262 COLLEGE BOTANY 



near the boiling point to that of the arctic regions. Some species 

 live on wet soil and rocks and others, known as endophytes, live 

 within cavities of some of the higher plants, especially the liver- 

 worts and ferns. They are sometimes so abundant as to produce 

 offensive odors when decaying, and have sometimes given trouble 

 by accumulating in great quantities in the reservoirs supplying 

 the water to large cities. 



They are always unicellular, but in many cases are held 

 together in very definite colonies. Each cell (with the exception 

 of the heterocysts) has the power of performing all the life 

 functions of a living plant and of producing a new colony. They 

 are always surrounded with a more or less prominent gelatinous 

 substance which is secreted by them and is of importance in hold- 

 ing them together. In most species the nucleus and cytoplasm 

 are not well defined. There are no chloroplasts and the coloring 

 matter is diffused throughout the cell. They always reproduce 

 by simple fission and so far as known there is no other method 

 of reproduction. Some of the most important genera are Glceo- 

 capsa, Oscillatoria, Nostoc, Rivularia and Stigonema. 



The genus Glceocopsa (Fig. 118, a) is composed of unicellu- 

 lar algae, which may be found on wet soil, stones or wood. Each 

 individual plant (i.e., cell) is morfe or less spherical, bluish- 

 green in color and surrounded by a clear jelly-like substance. It 

 reproduces by dividing into two daughter cells- or plants which 

 separate and divide again. Two or even four cells or plants 

 frequently cling together for some time before separating. 



The genus Oscillatoria, (Fig. 119,6) is composed of filament- 

 ous species which may be found on wet soil, stones or wood or in 

 shallow water. They frequently become detached and float on 

 the surface as scum masses. Each filament consists of disc- 

 shaped cells which are attached to each other by their flat sur- 

 faces. Although attached, there is no division of labor, each cell 

 performing all the functions of an individual plant. Each fila- 

 ment is surrounded by a thin jelly-like layer and has the power 

 of locomotion ; it sways back and forth or moves in either direc- 



