18 



PLANT STEUCTUEES 



together imbedded in the jelly-like matrix formed by the 

 wall material (Fig. 3). These imbedded groups of individ- 

 uals are spoken of as coIo7iies, and as 

 colonies become large they break up 

 into new colonies, the individual cells 

 composing them continuing to divide 

 and form new individuals. This rep- 

 resents a very simple life history, in 

 fact a simpler one could hardly be 

 imagined. 



19. Nostoc. — These forms occur in 

 jelly-like masses in damp places. If 

 the jelly be examined it will be found 

 to contain imbedded in it numerous 

 cells like those of Glcpoccqjsa, but they 

 are strung together to form chains of 

 varying lengths (Fig. 4). The jelly in 

 which these chains are imbedded is the 

 same as that found in Ol(eocaps((, being 

 formed by the cell walls becoming mucilaginous and swollen. 

 One notable fact is that all the cells in the chain are not 

 alike, for at irregu- 

 lar intervals there oc- 

 cur larger colorless 

 cells, an illustration 

 of the differentiation 

 of cells. These larger 

 cells are known as hef- 

 croci/sfs (Fig. 4, A), 

 which simply means 

 " other cells." It- is 

 observed that when 

 the chain breaks up 



into fragments each Fiu. 4. Xnx/m-, a blue-grecn alga, showing the 



fragment is composed chain-like ftlamenls, and the hcterocysts (.0 



*-^ ^ which determine uw oreaking up of the chiim. 



of the cells between - caldavell. 



Fig. 3, 



green alga, 



single cells, and small 



groups which have been 



formed by division and 



are held together by the 



enveloping mucilage. — 



Caldwell. 



