412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



her lowest form of plant life in them to help maintain a pure and 

 healthy atmosphere. 



Moreland (1937) writes of the puzzlement of the inhabitants in 

 certain sections of Louisiana when they observed unusual deposits 

 of thick, cobwebby, or paperlike material hanging from weeds and 

 other vegetation after the spring floodwaters of the Atchafalaya and 

 Ouchita Rivers had receded (pi. 1). In one section, near Jonesville, 

 La., the paperlike deposit not only clung to vegetation but covered 

 the ground and fences which looked as if they were covered with 

 snow. When an examination was made of the material which ap- 

 peared microscopically to be composed of unbranched filaments 

 interwoven similarly to the fiber of lens paper, it was found that the 

 substance was formed by the luxuriant growth of certain species of 

 green algae including filaments of Tribonema^ Oedogoniwm, and 

 Spirogyra. The overflow water which was almost free of sedi- 

 mentation had come early in the season when the temperature was 

 especially favorable for the growth of these algae. Undoubtedly, 

 the air over these flooded lands was purified by their presence. 



It has also been shown by scientists that certain algae play a role 

 in the nitrogen cycle in the soil. They do this directly by fixing 

 gaseous compounds and indirectly by supplying nitrogen-fixing bac- 

 teria, especially Azotob(wter, with available carbon compounds, pro- 

 duced by algal photosynthesis, and which are used by the bacteria 

 as sources of energy necessary for the fixation process. Both green 

 algae and blue-green algae can stimulate the activity of nitrogen- 

 fixing bacteria. Very recently it has been definitely established that 

 certain species of blue-green algae can fix atmospheric nitrogen in 

 the light though not in darkness. However, their practical impor- 

 tance in the nitrogen economy of the soil remains to be determined 

 since there is little known concerning the distribution and abun- 

 dance of these algae in the soil and the conditions which determine 

 natural fixation. 



Algae have for some time been recognized as nature's pioneers in 

 plant succession, and for that reason are now assuming importance 

 in man's efforts to control erosion. 



Treub describes the manner in which slimy layers of algae ap- 

 peared over the surface of cinders and rocks on the Island of Kra- 

 katau 3 years after a volcanic eruption which had denuded the island 

 of all visible plant life. By the growth, death, and decay of these 

 algae, the island surface was rapidly prepared for the growth of 

 mosses, ferns, and higher plants. Fritsch believes that an algal 

 covering on the surface of dry, sandy soil regulates the moisture of 

 the soil and thus provides a shelter for seed plants. 



