76 FOREST INFLUENCES. 



VEGETATIOIS^ AIS^D AIR TEMPERATURES. 



It appears from the preceding section that the tree shows, of itself 

 by its own action, no sign of warming the air and but little sign of cool- 

 ing it. Systematic observations show this to be true of the framework 

 of tlie tree, but no observations have been taken for the foliage and 

 otlier exterior parts — the parts where most of the vital activity of the 

 tree takes place. The great volume of chemical and physical changes 

 which are produced here and which result in the growth of the tree and 

 the formation of its fruits make it an open question (not yet decided by 

 direct observations, which are not easily instituted) whether the tree 

 does not affect temperatures sensibly. So far as any observations go— 

 those in the tree-crown for instance— they show little clear evidence 

 of such a meteorological effect, but it w ill be of interest to see what can 

 be concluded theoretically. The changes which take place at the leaf- 

 surface, due to vital activity, are produced by the trees in common with 

 the other and lower forms of vegetation and consequently the problem 

 to be solved is double, namely: Do the vital processes of vegetation, 

 in which heat is ntilized or given off, produce a sensible effect on air 

 temperatures, and is the effect, if any, different for trees and for mead- 

 ows, mixed growths, or crops? 



Vegetation has a complicated relation to heat in the various organic 

 processes. In the transpiration of water it utilizes large quantities of 

 heat, changing it from that sensible in temperature to the work of sus- 

 taining the w^ater in a condition of vapor. This latent heat becomes 

 again sensible when the water is condensed, but this may occur at a 

 distance from the place wiiere the w^ater is taken up. By unlocking 

 the combination of carbon and oxygen in carbonic acid, using the car- 

 bon and rejectin'g the oxygen, it reverses the action of combustion and 

 so takes up heat. On the other hand, in the various processes in which 

 oxygen is combined, it performs a process analogous to combustion and 

 gives out heat. This process is sometimes so active (as in the germina- 

 tion of some seeds and the flowering of some plants) that the tempera- 

 ture of the parts is raised several degrees above that of the outside air, 

 and it is sensibly warm to the hand. There are other more complicated 

 processes going on in the plant, the relations of which to heat can not 

 be foretold. 



The quantity of heat used in these processes varies greatly with the 

 season, the temperature, the condition of the plant, and so on. It can 

 be ascertained only approximately even in the processes best under- 

 stood. Yet the problem has so many features of interest that even 

 such an approximate solution is desirable. For instance, the German 

 forester Ney has attemi)ted to show^ that the unseasonable frosts of 

 mid-May in central Europe are due to the amounts of heat absorbed i 

 by j)lants at that season. I 



The transpiration of water by the plant is a vital process which ; 

 needs the stimulus of the sun's rays; it takes place in the green parts i 



