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, which are 
defined by Marie-Davy (1882) as “evaporation” and “ transpi- 
ration.” 
Evaporation.—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 and 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 minimum, 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 pressure of the sap that the sunhght manu- 
factures the first buds and leaves. The temperature of the air flowing 
among the branches and buds may have any value without 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, 
