jiiSSIMILATION OF CARBON BY PLANTS 203 



Lundegardh, a sandy soil, poor in humus, liberates about 2 kg. 

 of CO2 per hour per hectare; loam and clay soils, containing a 

 greater amount of humus, eliminate about 4 kg. ; and forest soils, 

 extremely rich in humus, produce from 10 to 25 kg. Moderately 

 fertilized soils produce on an average 5 kg. of carbon dioxide per 

 hectare per hour. 



When the consumption of carbon dioxide by the plant is 

 considered with the above figures in mind, the following values 

 are obtained. A field of 1 hectare sown to oats consumes in the 

 process of photosynthesis about 15 kg. of carbon dioxide per hour. 

 Five of these are supplied by the soil; the other 10 are obtained 

 from the atmosphere. In spite of the low percentage of carbon 

 dioxide contained in the atmosphere, an air layer 100 m. thick 

 and 1 hectare in area contains about 550 kg. of carbon dioxide. 

 If it is assumed that in summer the most intense assimilation 

 continues 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 supplied 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 takes place only in soils of 

 medium fertility. In 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. Conversely, in soils 

 very rich in humus, the loss of carbon dioxide may exceed the 

 accumulation of organic compounds by the plants. A specially 

 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. 



47. Dependence of Photosynthesis on Temperature. The 

 Interaction of Several Factors and the Law of Limiting Factors. 

 Diurnal Changes of Photosynthesis. — As has already been seen 

 in discussing respiration and growth of plants, temperature 



