172 THE STORY OF THE PLANTS. 



the pore closes. Excessive evaporation at such 

 times would, of course, damage or destroy the 

 foliage ; the plant desires rather to store up and 

 retain its stock of moisture. But after rain, and 

 in damp weather, the roots suck up abundant 

 water; and then it becomes desirable that evapo- 

 ration should go on, and the leaves and grow- 

 ing shoots should be supplied with liquid food, 

 as well as with the nitrogenous matter and salts 

 dissolved in it. Hence at such times the pores 

 open wide, and allow the water in the form of 

 vapour to exude from them freely. 



The object of this evaporation, again, is two- 

 fold. In the first place, it supplements root- 

 pressure as a means of raising water to the leaves 

 and growing shoots; and in the second place, by 

 getting rid of superfluous liquid, it leaves the 

 nitrogenous material and the food-salts in a more 

 concentrated form, at the very points where they 

 are just then needed for the formation of fresh liv- 

 ing protoplasm and other useful constructive fac- 

 tors of plant-life. But how does evaporation raise 

 water from the ground ? In this way. The liv- 

 ing contents of each cell on the upward path 

 have a natural chemical affinity for water, and 

 will suck it up greedily wherever they can get it. 

 Thus each part, as fast as it loses water by 

 evaporation, takes up more w'ater in turn from 

 its next neighbour below ; and that once more 

 withdraws it from the cell beneath it ; and so on 

 step by step until we reach the actual absorbent 

 root-hairs. Root-pressure by itself could not 

 raise water as high as we often see it raised in 

 great forest trees and tropical climbers ; it has 

 not enough mechanical motor power. But here 

 evaporation comes in, to aid it in its task ; and 



