ENVIRONMENTAL FACTORS 29I 



temperature of the wet and dry thermometer bulbs giving a measure 

 of the deficiency of water-vapour below saturation point in the air 

 tested, while the saturation-deficit can be calculated directly from 

 these difiFerent readings. Although this instrument is customarily 

 swung through the air to simulate wind and replace any saturated 

 layers that may form around the wet bulb, from our point of view 

 it is best kept stationary as plants are. Even the hygrometer gives 

 readings only at a particular time ; often it is more important to 

 know the effect over a given period, and this may be measured by 

 using an ' atmometer ' (evaporimeter), which indicates the water- 

 loss by evaporation from a given area of porous pot. This integrates 

 the water-vapour content of the air with temperature, wand, and the 

 time-factor, and may be compared with a plant which it is placed 

 beside. Or, for continuous recording, a tracing may be made by 

 an automatic hygrograph, such as those using strands of human 

 hair which is highly sensitive to changes in atmospheric humidity. 

 Batteries of such instruments will often show marked differences in 

 different strata of a forest, for example — indicating local variation 

 which may be of great importance in varying the ' pull ' exerted on 

 different plants, on the same plant at different times, or even on 

 different parts of the same plant at a particular time. 



(5) Wind. Owing to the friction of the soil surface, rocks, 

 buildings, and above all major physiographic features and masses 

 of vegetation, winds tend to increase in velocity with height above 

 the ground. Wind commonly affects other ecological factors in a 

 given spot — for example, water content and temperature, through 

 its effect on evaporation — but can also have a direct influence on 

 vegetation, especially by uprooting trees or by breaking off branches 

 or other portions. It has a similar, usually drying, effect upon the 

 soil, or may occasionally act in the opposite direction by bringing 

 up moister air which reduces transpiration and evaporation, and 

 may actually lead to deposition or precipitation. Most widely 

 important to plants, however, is the manner in which wind increases 

 water-loss, by constantly bringing unsaturated air into contact with 

 leaves and young shoots. Mechanically, wind can also cause 

 erosion of soil and abrasion of vegetation through carriage of 

 particles, and physiologically it can decrease growth by replacing 

 damp by dry air, and consequently increasing the transpiration and 

 reducing the turgor of organs on which it impinges. This explains 

 the frequent growth of trees and shrubs chiefly away from the 

 direction of the prevailing wind in exposed situations {cf. Fig. 82, A), 



