18 



PLANT STRUCTUKES 



together imbedded in the jelly-like matrix formed by the 

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

 uals are spoken of as colonies, 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 Glwocapsa, 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 Glceocapsa, 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 het- 

 erocysts (Fig. 4, A), 

 which simply means 

 "other cells." It is 

 observed that when 

 the chain breaks up 



into fragments each FIG. 4. Nostoc, a blue-green alga, showing the 

 fragment is Composed chain-like filaments, and the heterocysts (A) 



which determine the breaking up of the chain. 

 Of the Cells between -CALDWELL. 



PIG. 3. Gloeocapsa, & blue- 

 green alga, showing 

 single cells, and small 

 groups which have been 

 formed by division and 

 are held together by the 

 enveloping mucilage. 

 CALDWELL. 



