442 General Botany 



bodies like the chloroplasts of higher plants. The cells lack 

 true nuclei. One of the striking characteristics of the group 

 is the abundant formation of mucilage by the cells, leading in 

 many instances to the production of gelatinous colonies of cells 

 and to simple and branched filaments. About 1200 species 

 have been described. 



Occurrence. The blue-green algae occur in abundance in all 

 parts of the earth. They are the prominent algae of the tropics 

 and the polar regions, and they may impart their color to the 

 landscape. Most of these algae are in fresh water ; a few are 

 found in salt and brackish water along coasts. Many of the 

 forms are violet, red, gray, or brown in color. The Red Sea 

 owes its name and color to a red species of '' blue-green " algae. 

 The so-called " water bloom " is frequently a sudden develop- 

 ment and accumulation of certain blue-greens near the surfaces 

 of lakes and ponds. Pond waters at such times have a distinct 

 greenish color and have been known to poison cattle and horses. 



Blue-greens occur on all moist soils, and in some parts of the 

 tropics as epiphytes also. Some of these algae form papery layers 

 on the soil surface which are very hygroscopic and aid in re- 

 taining the soil water. The soil algae are commonly associated 

 with the nitrogen-fixing bacteria, and as they die they set free 

 carbon compounds useful to the bacteria. The bacteria, in 

 turn, at their death liberate nitrogen compounds useful to the 

 algae and the higher plants. 



Blue-greens often become troublesome weeds on soils in green- 

 houses — our artificial tropics. 



Resistance to unfavorable conditions. Some blue-greens 

 thrive in conditions where no other autophytes can live. This 

 resistance is partly due to the high water-retaining capacity 

 of their gelatinous walls, and perhaps partly to the nature of 

 their proteins and their simplicity of organization. 



Blue-green algae have been known to remain alive in dry soil 

 samples for a period of 50 to 70 years. They withstand being 

 frozen for several months. 



