CHAPTEE VIII 



THE ATMOSPHERE 



Its the Introduction we pointed out that the air was a 

 mixture of gases, and we referred briefly to the part played 

 by these gases in the life of plants. This was dealing 

 with the atmosphere from the chemical side. It remains 

 now to say something of the physical effects of air upon 

 plants — that is to say, of air in motion — wind. 



Wind.— The effect of wind upon vegetation is twofold : 

 (1) It promotes evaporation, and therefore increases 

 transpiration ; and (2) it lowers the temperature of the 

 bodies over which it blows. The second effect is really 

 a consequence of the first ; for a body which is losing 

 water by evaporation is at the same time losing heat and 

 becoming cooler. Heat is required to change any liquid 

 into vapour ; where the heat is not imparted from without, 

 the body from which the water is evaporating provides it, 

 and its temperature falls. Common experience testifies 

 in a very simple way to the truth of this. A person, 

 exerting himseK greatly, becomes hot and perspires. If 

 he takes off his hat, a pleasant coolness accompanies the 

 disappearance of the perspiration from his head. Again, 

 if ether is poured upon the hand, it evaporates away in 

 a few seconds, but heat for the purpose is drawn from 

 the hand, which therefore experiences a sensation of 

 cold. 



Wind promotes evaporation by constantly and rapidly 

 renewing the air in contact with the evaporating surfaces. 

 Wet clothes dry quicker when a breeze is blowing than in 

 a calm. Plants suffer in the same way, and when the 

 wind is strong, they are in danger of losing more water 

 than they can afford. This danger is increased by the 

 drying and the cooHng of the soil, and the decrease in 



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