INTRODUCTION 23 



the process by which the root-hairs absorb water, or 

 osmosis, is retarded by cold, as heat in general pro- 

 motes activity in plant-life, increasing the energy of 

 the plant, whilst cold reduces it. If also the soil is sour 

 or acid and contains a large percentage of salts (3-5 

 per cent.) the same effect is brought about and 

 absorption ceases. Thus, even if there is plenty of 

 water, if the atmosphere be cold or the soil cold, or 

 the soils sour or acid, the same effect is brought 

 about as if the soil were dry. 



This is physiological drought. If the soil contains 

 little water, also, absorption is slow and physical 

 drought occurs. This is to be noticed in deserts, 

 steppes, rocky regions, sandy soils, chalk hills, sea 

 coasts, bogs, etc. 



When the rate of transpiration is too great the 

 plant suffers as much as if it were unable to absorb 

 water. In general this is due to the evaporation of 

 water. Transpiration is rapid in a dry atmosphere 

 and ceases when the air is saturated with water 

 vapour. The amount of moisture that can be re- 

 tained in the air is raised when the temperature is 

 high. Thus a high temperature favours rapid tran- 

 spiration. 



Wind also has a similar drying effect. At high 

 altitudes the air is rarefied and evaporation is rapid. 

 Wide leaves transpire freely, narrow less, in relation 

 to volume. Transpiration is accelerated by intense 

 illumination, but evaporation is not. 



In order to counteract such a state of instability. 



