Deserts 89 



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 inde- 

 pendent 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 surrounded 

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

 their temperature soon rises or falls, according as the sur- 

 rounding air is hot or cold. But if while alive, the tem- 

 perature of their sap were affected by climate, or by the 

 changes of summer and winter, day and night, then not 

 only would it be constantly frozen in the Far North, and 

 not far short of boiling in the tropics, but the sap of an 

 acacia of the desert might freeze by night and almost boil 

 by day — a sudden and violent change, which, as has been 

 shown, wears out the very rocks. 



But to return to the ''deserts," by which we are to 

 understand those regions where water is scarce, drought 

 frequent, and where vegetation, though seldom or never 



