BLUE-GREEN ALGAE 297 
Blue-green Algae. Cyanophyceae 
The Blue-green Algae are the simplest forms of Algae and are 
the simplest known plants that make their own food. They 
are so named because of their bluish green color which is due 
to the presence of chlorophyll and a blue pigment called Phyco- 
cyanin. Although their size is microscopical, they form aggre- 
gations that are often quite conspicuous. There are about 1200 
species of Blue-green Algae, and they are widely distributed, 
occurring nearly everywhere in fresh and salt water and also on 
wet soil, rocks, and logs. On wet surfaces they form bluish 
green slimy layers or jelly-like lumps, and in sluggish streams 
and ponds they form bluish green scums or mats which float on 
or near the surface of the water. They thrive best where there 
is organic matter and consequently prefer stagnant to running 
waters. Some forms are so resistant to heat that they can live 
in hot springs where the temperature is near the boiling point 
of water. Some, called endophytes, live in the cavities of some of 
the more highly organized plants, such as the Liverworts and 
Ferns. Some are associated with Fungi in the formation of 
Lichens. The Blue-green Algae are of only slight economic im- 
portance. When allowed to accumulate, they impart offensive 
odors to water supplies, but are easily controlled by use of 
copper salts. It is claimed that livestock are sometimes killed 
by drinking water that has become foul with Blue-green Algae. 
The plant body in the Blue-green Algae is a single cell or a 
colony of cells so joined as to form a filament or plate. When 
cell division is in only one direction and the cells formed do not 
separate, then as a result of a number of successive cell divisions 
a chain or filament of cells is formed. When cell division is in 
more than one direction and the cells do not separate, then 
colonies of other shapes are formed. Colonies, although they 
may resemble multicellular plants, are aggregates of essentially 
independent cells. One notable feature of the plant body of the 
Blue-green Algae is the secretion of a gelatinous substance which 
forms a sheath about the plant. As plants grow and multiply, 
the gelatinous secretion accumulates and commonly forms a 
matrix which holds the plants together in slimy layers or jolly- 
like lumps. The gelatinous sheath holds water and thus protects 
the plants from drying out. Another notable feature of this 
