624 TEXTBOOK OF BOTANY 



at temperatures between 150° and 170° F. Other species may remain 

 frozen in ice for many months without injury. Blue-greens may become 

 serious pests on soil in greenhouses by forming a gelatinous layer that 

 prevents aeration of the soil beneath. They are very common in ponds 

 in the far north. 



The blue-greens constitute an important part of the algal population 

 of soils, and their spores can withstand complete desiccation for many 

 years. They may grow as parasites within the bodies of man and other 

 animals; as epiphytes on other algae and on the vegetative parts, particu- 

 larly leaves, of many seed plants; and as hosts of fungi in a few kinds 

 of lichens. Species of blue-green algae regularly occur in the leaves of a 

 water fern (Azolla), and on the scales of some liverworts. One blue- 

 green species grows in the aerial roots of some cycads, and many others 

 may be found within the gelatinous envelopes of other algae. 



In general, this group of algae may be considered as the characteristic 

 plants in bodies of water high in organic matter, but not necessarily so. 

 When fresh-water streams are polluted by sewage, by poisonous wastes 

 from various manufacturing processes, and by drainage from coal mines 

 and oil wells, the blue-greens are the last to disappear. In fact, in such 

 habitats certain bacteria are about their only living associates. 



Blue-green algae are either unicellular, colonial, or multicellular. Cells 

 of the colonial species may be loosely or definitely aggregated into glob- 

 ular, saccate, plate-like, or irregular colonies with a few to many cells 

 held together by mucilage. The cells of some other species are joined 

 end to end in definite regular or irregular filaments; the filaments may 

 be branched or not. Division of the cells in only one plane results in a 

 chain of cells; division in two planes, a plate-like colony; and division 

 in three planes, a solid and often nearly cubical aggregate. 



The granules of food that accumulate in the vegetative cells are not 

 definitely known chemically. They undoubtedly are conversion products 

 from sugar formed in photosynthesis; they are not starch. Glycogen, or 

 possibly glycoproteins, and droplets of oil may accumulate, especially in 

 the spores. 



The plants increase in number by vegetative multiplication. Any vege- 

 tative cell in some plants, or only certain vegetative cells in other plants, 

 may develop thick walls and become dormant. After a period of time, 

 new plants grow from these thick-walled cells. New filaments also result 

 from the fragmentation of old filaments into small groups of cells, fol- 

 lowed by repeated cell division. 



