34 BRITISH PLANTS 



be present, but if it is not of the right, sort it is not 

 absorbed. This kind of dryness is called physiological 

 dryness, and, in ecology, when we speak of dryness in 

 external conditions, we mean not only physical dryness, 

 but this physiological dryness as well. 



Causes which tend to Increase Transpiration. 



These are, with one exception, the same as promote 

 evaporation from the surface of any moist body exposed 

 to the air : 



1. A Dry Air, which promotes evaporation from all 

 the water-surfaces in contact with it. When the air is 

 very dry, evaporation is very rapid ; as the amount of 

 water-vapour in the air increases, the rate of evapora- 

 tion decreases, and when the air is saturated, it ceases 

 altogether. 



2. A High Temperature, which increases evaporation 

 by increasing the amount of water- vapour the air can hold. 



3. Wind. — The faster the air in contact with the 

 evaporating surfaces is renewed, the more quickly the 

 water is evaporated. Wet clothes dry more quickly in 

 a wind than in a calm. 



4. Rarefaction of the Atmosphere. — ^Evaporation of 

 water increases with the diminution of the air-pressure 

 on its surface. On a high plateau, water exposed in a 

 bowl will disappear more quickly, other things being 

 equal, than on a lowland plain. 



5. Light. — Intense illumination does not increase 

 evaporation if the temperatm'e is unaltered, but it 

 does increase transpiration. The phenomenon is clearly 

 a vital or physiological one, for light only ipromotes loss 

 of water from a living plant, not a dead one. 



The rate at which a body loses water by evaporation 

 depends also upon the Extent of Surface exposed to the 

 Air. Half a pint of water spread out over a table will' 

 soon dry up, but if enclosed in a jug, with only a small 

 evaporating surface exposed, it wiU take a long time to 

 disappear, even in dry weather. It is the same with a 

 leaf. A small, thick leaf may contain as much tissue 

 and as much water as a large, thin leaf, but the latter will 

 lose water more quickly than the former, because the 

 amount of exposed surface is greater. 



