254 SOIL CONDITIONS AND PLANT GROWTH 



roots and aid in the nutrition of the plant. These were first 

 investigated by Frank and have received considerable attention 

 from mycologists. 1 



Organisms Capable of Bringing About Changes of Im- 

 portance to the Plant. 



i. Alga. 



All soils hitherto examined contain algae including diatoms, 

 green and blue-green algae, and the flora seems to be par- 

 ticularly well-developeo! in cultivated soils. F. Esmarch 2 

 studied the soil flora of the German African colonies and of 

 Schleswig-Holstein, W. W. Robbins 3 that of the Colorado, 

 and J. B. Petersen 4 the diatoms of the Danish soils ; Dr. B. 

 Muriel Bristol of Rothamsted 5 the flora of certain English 

 soils. The algae occur to a depth of 12 inches and in some 

 cases even 2 feet : the species in the lower layers are almost 

 identical with those at the surface. 



It is now known that certain algae can develop in darkness 

 provided suitable nutrients are present, and assuming these 

 conditions there is nothing against the possibility of a trophic 

 alga-flora in the soil. Esmarch attempted to obtain positive 

 evidence by burying algae in the soil and making microscopic 

 examinations after given intervals of time ; in these circum- 

 stances the blue-green algae retained their colour for some 

 weeks, but ultimately became yellowish, and the filaments 

 disintegrated. 



1 B. Frank's paper is in Botan. Ztg., 1891, 9, 244-253, where references to his 

 earlier papers are given. Recent investigations have been made by M. C. Rayner, 

 Annals of Botany, 1915, 29, 97-133. 



2 F. Esmarch, Beitrag zur Cyanophyceen-Flora unserer Kolonien (Jahrbuch 

 der Hamburglschcn wisscnsch. Anstalten, xxviii., 3 Beiheft, 62-82, 1910). Unter- 

 suchungen fiber die Verbreitung der Cyanophyceen aufund in verschiedenen Boden 

 (Hedwigia, Band lv., Heft 4-5, September, 1914). 



3 W. W. Robbins, Alga in some Colorado Soils (Bulletin 184, Agri. Exp. 

 Sto., Colorado, June, 1912). 



4 J. B. Petersen, Danske aerofile alger (D. Kgl. Danske Vidensk. Selsk. 

 Skrifter, 7, Raekke, Naturv. og Mathem., Afd. xii, 7, 1915). 



5 B. M. Bristol, On the Alga-Flora of some Desiccated English Soils (Annals 

 of Botany, 1920, 34, 35-80). 



