170 CRYPTOGAMS 



BLUE-GREEN ALGJE 



409. On wet walls of stone and on undisturbed moist 

 earth may often be found small, rounded, jellylike masses 

 of a greenish or bluish color. A bit placed under the 

 microscope shows a great number of chains of rounded 



cells (Fig. 282), embedded in the 

 gelatinous matter. Certain cells 

 of each chain are somewhat 

 larger and lighter colored than 

 the rest. When a chain breaks 

 282. A chain of Nostoc cells: in pieces, as occasionally happens, 



h, heterocyst; d, recent , . ,, . , , 



divisions. separation usually takes place 



next to one of these enlarged 



cells, or heterocysts. The fragments finally develop into 

 chains of the original character. The cells increase in 

 number by transverse division (Fig. 282, d). Cell divi- 

 sion is, in fact, the ordinary process by which the plants 

 of this group multiply. 



410. If the substratum on which the plants are grow- 

 ing dries up, the investing mass of gelatinous substance 

 hardens in proportion as it parts with water, and so be- 

 comes a protective coating which enables the plant to 

 withstand extreme drought. 



411. The plant here described and figured (Nostoc) is 

 representative of the Blue-green Algae in color, in the 

 filamentous arrangement of the cells, in the method of 

 multiplication by transverse fission, and in throwing off 

 mucilaginous matter from the Avails to form sheaths and 

 embedding masses. In some species, however, the cells 

 are found in small groups, not filamentous ; and in others 

 the gelatinous coating is either very thin or entirely 

 wanting. 



412. Oscillatoria (Fig. 283) is, like many of the group, often 

 aquatic, either floating freely or gathered in small tufts. The filaments 

 have a characteristic motion of bending slowly from side to side 

 whence the name Oscillatoria. They also possess some means of 

 locomotion, by which they slip along over the substratum, while at the 

 same time slowly revolving upon the longer axes of the filaments. 



