FOREST IKFLUEKCES. 33 



evaporation, and that plants are liable to suffer as much 

 by winter drought as by summer drought. 



This is certain : that since summer and winter drought — 

 that is, rapid evaporation due to continuous dry winds — ^is 

 the bane of the farmer on the plains, rationally disposed 

 timber belts will do much to increase available water supply 

 by reducing evaporation. 



Evaporation, of course, goes on much less rapidly within 

 than without the forest. How great this difference is we 

 have no exact figures to tell, but it is certain that it is 

 much more than in Bavaria, where the following result 

 was obtained: In an experiment, which was carried on to 

 determine the amount evaporated from April to October, 

 it was found that from a certain area without the forest 

 40.8 centimeters were evaporated, within pine wood 15.9 

 centimeters, and within deciduous woods 6.2 centimeters. 

 This shows that the evaporation was six and one-half times 

 as great in the open field as in deciduous woods. 



Transpiration. Another factor by which forests dissipate 

 water supplies and which has been referred to (page 8) is 

 transpiration. The quantity of wat^r so used is as variable 

 as the amount of precipitation, and in fact, within certain 

 limits, depends largely upon it; that is to say, a plant 

 will transpire in proportion to the amount of water which 

 is at its disposal. Transpiration is also dependent on the 

 stage of development of the plant, on the nature of its leaves 

 and amount of foliage, on temperature, humidity, and cir- 

 culation of the air, on intensity of the sunlight, and on 

 temperature and structure of the soil, and on other meteor- 

 ological conditions. Rain and dew reduce transpiration, 

 wind increases it. 



The amount of transpiration depends considerably upon 

 the tliickness of the leaves; therefore the surface of the 

 foliage is not a reliable measure, but should be compared 

 with the weight. 



