ASEXUAL REPRODUCTION 



319 



its methods of reproduction. In the first place not all the 

 cells are alike. Examined at a favorable period in its de- 

 velopment three sorts of cells will be found. The larger 

 number of cells are of the ordinary vegetative type which 

 go right along performing their work of food manufacture, 



3 Gloeotrichia 



Fig, 79. — Other Blue-green Algae. 1. Nostoc filaments show- 

 ing dividing cells, heterocysts, h, which serve to break the filaments 

 into sections and so multiply their number, and enlarged spore 

 cells, sp., stored with abundant food and surrounded by a firm wall 

 for protection during the resting period. 2. Gloeocapsa, a simpler 

 form which forms small irregular groups of cells instead of filaments, 

 the whole enclosed in a mass of gelatinous wall material. 3. Gloeo- 

 trochia, a somewhat more complicated type with very large spores, 

 sp., formed by the union of several adjacent cells. The heterocyst 

 forms next to the spore and so serves to detach the spore from the 

 filament. The several figures show various stages in the growth of a 

 filament. 



growth, and cell division. At intervals in the filament 

 are to be seen a second type of cells which are larger, 

 have thick walls and very scanty cell-contents. Some of 



