ABSORPTION OF CARBON 45 



for about 8 hr., then the oat plants will consume in the course of a 

 day 120 kg. of carbon dioxide per hectare. Of these, 40 are sup- 

 plied by the soil and 80 absorbed from the air. This constitutes 

 but 15 per cent of the total amount contained in a layer 100 m. 

 thick. Lundegardh observed that during the day the carbon- 

 dioxide content of the air is considerably lowered. At night the 

 "respiration" of the soil once more restores the balance, producing 

 during 16 hr. the 80 kg. of carbon dioxide lost from the air. 



This balancing of carbon dioxide without profit or loss takes 

 place only on soils of medium fertility. On poor ground, plants 

 absorb more carbon dioxide than is lost by the soil, hence with 

 the growth of the plants the soil becomes enriched in humus. On 

 the contrary, in soils very rich in humus, the loss of carbon dioxide 

 may exceed the accumulation of organic compounds by the plants. 

 An especially great abundance of carbon dioxide has been found in 

 forests under the cover of trees where in the lower layers of air 

 carbon dioxide may reach a concentration of 0.08 per cent, instead 

 of the average 0.03 per cent. This high percentage of carbon 

 dioxide compensates the shade plants to a certain degree for the 

 lack of light. 



14. Dependence of Assimilation on Temperature. The Inter- 

 action of Several Factors in the Process of Assimilation. The 

 Law of Limiting Factors. — Temperature is an external factor 

 exerting a profound influence not only on all vital processes, but 

 also on processes of a strictly chemical character. According to the 

 well-known rule of Van't Hoff, the rate of chemical reactions 

 approximately doubles with every increase of 10° C. in tempera- 

 ture. This acceleration of a reaction with the increase of tem- 

 perature by 10° is termed the "temperature coefficient" and is 

 designated by Qio- Van't Hoff's rule, therefore, may be expressed 

 by the formula Qio = 2. 



The assimilation of carbon dioxide, as well as a large number of 

 other vital processes, is subject to the rule of Van't Hoff only within 

 comparatively narrow limits of temperature between and 30 to 

 35° C. With further increase of temperature the acceleration of 

 the process is markedly delayed, after which a rapid falling off 

 takes place, the bend in the coefficient curve often showing an 

 acute angle (Fig. 19). With 40 to 50°, the process completely 

 ceases. Therefore, in the temperature coefficient curve of assimila- 

 tion (and other vital processes) three principal or, as Sachs called 



