Deserts 113 



evaporation, and water in a dead root, would speedily 

 grow warm under similar circumstances. Water in a 

 porous vessel keeps cool, indeed, in the hottest sun, 

 because the vessel is porous, and water is constantly 

 passing through it and being turned into vapour ; with 

 the result that the air immediately surrounding the 

 vessel is being constantly cooled. The water is turned 

 into vapour by means of the heat abstracted from the 

 air. 



But the water in the roots of the eucalyptus is not 

 kept cool by evaporation ; else, in time of drought, it 

 would be evaporated altogether. Besides, the juice of 

 the hard, leathery-skinned pomegranate is cool on the 

 hottest day ; so, too, is that of the melon, with its 

 thick rind; and the abundant juice of the thick-skinned 

 mango feels as cold as iced water, even under the 

 blazing sun of Ceylon; though the evaporation from 

 any one of these must be very slight. indeed. 



Moreover, the coolness lasts only while the fruit 

 remains on the plant, and disappears in a few minutes 

 after it is gathered. It must, therefore, be quite 

 independent of evaporation, and the temperature of a 

 living plant's juices must be like the temperature of 

 the blood in men and animals, quite independent of 

 climate. 



The ordinary temperature of the blood of human 

 beings (98° F.) remains the same whether they live 

 under the equator or in the Arctic regions. 



And so it is with plants. They are cold-blooded, so 

 to say, and cold-blooded they remain, even when sur- 

 rounded by hot air, as long as they are alive. When 

 they are dead their temperature soon rises or falls, 

 according as the surrounding air is hot or cold. But 



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