18 



PLANT STRUCTURES 



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 Glmoccq^sa^ 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 liet- 

 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 

 frao-mPTif iscomnosed chain-like filaments, and the heterocysts {A) 



iragment IS composed ^^,^.^j^ determine the breaking up of the chain. 



of the cells between -caldwell. 



Fig. 3. Glceocapsa, a blue- 

 green alga, showing 

 single cells, and small 

 groups which have been 

 formed by division and 

 are held together by the 

 enveloping mucilage.— 

 Caldwell. 



