76 The Great World's Farm 



Then there is the sweet woodruff, whose lance-shaped 

 leaves grow in whorls of eight, for the plant dwells in 

 moist, shady places, where there is no risk in having many 

 leaves. But look from this to another member of the 

 family, the quinsywort, and what do we find? The 

 leaves are very narrow, and there are but half as many of 

 them. Why? Because this little plant grows on dry 

 banks, where many and large leaves would be dangerous 

 to its welfare. 



It has been already mentioned that the pine family, 

 which thrive in dry, sandy soils, have hard, needle-like 

 leaves, with few pores, and therefore give off but little 

 water, either by evaporation or transpiration; and it is for 

 this reason that the air in a pine forest in summer has 

 none of the coolness which one finds in a forest of what 

 the Germans call * 'leaf-trees." The needles of the pine 

 they do not consider worthy the name of leaves. 



Leaf-trees are continually cooling the air by the moist- 

 ure which they give up to it; but the pine-needles have 

 so few pores, and are so very much protected, that the 

 little water they part with is not enough to produce any 

 appreciable effect upon the air. 



It is, perhaps, hardly necessary to do more than 

 remind our readers that the evaporation of water is always 

 accompanied by the absorption of heat; or, in other 

 words, that water cannot be converted into gas or vapor, 

 which it is when evaporated, without using up heat. 

 Whether it be the heat of a fire or the heat of the sun, it 

 is all the same. A certain amount of heat is required to 

 make water pass from the liquid to the gaseous state, and 

 if this heat be taken from the air, the air is necessarily by 

 so much the cooler. 



