94 Water 



its being. Evaporation may continue in a dead plant, 

 but only a living plant transpires. 



Both processes are affected by the weather, however, 

 and both in a similar way. 



Nothing, we know, dries on a very damp day, 

 because the more moisture the air contains, the less it 

 can take up ; or, in other words, evaporation proceeds 

 slowly in moist air. So, too, transpiration almost or 

 quite ceases in damp weather, or when the leaves are 

 wet. But both go on more briskly in the sun, in dry 

 air, and more especially in a drying wind. 



The leaf-pores by which transpiration proceeds are 

 usually more abundant on the under surface — the 

 shady side — of the leaves, and are few or altogether 

 wanting on the upper surface, where they would be 

 exposed to the sun, and water might pass off too 

 rapidly. In moist, shady situations there is no danger 

 of too much transpiration, and plants growing in these 

 not only have more leaf-pores than others, but can 

 also have them without risk, both on the under and 

 upper surface of the leaves, for here transpiration goes 

 on more slowly, and the loss of water is also easily 

 made up. 



Thick, fleshy leaves have the fewest leaf-pores, and 

 thick, fleshy leaves are particularly characteristic of 

 hot countries, where plants can afford to lose but 

 little of the scanty supply of water which comes to 

 them. 



Many leaves which are alike on both sides have 

 about an equal number of pores above and below ; but 

 when there is any difference, as, for instance, where 

 one side is dull and the other glossy, the dull side, 

 which is also the under side, has the larger number of 



