4 THE FACTORS [Part I 



2. Abundance of soluble salts in the soil. A small quantity of salt favours 

 absorption, whilst a large quantity impedes it. The degree of concentration 

 at which the retardation commences varies with the species of plant, but 

 rarely exceeds 0-5 °/ o . Mixtures of salts impede absorption more than pure 

 salts, and certain kinds, for example sodium chloride, act more energetically 

 than others, for example saltpetre 1 . 



3. Richness of the soil in Jiumous acids. 



4. Low temperature of the soil. A frozen soil is quite dry to all plants ; 

 one at a temperature slightly above freezing-point is nearly dry to most 

 plants. The minimum temperature for a normal absorption of water, that 

 is to say for an absorption of water sufficient to compensate for the loss by 

 transpiration through open stomata, varies with the species of plant, being 

 generally much higher for those of warm zones than [for plants whose lives 

 are spent in colder zones. 



Factors favouring Transpiration. 



1. A dry atmosphere. Although transpiration is a physiological and not 

 a physical process, yet it behaves in respect of this factor essentially 

 like evaporation : it constantly increases in proportion to the dryness of 

 the air. 



2. High temperature of the atmosphere. Transpiration increases with the 

 temperature up to a maximum which varies with the species, and beyond 

 which pathological changes cause a diminution. 



3. Rarefaction of the air. A reduction in the atmospheric pressure 

 accelerates transpiration, not directly, as in the case of evaporation, but 

 indirectly by accelerating the diffusion of the water-vapour. 



4. Light. Transpiration is greater in the presence of light than in 

 darkness, and it increases with the intensity of the illumination. The more 

 effective rays of light, according to Wiesner, are, first, the blue ; secondly, 

 the red ; while the green rays exercise only a feeble influence. 



Of the factors that accelerate transpiration, temperature and light are the 

 weakest, and do not in themselves suffice to produce decided xerophilous 

 characteristics. 



All plants with an environment involving cither one or other or a 

 combination of any of the above factors, excepting light and temperature, 

 possess the structure and characteristics of xerophytes. 



Xerophilous Structure. 



As has been already stated, many plants are able to thrive under very 

 diverse conditions of water-supply by altering their structure in relation 

 to their environment. The necessary investigations have been made as 

 regards four of the above factors — dryness of the substratum, atmospheric 



1 See Part I, Chap V, The Soil. 



