THE rilALLOPHYTA 265 



Cyanophyceae or Blue-green Algae. — These are the simplest 

 and lowliest of all green plants. The body consists of a single 

 cell, but in most species the cells tend to hold together in colonies. 

 The cell itself is very simple, lacking the nucleus, sap-cavity, and 

 chloroplastids so characteristic of other green plants. Its cyto- 

 plasm may be perfectly homogeneous, with the pigment evenly 

 dispersed, or a colored outer zone and a colorless inner one may be 

 roughly distinguishable. The latter perhaps represents a nucleus. 

 The pigment, which seems to be dissolved directly in the cyto- 

 plasm and never confined to definite plastids, is usually (though 

 not always) blue-green in color and is probably a combination of 

 chlorophyll with a blue pigment, phycocyanin. Both of these 

 may be concerned with photosynthesis but this is as yet uncertain, 

 for our knowledge of the photosynthetic process in the blue- 

 green algae is far less complete than it is for higher plants. The 

 cell-wall is typically thick and mucilaginous, and in many species 

 a group of cells becomes embedded in the gelatinous mass derived 

 from their walls so that a large, jelly-like colony results. On the 

 occurrence of unfavorable conditions for growth, heavy-walled 

 "resting cells" may be produced. Cell division is very simple 

 and shows none of the elaborate phenomena of mitosis, the cell 

 merely becoming constricted by the growth of a new wall until 

 complete separation into two cells takes place. Little differentia- 

 tion is evident, although peculiar large, dead cells frequently 

 appear in certain species. Reproduction consists merely in cell 

 division or "fission," and no instances of sexuality have ever 

 been observed in the class. In this respect the blue-greens differ 

 from other algae but resemble bacteria, and these two groups 

 have therefore sometimes been placed together as a separate 

 division of the plant kingdom, the Schizophyta or Fission Plants, 

 divided into the classes Schizophyceae (Cyanophyceae) and 

 Schizomycetes (Bacteria). 



The blue-green algae live in both salt and fresh water and are 

 able to grow at higher temperatures than can any other plants, 

 often thriving in the water of hot springs at temperatures up to 

 60°C. Most species prefer water which is dirty and full of 

 organic matter, and some may even be found on damp soil, 

 rocks, and other places which are exposed to the air. 



One of the simplest examples is Gloeocapsa (Fig, 141, B), a 

 minute, single-celled alga with a very gelatinous wall. As an 

 individual divides, the resulting cells of the first generation, and 



