THE BLUE-GBEEN ALGJE (CYANOPHYCE^) 183 



167. Nostoc : structure and nutrition. In regions such as 

 those mentioned as the living place of Grloeocapsa, and also 

 upon damp soil or floating upon standing water, there may 

 be found the jelly-like balls of Nostoc. Instead of the rather 

 ragged fragments of the Grloeocapsa masses, Nostoc usually is 

 found in irregularly rounded compact balls (Fig. 153, A), 

 which have a dark bluish-green color. The ball is a colony 

 of plants, but when it is taken in the fingers no evidence of 

 the existence of single plants can be obtained. 



Under magnification the ball is seen to be composed of 

 granular jelly, through which are interwoven thousands of 

 chains of cells, each of which is a Nostoc plant. Usually two 

 kinds of cell are found in the chain, one or more larger ones 

 called heterocysts (which simply means " different cells " ), and 

 ordinary cells (Fig. 153, 5), each one resembling a Grloeocapsa 

 plant. These ordinary cells of Nostoc manifest an evident dif- 

 ference from those of Grloeocapsa; the jelly-like mass about 

 Nostoc plants is not in layers about each cell, as is true in 

 G-lceocapsa. The cells are loosely held together in chains by 

 attachments of their walls, thus making a slightly more com- 

 plex plant than G-loeocapsa. 



In nutrition, Nostoc may absorb directly through the cell walls 

 the materials that are needed for photosynthesis, or it may, per- 

 haps, absorb organized foods, since much food of this kind is in 

 the water in which the plants live. Since the jelly mass is often 

 quite large, obviously there must be absorbed through the outer 

 part of the Nostoc ball the food needed not only for the outer- 

 most plants but also for those that are more deeply situated. 



In times of drought the gelatinous nature of the Nostoc balls 

 results in extremely slow drying, though if the drought be long- 

 continued the whole ball may become so dry as to crumble be- 

 tween the fingers. Even when as dry as this, not all of the 

 plants are killed, and a large number of them proceed to grow 

 when furnished the needed moisture. This covering of jelly 

 and the consequent slow drying seems to be of the greatest 

 importance in the life of both Nostoc and Grloeocapsa. 



