CHAPTER III. 



— ♦ — 



OF ATMOSPHERICAL MOISTURE AND TEMPERATURE.* 



The constituents of the atmosphere that surrounds us are 

 either the same in different regions, or the differences, if any, 

 are not appreciable By chemical processes. It is far otherwise, 

 as regards temperature and humidity, which are so intimately 

 connected that they cannot be considered apart from each 

 other. 



From what has been already stated (Book I. Chap. V.), it is 

 apparent that of the vital functions of plants none are more 

 important than perspiration and evaporation ; and that, while a 

 certain amount of loss of fluid particles is necessary to them, 

 a great excess or diminution of the loss must be injurious. 

 Although the solar rays appear to be the immediate cause of 

 perspiration, which proceeds in proportion to their intensity, 

 yet this action is necessarily modified by the state of the 

 medium, that is, of the atmosphere, which surrounds them ; in 

 proportion to its heat and dryness will their power be aug- 

 mented, and in proportion to its cold and moisture diminished. 

 The physiological effect of an excessive augmentation of 

 perspiration is to dry up the juices and to destroy the texture 

 pf the leaves ; on the other hand, an excessive obstruction of 

 that function prevents the decomposition and assimilation of 

 fluids, and the formation of new organised matter, as well as of 



* TMs suljjeot Has already been folly treated by the late Professor Daniell, in his 

 excellent paper " On Climate with, regard to Horticulture," published in the Tramsac- 

 tions of the fforticvltwal Society, vol. vi. p. 1. It is impossible to discuss the same 

 topic •without profiting largely by this important treatise, which I have much foDowed 

 in the present chapter. 



