NOTES. 437 



(5) Air Temperature in General. — From the preceding 

 generalizations it appears that the forest aflfects the tempera- 

 ture just as any collection of inorganic obstacles to sunshine 

 and wind ; but as an organic being the forest may be also an 

 independent source of heat. Careful observations of the 

 temperature within the trunk of the tree and of the leaves of 

 the tree show that the tree temperature is affected somewhat 

 by the fact that the water rising brings up the temperature of 

 the roots, while the food material from the leaves brings their 

 temperature down, and the tree temperature, considered as 

 the result of the complex adjustment, is not appreciably 

 affected by any heat that may be evolved by the chemical 

 processes on which its growth depends. It is not yet clear 

 as to whether the chemical changes that take place at the sur- 

 face of the leaves should give out any heat ; it is more likely 

 that heat is absorbed ; namely, rendered latent, especially in 

 the formation of the seed ; the process of germination usually 

 evolves this latent heat ; the immense quantity of water tran- 

 spired and evaporated by the forests tends to keep the leaves 

 at the same temperature as that of the surface of water or 

 moist soil. 



(6) Humidity of Air. — The annual evaporation within the 

 forests is about one-half of that in the open field ; not only is 

 the evaporation within a forest greatest in May and June, but 

 the difference between this and the evaporation in the open 

 field is also then a maximum, which is the saving due to the 

 presence of the woods. The average annual evaporation 

 within the woods is about 44 per cent of that in the field. 

 Fully half of the field evaporation is saved by the presence of 

 the forest. 



The quantity of moisture thrown into the air by transpira- 

 tion from the leaves in the forest is sometimes three times 

 that from a horizontal water surface of the same extent, and 

 at other times it is less than that of the water. The tran- 

 spiration from leaves in full sunshine is decidedly greater 

 than from leaves in the diffused daylight or darkness. The 



