i8 INTRODUCTORY [chap. 



the plant, of water w'nich enters by the root hairs and 

 is evcntnallv evaporated from the leaves and other 

 growinjT sur''aces of the plant It has been shown 

 that under the normal conditions the plant transpires 

 from 200 to 500 pounds of water for every pound of 

 dry matter that is simultaneously produced ; the lower 

 number being nearer the factor prevailing in our humid 

 atmosphere, and the higher one holding for drier coun- 

 tries. With this water enter the nutrient constituents 

 of the soil and of fertilisers applied to it; but the 

 process by which they enter is rather more complex 

 than one of the simple intake of a solution. 



The passage of the dissolved substances into the 

 plant takes place by the purely physical process 

 of osmosis, the walls of the root hairs (which con- 

 sist of single elongated cells) acting as membranes 

 through which water or salts will pass independently, 

 according to the relative concentration of the solu- 

 tions inside or outside the cell. Should the cell sap 

 be more concentrated than the soil water outside, 

 pure water will pass through the wall until a certain 

 osmotic pressure (causing turgor in the plant) is 

 reached, which varies with the concentration. If, on 

 the contrary, the soil water became more concentrated 

 than the cell sap, water will leave the cell, the plant 

 will become flaccid, and even die if the withdrawal of 

 water be too great. It is in this way that plants 

 become "scorched" or "burnt" by too concentrated 

 solutions of any kind of soluble salts, such as are formed 

 when a little soluble manure, salt, etc., falls upon the 

 surface of a leaf 



Not only will water pass in or out of the cell, but 

 an equilibrium will be attained between the cell sap and 

 the external soil water for each constituent present in 

 the latter. If, for example, sodium or potassium chlorides 



