300 



THALLOPHYTES 



tive to the ordinary cells of the filament; and, when favorable 

 conditions for growth return, the protoplast of the resting cell 

 breaks through the heavy wall and develops a new filament. 



In Rivularia (Fig. 258), another filamentous form, the filament 



is apparently differentiated into a basal and apical region. A 



heterocyst is the basal cell and the cells decrease in size toward 



the apex, so that the filament has a whip-like 



appearance. 



Besides the features just mentioned in con- 

 nection with the plant body, there are some 

 other minor ones which some particular species 

 of Blue-green Algae have. For example, in 

 one species the cells of the colony arrange 

 themselves so as to maintain a regular rec- 

 tangle. In some forms the colony forms a 

 branched filament. 



Food is manufactured, and water and min- 

 eral matters are absorbed by these simple 

 plants in essentially the same way as in the 

 more complex plants, but each cell must 

 manufacture food and absorb water and 

 mineral matters for itself. Since these plants 

 live in water or on a moist substratum, they 

 are able to absorb water and mineral matters 

 from their immediate surroundings. Having 

 chlorophyll, they are able to carry on photo- 

 synthesis and thereby provide themselves 

 with carbohydrates. Although the function 

 of phycocyanin is not known, it is probable 

 that it assists some in connection with photo- 

 synthesis. Sometimes there is an additional reddish pigment 

 developed, which may have something to do with enabling the 

 plant to utilize the sun's rays in the manufacture of food. The 

 reddish pigment is so abundant in a few forms that the plants 

 appear red in mass, as in one group which forms floating colonies 

 in salt water and has given the name to the Red Sea. 



Reproduction in the Blue-green Algae is chiefly by cell division. 

 They form no sex cells and, therefore, depend entirely upon 

 vegetative methods of reproduction. By cell division new cells 

 are formed, which may, according to the species, separate as new 



Fig. 258. — a 

 single colony of 

 Rivularia consist- 

 ing of a large hete- 

 rocyst and many 

 vegetative cells 

 which decrease in 

 size away from the 

 heterocyst. X 540. 



