CLIMATIC FACTORS 111 



cent. This high percentage of carbon dioxide compensates, to a certain 

 extent, for the low hght intensity of the forest floor. 



Gut (1929) found in general a diminution of the carbon dioxide con- 

 tent of the forest atmosphere during the early hours of the day, a 

 shght increase in the afternoon, which generally continues into the 

 evening, and a restoration to the normal by the "respiration" of the 

 soil during the night. In the deciduous forest there is sometimes a 

 decided but transient increase in the carbon dioxide content toward 

 evening, which might be explained by the accumulation of the products 

 of metabolism in the leaves to the extent that it hinders the rate of 

 further carbon assimilation. In the forest the lowest content of 

 atmospheric carbon dioxide, as well as its greatest variations, occurred 

 in the spring; in the autumn its highest content was reached. 



It is impossible at present to estimate the ecological importance of 

 carbon dioxide and its variations. It seems certain that the normal 

 atmospheric content of carbon dioxide represents a minimum rather 

 than an optimum amount for plants. The determinations of its 

 variations are still too fragmentary to have great ecological value. It 

 must suffice to call the attention of phytosociologists to the possible 

 importance of this factor. 



The determinations of Lundegardh in the Swedish oak forests, con- 

 firmed by Gut, show that the carbon dioxide content of the air is 

 subject to greater fluctuations in the beech than in the pine forests. It 

 therefore appears that in the deciduous forest the gas exchanges are 

 more active but there is a slower rate of growth. 



The sources of atmospheric carbon dioxide are combustion; the 

 respiration of man, animals, and plants; volcanic emissions; and the 

 gases arising from the sea and from the soil. 



Determination of Carbon Dioxide Content of the Air. — Of the 

 numerous methods for the determination of the carbon dioxide content 

 of the atmosphere the only one that appears to be practicable for 

 phytosociologists is the volumetric method of Gut. Even this is 

 complicated and diflficult. For details the reader is referred to the 

 description by Gut (1929, pp. 18-29). 



4. WATER 



Water is the plenary agency that sets in motion the nutrients of the 

 soil and makes them available to plants. More than any other factor 

 of the habitat, water affects the inner and outer morphology of those 

 plant organs that together determine the physiognomy of vegetation. 

 In the large, water causes the divisions within the vegetational zones 

 of the earth due to temperature. On a smaller scale, it regulates the 



