88 TRUCK-FARMING AT THE SOUTH. 



prevents its departure at night. Hence the clearest, 

 driest nights are the coldest. Hence the driest regions, 

 like the desert of Sahara, have the coldest nights, and the 

 cold of high elevations is due to the same cause. 



Prof. Tyndall says: " The removal, fora single sum- 

 mer night, of the aqueous vapor from the atmosphere 

 that covers England, would be attended by the destruc- 

 tion of every plant which a freezing temperature would 

 kill." Humidity and temperature are therefore inti- 

 mately connected. 



Although the heat of the sun causes evaporation from 

 plants, its amount is governed by the humidity of the 

 air and the velocity of the wind. If the gardener could 

 regulate the moisture of the atmosphere surrounding his 

 crops, and make it most favorable for keeping up the 

 proper evaporation, by applying water artificially and 

 only in circumscribed limits, to their roots, he could be 

 assured of success. While he may do so in his green- 

 house, there are no means of regulating the heat and 

 moisture of the open air. It is therefore that watering 

 out-door crops, in our hot climate, is more often pro- 

 ductive of harm than of benefi t. 



When the earth is naturally moistened by rain, the 

 whole air is saturated with moisture, preventing a too 

 rapid perspiration from the leaves and the evaporation 

 from the soil. If watering is done at all, it should be in 

 cloudy weather; but it is most frequently injudiciously 

 practised in dry, hot weather, and so circumscribed in ex- 

 tent, that it can have little or no effect upon the atmos- 

 phere. The roots are temporarily excited, and the dry, 

 hot air robs the plant of the moisture through the leaves 

 as rapidly as it can toe pumped up by the roots. As soon 

 as the temporary supply is exhausted, the plant not only 

 returns to its former state of suffering, but is left more 

 susceptible to injury than before. If the watering is 

 repeated, the emission of rootlets near the surface is en- 



