THE GEEAT GEOUPS OF AUijK 



233 



together imix'dded 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 simjjler 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 (rkeocapsie, but they 

 are strung together to form chains of 

 varying lengths (Fig. 202). The jelly in 

 which these chains are imbedded is the 

 same as that found in Glceoaqjsn, being 



formed l)y 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, A), 

 which simply means 

 " other cells." It is 

 observed that when 

 the chain breaks up 



Fig. 303. Ifostoe. a blue-green alga, showing the mto fragments Cach 

 chain-like fllaments, and the heterocysts 1,1) fragment isCOmpOSed 

 which determine the breaking up of the chain.— '"■ n i , 



c.LD,vELL. ot the cells between 



Fig. 201. Gkvocajisa, a 

 tjliiL'-gi-een alga, show- 

 ing single cells, and 

 small groupg wliich have 

 been formed by division 

 and are held together by 

 the enveloping muci- 

 lage. — Caldwell. 



