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. 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 will take a long time to 
disappear, even in dry weather. It is the same thing 
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. 
6. Light. — Intense illumination does not increase 
evaporation, if the temperature is unaltered, but it 
does increase transpiration. ‘The phenomenon is clearly 
a vital or physiological one, for light only promotes loss 
of water from a living plant, not a dead one. 
