RESPIRATION AND OXYGEN. 



51 



Bergmann (1920 : 22) has found that Philotria decreases the car- 

 bon dioxid and increases the oxygen during the day, as shown by 

 tables 16 and 17. 



TABLE 16. Gas-content per liter of water with and without Philotria. 



TABLE 17. Gas-content per liter of water with Philotria. 



Esmarch (1910, 1914) first called attention to the algal flora of 

 soils, and pointed out that algae occur in the lower layers as well as 

 in the surface-soil. He concluded that cultivated soils are richer 

 in blue-green algae than uncultivated ones. A later study dealt with 

 the depth and distribution of blue-green algae in the soils of Schleswig- 

 Holstein. He found that blue-green algae in cultivated soils are not 

 confined to the surface layers, but that many occur to depths of 10 

 to 25 cm. and some as deep as 40 to 50 cm., ranging throughout the 

 entire depth. This is due to cultivation, percolation of water, and 

 the burrowing of worms and other small animals. It was also ob- 

 served that blue-green algae inclosed in the soil keep green for a time, 

 but gradually become yellowish, and after a further interval the 

 filaments become distorted and disintegrate. This period is 3 to 6 

 weeks long, or, in one case, as much as 10 weeks long. The author 

 thought that in certain soils the presence of blue-green algae is an 

 important factor in the fixation of nitrogen by bacteria. 



Robbins (1912) found 21 species of algae in Colorado irrigated soils, 

 all belonging to the blue-green slimes, except Pleurococcus viridis and 

 Navicula sp. The algae occurred in all kinds of soil, from sandy 

 loam to heavy adobe, and were probably derived for the most part 

 from the irrigation-water. The most important forms were Phor- 

 midium tenue, Nostoc sp., Anabcena sp., Nodularia harveyana, and 

 Stigonema sp. The organic matter furnished by the algae was 



