414 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



TERRESTRIAL ALGAE 



It is difficult to differentiate between aerial and terrestrial algae 

 on the basis of source of water, since many of the terrestrial algae 

 live below the surface of the soil. The number of soil-inhabiting 

 algae is very large; some of the forms are strictly terrestrial, but 

 most of the species are also known in aquatic habitats. In many 

 parts of the world the terrestrial algae form a conspicuous coating 

 on the soil only during prolonged rainfall, when the soil is con- 

 tinually saturated with water. In middle- western sections of the 

 United States, the extensive development of soil algae is restricted 

 to especially rainy years. In California, there is a regular de- 

 velopment of them during the rainy winter months. 



These algae usually grow on the soil in small patches, a few 

 inches in diameter, a number of species in each patch. Sometimes, 

 however, they cover large areas an acre or more in extent that con- 

 sist very largely of a single species. The texture and chemical com- 

 position of the soil determines the particular species present as may 

 be seen by comparing the algae growing in a well-beaten path with 

 those growing on bare loose soil next to the path. The influence of 

 the chemical environment may be exemplified by the restriction of 

 Zygogonium to acid soils and the development of PrasioJa on damp 

 soil rich in nitrogenous matter. 



Nature reserves a utilitarian purpose that is threefold for her 

 terrestrial algae. They aid in the erosion of exposed surfaces, their 

 decay affords the first available supply of humus, and they provide 

 a moisture-retaining substrate for the spores and seeds of higher 

 plants. Microscopic green algae such as Gloeocapsa^ Gloeothece^ and 

 Aphanoca'psa in visible green masses are generally the first algae 

 to appear in the colonization of rock surfaces. These algae are fol- 

 lowed by filamentous mats of Scytonema, Hapalosiphon^ and 

 Stigonema. 



After heavy rains, as in California, the green soil-dwelling algae 

 appear as if by magic on the earth. It would almost seem to a 

 casual observer that they came down from the sky with the rain- 

 drops. Like the aerial algae, the soil-dwelling algae are able to 

 withstand prolonged desiccation. Many of these algae survive the 

 dry season in a resting condition. Their cell walls become enlarged 

 during the dryness, and the cell contents are thus protected in the de- 

 creased center of the cell. The length of time that soil algae can 

 survive desiccation is almost beyond belief, for some of them are 

 able to resume growth after drying for 50 years. 



Certain algae are able to build up chlorophyll and phycocyanin 

 and to grow in the dark, provided sufficient food material is avail- 

 able. Green filamentous algae have been found growing on the 



