THE PHYLA OF ALGAE 



HE organisms which constitute what are commonly known 

 as freshwater algae are extremely 4ive r<3e m form, color, 

 habit, and in theii habitats. Actually, there are as many 

 as eight separate phyla or divisions of the plant king- 

 dom included under the name "algae," (nine if Cryptophy- 

 ceae of uncertain position are given the status of a division). Hence 

 to undertake writing descriptively about freshwater algae, one is con- 

 fronted with almost as great a task as if he were treating all the phyla 

 of land plants, fungi, mosses, ferns and the seed plants, plus three 

 additional groups all in one work. To be sure all the phyla of 'algae' 

 do not include as many families and genera as do some of the higher 

 plants, but the green algae alone include some ten or twelve thou- 

 sand species, distributed among about 375 genera. The task of the 

 writer in describing freshwater algae is not made easier by their rela- 

 tively small (mostly microscopic) size. Hence it is necessary to employ 

 special descriptive terms in order to differentiate these minute organ- 

 isms, and to classify properly the classes and phyla to which they 

 belong. 



In such a treatment as is presented here, only the most common 

 and best known genera can be given a place. The reader should 

 have thi.« in mind when using the key. He should avail himself of 

 other less abridged or monographic works if satisfactory identification 

 of a plant in which he is interested does not appear possible by the 

 use of the following key. 



The illustrated key is intended to assist the beginning student 

 and the non-specialist (equipped with a microscope) in identifying 

 the genera of freshwater algae which occur in the United States. 

 Such a key cannot be made as easy to use as are many keys to 

 larger organisms. But an attempt has been made to overcome some 

 of the usual difficulties by keying out some genera at several dif- 

 ferent points, especially those which are so variable that selection 

 of any one set of differentiating criteria for them is almost impos- 

 sible. At least, a beginning student or one with little familiarity with 

 the algae, must exercise patience until he has developed some de- 

 gree of judgment and has become well-acquainted with the meaning 

 of terms, and until he has discovered to what degree a plant may 

 vary from the usual character employed in making an identification. 

 Many times he will find it profitable, if not necessary, to "back-track" 

 in using the key and follow down both dichotomies of choice before 

 arriving at a satisfactory determniation. In making use of the illus- 

 trations it must be remembered that only one or two species of a 

 genus are illustrated, and that the plant in which one may be inter- 



