19 



Evidently this whole process of respiration depends largely upon 

 the temperature of the air and is more active as the temperature 

 increases. It goes on both in darkness and in light, but with this 

 difference — that in darkness more carbonic-acid gas is given out than 

 the oxygen that is absorbed, whereas, on the other hand, under the 

 influence of light more oxygen is given out than the carbonic-acid 

 gas that is absorbed. Both these processes are stimulated by heat. 

 The assimilation or nutrition of the plant depends upon this me- 

 chanical influence of light in disengaging oxygen and " fixing " the 

 carbon of the gas in the cells of the plant. Plant respiration is 

 accompanied by two distinct but correlated phenomena, Avhich are 

 defined by Marie-Davy (1882) as "evaporation" and "transpi- 

 ration." 



Eva-pofation. — This is a purely physical phenomenon. All bodies 

 lose water from their external surfaces when in contact with dry air, 

 and do so faster in proportion as the wind is stronger and the air 

 is drier. Evaporation takes place for dead and living surfaces alike. 



Transpiration. — This is a physiological and not a purely physical 

 phenomenon. It occurs only in living plants and under the influence 

 of light ; it is independent of the dryness of the air a'hd is only indi- 

 rectly dependent on temperature. It is intimately connected with 

 assimilation, since by its means materials are furnished to complete 

 the work of the growth of the plant. 



DRYNESS, TEMPERATURE, AND VELOCITY OF THE WIND. 



The evaporation from the leaves, the flow of sap, and the develop- 

 ment of the plant depend almost as much on the wind and the dry- 

 ness of the air as they do on the temperature of the air considered by 

 itself, since all these are necessary in order to bring the supply of 

 nutritious water up to the leaf. Therefore, the temperature of the 

 air must not be considered as the only important climatic element con- 

 trolling vegetation. At the time of the bursting of the buds in the 

 spring, when no leaves are on the trees and when the respiration of 

 the plant and the evaporation are at their miniminn, the temperature 

 and dryness of the air have their least influence, while the tempera- 

 ture and moisture of the soil may have their maximum relative im- 

 portance. These latter are the elements that determine how much 

 water shall be absorbed and pushed upward as sap. It is under the 

 influence of this upward ])ressure of the sap that the sunlight manu- 

 factures the first buds and leaves. The temperature of the air flowing 

 among the branches and buds may have any value Avithout seriously 

 affecting the development of the plant, provided it is above freezing 

 and below a destructive temperature, such as 120° F., and above a 

 destructive dryness, such as 5 or 10 per cent of relative humidity. 

 Ordinarily a warm spring day implies a warm, moist soil and a warm, 



