226 



PLANT SOCIOLOGY 



That is, the soil was well aerated. In a wet deciduous forest he 

 obtained rather different results (Table 26, II). 



Table 26. — Increase in the Carbonic Acid Content with Depth op Soil: 

 I According to Fodor; II According to Romell 



At 45 cm. depth the accumulation of carbonic acid in the deciduous 

 forest is so great that only a few willows and alders, Alnus incana and 

 A. glutinosa, can withstand it. Carbonic acid concentrations of 1 to 

 2 per cent have toxic effects for many plants; at 4 per cent the seeds of 

 Brassica alba (temperature 3°C.) fail to germinate (Kidd, 1914); 

 at 25 per cent of CO2 the root growth of Covillea tridentata, Krameria 

 canescens, Mesembryanthemum sp. is completely stopped in a few hours 

 (Cannon). The forester thus finds it necessary to determine the 

 specific resistance of the trees against carbonic acid poisoning and the 

 means to combat this danger. Nothing is known of the part which 

 the individual wild plants play in the accumulation of CO2. Parker's 

 (1924) experiments with cultivated plants lead us to expect different 

 effects from the different species. While buckwheat released very little 

 CO2, bean roots gave off considerable quantities. 



How widely the carbonic acid content varies in closely adjacent 

 areas is shown by the observations of Lundegardh (1925). In the 

 Car ex vesicaria bog with Peucedanum palustre he measured 1.24 per 

 cent; and only 2 m. away, at the same depth (15 cm.) in the alder 

 thicket with Oxalis, Maianthemum, etc., he obtained a value of 0.24 to 

 0.50 per cent of CO2. 



Valuable data for the estimate of soil aeration are afforded by 

 purely physical methods for the determination of pore volume and air 

 capacity. At the same time, also, water content and water capacity of 

 the soil may be determined (see p. 231). 



Pore Volume. — Pore volume means volume of the air-filled hollow 

 space of the dry soil. If a graduate cylinder of 1,000 cc. is filled with 



