328 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



ABSORPTION AND EMISSION OF WATER FROM THE LEAVES OF 



PLANTS. 



M. Boussingault showed some years ago that plants absorb from the 

 earth and exhale to the air an enormous quantity of water. He calcu- 

 lated that a field of cauliflower, 1 hectare in extent, can emit in twelve 

 hours 20,000 kilograms. M. Deherain states that a young blade of 

 wheat evaporates in one hour a weight of water equal to its own. 

 Eucalyptus globulus is supposed to be capable of evaporating eleven 

 times the rainfall of the area which it covers, provided, no doubt, that 

 the rainfall is not excessively large. Oaks are also great evaporators 

 and grow best in wet clay. M. Fautrat, inspector of forests, has 

 found that the quantity of vapor in the air over forests is much greater 

 than in the air over the open country. But exact comparative obser- 

 vations of the amount of water evaporated within and without forest 

 areas in various climates are wanting. Forests have been planted in 

 certain parts of southern France with excellent results in the improve- 

 ment of health, and malaria has diminished in several instances in 

 consequence of judicious planting. The question of planting in con- 

 nection with human health is a very important one, and the influence 

 of forests and trees on the steadiness of the water supply makes it very 

 necessary that forests should be carefully guarded by the State in 

 many countries. Vegetation, large or small, should never be hastily 

 destroyed. Trees and hedges are very useful in breaking the force of 

 strong winds, in giving shelter to animals, and promoting the growth 

 of fruit trees and vegetables, and they add greatly to the amenity of 

 the country. 



The exact conditions of climate most suitable to each kind of useful 

 crop, tree, or plant, have yet to be determined, though they are in 

 many cases fairly well known. The development and selection of 

 hardy specimens would be aided by trial of the effect of transplanting 

 or obtaining seed from various climates of each species examined. The 

 gradual acclimatization of plants might, under scientific inquiry, be 

 found to be capable of furnishing better results than have hitherto 

 been obtained. 



The amount of water collected by trees from the air in misty and 

 damp weather has not been determined, although in some districts-, 

 especially where warm, moist winds from the sea prevail, with frequent 

 mist, it must be considerable. 



The exact manner in which the spores of dry rot, potato disease, 

 vine diseases, rust, and other plant fungi are conveyed through the air, 

 and how far they may be carried in a potent state through dry and 

 moist air, requires investigation; also the influence of ozone, of sun- 

 light, and of drought upon them when deposited on their host. 



The relation of the air supply, air temperature, and moisture to the 

 microbe life in the soil, in connection with the growth of crops, with 

 biological chemistry, with soil emanations, and with diseases. 



