60 PRACTICAL BOTANY. 



in greater or less abundance in the gatherings from stag- 

 nant pools and ditches and in masses on moist rocks. 

 There are five orders in the Cyanophyceee. They are 

 the Chroococcacece, Nostocacece, Rivulariece, Oscillatoriece, 

 and Scytonemece. 



In the Chroococcaceae the cells are enveloped in a trans- 

 parent gelatinous covering. The cells are found singly, 

 or associated in different small groups, which result from 

 the repeated division of single cells. Groups of three are 

 common. These plants occur as slimy growths in damp 

 places and in stagnant water, usually associated with other 

 kinds of plants. The two most common forms are Chro- 

 ococcus and Grloeocapsa. These are very simple organisms. 

 They are distinguished from other organisms by the uni- 



FIG. 30. Gloeocapsa, magnified. 



formity in the distribution of the minute blue-green 

 granules through the protoplasm and by their gelatinous 

 envelopes. In Gloeocapsa the envelope has two distinct 

 layers, which in the older forms are quite dark, facts that 

 serve to distinguish it from Chroococcus. The cells divide 

 by partitions running in all directions through the parent 

 cell and thus give rise to groups of cells. 



The Nostocaceae form filaments which distinguish them 

 from other plants containing the blue-green coloring mat- 

 ter characteristic of this class. The filaments in some of 

 the species are more or less coiled in gelatinous envelopes. 

 The cells of the filaments are not uniform throughout, but 

 there occur at intervals larger cells that receive the name 

 heterocysts. There are three modes of reproduction. 



