THE GREAT GKOUPS OF ALG^ 



23a 



together imbedded in the jelly-like matrix formed by the 

 wall material (Fig. 201). These imbedded groups of indi- 

 viduals 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 represents a very simple life his- 

 tory, in fact a simpler one could hard- 

 ly be imagined. 



160. 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 Gloeocapsa^ but they 

 are strung together to form chains of 

 varying lengths (Fig. 202). Th 3 jelly in 

 which these chains are imbedded is the 

 same as that found in Glmcapsa^ 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. 202, J), 

 which simply means 

 "other cells.-' It is 

 observed that when 

 the chain breaks up 



Fig. 202. Nostoc, a blue-green alga, showing the into fragments each 



chain-like filaments*, and the heterocysts (.1) fragment isCOmpOSCd 

 which determine the breaking up of the chain. — *^ 



Caldwell. of the CClls bctWCCn 



Fig. 201. Glcocapsa, a 

 blue-green alga, show- 

 ing single cells, and 

 small groups which have 

 been formed by division 

 and are held together by 

 the enveloping muci- 

 lage.— Caldwell. 



