216 PRINCIPLES OF SOIL MICROBIOLOGY 



cient to separate the various forms and to cultivate them under artificial 

 conditions, even if they are contaminated with fungi or bacteria; but 

 for physiological studies, especially in the assimilation and transforma- 

 tion of various elements, as in organic matter decomposition, nitrogen 

 fixation and symbiotic action, it is important to obtain them free from 

 any contaminating organisms. 



Methods of isolation of impure cultures of algae. The methods of 

 isolation of algae from the soil fall into enrichment culture and pure 

 culture methods. The enrichment culture methods consist in making 

 conditions favorable for an abundant development of algae, for iden- 

 tification purposes; the pure culture methods deal with the processes 

 of obtaining the organisms free not only from fungi and bacteria, but 

 also from other algae, for physiological studies as well as for a more care- 

 ful study of their morphological characteristics. 1 The enrichment cul- 

 ture is also the preliminary step in the isolation of the pure culture. Since 

 these organisms require light for the autotrophic assimilation of carbon 

 dioxide (photosynthetically), this need of light is utilized for enrichment 

 purposes. A small quantity of soil added to a large flask containing 

 0.02 per cent K 2 HP0 4 in tap water and exposed to light will soon allow 



1 Wettstein, F. von. Zur Bedeutung und Technik der Reinkultur fur die 

 Systematik und Floristik der Algen. Osterreich Bot. Ztg. 70: 23-28. 1921. 



PLATE XII 

 Soil Algae 



75. Pleurococcus (from Robbins). 



76. Mostoc commune (from Robbins). 



77. Microcoleus vaginatus (from Robbins). 



78. Phormidium species (from Robbins). 



79. Anabaena sp. (from Robbins). 



80. Nodularia sp. (from Robbins). 



81. Chlamydomonas communis: 1-3, motile vegetative cells; 4, resting cell in 

 which division is about to take place; 5-6, longitudinal fission into four zoo- 

 gonidia; 7, oblique fission, X 960 (from Bristol). 



82. Ulothrix tenuissima X 550 (from Bristol). 



83. Bumillaria exilis: a and b, vegetative filaments showing variable number 

 of chloroplasts, X 550; c and d, filaments showing stages in formation of zoo- 

 gonidia, X 960 (from Bristol). 



84. Cylindrospermum muscicola: a, typical filament in different stages of 

 spore-formation; b, spnre formed in an irregular position, X 550 (from Bristol). 



85. Some typical soil diatoms: 1-2, Navicula borealis; 3-5, N. balfouriana; 6-9, 

 N. intermedia; 10-12, N. brebissonii, var. diminuta; 13, N. elliptica, var. oblongella; 

 14, N. elliptica, var. minima; 15-17, N. terricola, X 960-1150 (from Bristol). 



