282 PLANT PHYSIOLOGY 



immense system enables the plant to utilize perfectly even a 

 scantily distributed soil moisture and equally meageriy dis- 

 tributed mineral substances. 



The amount of water consumed by a plant during its life is 

 not a constant magnitude but varies to a considerable degree 

 according to the climatic conditions. In a hot dry climate, 

 plants consume about two to three times more water than in 

 humid clmiates. Similarly, under these conditions, they develop 

 a relatively larger root system and a smaller evaporating leaf 

 surface. Plants consume water least of all in humid shady 

 forests, and it is just under these conditions that the larger and 

 thinner leaves are found. Conversely, the root system of shade 

 plants develops weakly, as does also the conductive system. 

 That is why transpiration is sharply increased when the trees 

 are cut down; shade plants desiccate and give way to light plants, 

 which are better adapted to these new conditions. 



Water expenditure and absorption are most intimately con- 

 nected with each other and represent two phases of the same 

 process, but here for convenience they will first be examined 

 separately. 



65. The Absorbing and Pumping Activity of the Root System. 

 Root Pressure, Guttation, and Bleeding of Plants. — Land plants 

 of simple structure, for example, the mosses and lichens, absorb 

 water through their whole surface. In higher plants, special 

 organs, the fine tips of roots or rootlets, serve this purpose. The 

 cells of this part of the roots have thin walls made up of cellulose 

 and callose. In the majority of cases, the cells have special 

 outgrowths, the root hairs, which greatly increase the absorbing 

 surface of the plant. Root hairs penetrate between the soil 

 particles and absorb the water contained in them. 



If the plant did not lose water continually by transpiration, 

 the root cells would soon be saturated with water, and absorption 

 would cease. The loss of water by the plant must, therefore, be 

 regarded as the principal process inducing an unsaturated con- 

 dition in the plant and causing a considerable suction tension. 

 The plant as a whole presents, as it were, a self-regulating osmotic 

 apparatus. The suction tension originating in the leaves is 

 finally transmitted through a series of mechanisms to the root 

 system, where it creates a water deficit that is necessary for the 

 initiation of absorption. 



