ELEMENTS OF DISSIPATION. 31 



"It may not be necessary to state that the damage done to 

 crops by the cold, dry winter winds is mainly due to rapid 

 evaporation and that plants are liable to suffer as much by 

 winter drouth as by summer drought. 



"This is certain that since summer and winter drouth 

 drouth that is, rapid evaporation due to continuous dry 

 winds, is the bane of the farmer on the plains, rationally dis- 

 posed 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 in this 

 section 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 centi- 

 meters 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 14) is 

 transpiration. The quantity of water 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 circulation 

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

 and stiucture of the soil and on other meteorological condi- 

 tion. Rain and dew reduce transpiration, wind increases it. 

 The amount of transpiration depends considerably upon 

 the thickness of the leaves, therefore, the surface of the foli- 

 age is not a reliable measure but should be compared with the 

 weight. 



In some European experiments carried on during the 

 period of vegetation the amount of water transpired by the 

 different species per pound of dry matter in the leaves was as 

 follows: 



