220 



INTRODUCTION TO BOTANY 



The bluish-green balls (fig. 172, A) of Nostoc plants are 

 found upon damp soil or floating upon stagnant water. Under 

 magnification 1 the Nostoc ball is seen to be composed of 



granular jelly, inter- 

 woven by many chains 

 of cells, each of which 

 is a Nostoc plant (fig. 

 172,j5). These chains 

 often divide into two 

 or more shorter chains, 

 breaking where there 

 are large, dead cells 

 (heterocysts), and each 

 chain proceeds to live 

 as a new plant. Nos- 

 toc may absorb the 

 materials needed for 

 photosynthesis directly 

 through the cell walls, 

 or it may perhaps ab- 

 sorb organized foods as 

 do the bacteria, since 

 much food of this kind 

 is present in the water 



xrVCs-^ .'''., >? i " 



in which the plants live. 

 FIG. 172. Nostoc In times of drought the 



jelly balls dry slowly 

 and may become dry 

 enough to crumble eas- 

 ily, but when a favora- 

 ble amount of moisture 

 returns, the plants within the ball may proceed to grow. 



At the left (A) are several of the Nostoc balls, 

 which appear as glistening, rounded masses (nat- 

 ural size). At the right (B), inclosed in gelatinous 

 material, are a few chains of Nostoc plants which 

 have been taken from one of the balls and greatly 

 magnified. In the chains several of the enlarged 

 heterocysts may be seen 



1 When beginning the study of the algse it is often better to use a good 

 specimen under a demonstration microscope than to attempt individual 

 microscopic work. If preceded by preliminary demonstration work the fol- 

 lowing individual studies will be more intelligible and successful. 



