INTRODUCTION 3 



At one extreme it is represented by unicellular microscopic forms, 

 and at the other by species of complex structure, some of which 

 are of great size. The members of the group are chiefly aquatic, 

 and include the pond scums and sea weeds familiar to all students 

 of nature. 



Though chlorophyll occurs in general throughout the algae 

 the thallus is distinctly green only in the Chlorophyceae. In 

 the Cyanophyceae, Phaeophyceae, and Rhodophyceae other color- 

 ing matter is present in sufficient amount to obscure the green, 

 and give the thallus another characteristic color. Older algolo- 

 gists used the color of the thallus as a basis for the primary sepa- 

 ration of the group, and recognized as major subdivisions, 

 blue-green algae, green algae, brown algae, and red algae. 

 Though these groups are still recognized as constituting in a 

 broad way a convenient separation, there is a pronounced 

 tendency in modern classification toward basing separations 

 wholly on morphology {e.g., West and Fritsch 1927). Where 

 color fails to coincide completely with morphology rearrangement 

 has occurred. It is to be expected that future research will 

 result in even more extensive revision and the disappearance 

 of the old absolute separation based on color. 



In some of the more recent treatments, the blue-green algae 

 (Cyanophyceae) are removed from the algae, are renamed the 

 Schizophyceae, and together with the Schizomycetes (bacteria) 

 are made to comprise a lower primary subdivision of the Thallo- 

 phyta termed the Schizophyta. The group thus constituted is 

 regarded as occupying an isolated position in the phylogenetic 

 arrangement, and its members are believed to be very primitive. 

 Both in the bacteria (fission fungi) and blue-green algae 

 (fission algae) reproduction is accomplished almost exclusively 

 by means of simple vegetative cell cleavage or fission. Though 

 spores of a simple type occur in both groups, complicated repro- 

 ductive structures and processes, comparable to those of other 

 thallophytes, are' lacking. The nucleus, if not actually absent, 

 is at least far less definitely organized than in other plants. 



The blue-green color of the Cyanophyceae is believed to be 

 due to the presence in the protoplasm of two associated pigments, 

 one blue, the other green. The blue coloring matter has been 

 named phycocyanin. That the green is chlorophyll seems not 

 entirely certain. It does not occur in distinct chloroplasts, 

 and may be in chemical combination with the blue pigment. 



