224 



PLANT SOCIOLOGY 



all by this treatment. The effect of aeration upon the germination of 

 seeds of Impatiens was studied by Hunter and Rich (1925), who 

 show better germination for well-aerated soils (Fig. 115). They 

 have also shown by experiment that the development of the root 

 system is directly dependent upon soil structure and that thus the 

 growth of the organs aboveground is indirectly affected. 



Well-aerated soils always show a high oxygen and a lov>^ carbonic 

 acid content. The slower the gas exchange proceeds the more the 

 carbonic acid accumulates. Consequently, the oxygen content drops, 

 and the floristic composition of the vegetation changes accordingly. 



Fig. 115. 



-Germination of seeds of Impatiens in well-aerated (A) as compared with 

 non-aerated (B) soils. {After Hunter and Rich.) 



Oxygen Content of the Soil. — The oxygen content of the soil and its 

 ecological effect have been examined by Hesselman (1910), Russell and 

 Appleyard (1915), Cannon (1924), and still more critically by Romell 

 (1922). Romell determined both the oxygen and the carbonic acid 

 content, the analyses being made with Krogh's microanalytic apparatus. 



Romell established beyond question the fact that soil aeration by 

 diffusion occurs to a notable extent in natural soils and more particu- 

 larly in forest soil. This is opposed to the prevailing view which 

 follows Ramann and Mitscherlich. The oxygen deficit and the surplus 

 of carbonic acid are at any given depth directly proportional to soil 

 activity, other things being equal. With equal activity (CO2 produc- 

 tion) and uniform distribution of activity in the soil, these values arc 

 inversely proportional to the air content of the particular soil. In 

 coarse soils the size of the grains is of secondary importance for aera- 

 tion. Aeration is considerably decreased, however, in finely dispersed 

 clay soil (down to about one-hundredth normal). Of course, soil 

 water presents the greatest obstacle to aeration. When the pores are 



