremely simple vegetative growth being accompanied with very 

 elaborate reproductive mechanism in Oedogonium and Vaitcheria, 

 for instance ; remarkable variety of external form in the various 

 species of Caitlerpa, with no reproductive process whatever 

 known. In many genera, even of conspicuous plants, we still 

 have to write " reproduction not well known " or even " repro- 

 duction unknown." The division of the orders into families 

 will appear as the}' are reached in the following pages, and 

 requires no comment here. 



