Water 93 



leaves, but also on some mealy-looking ones, as, for 

 instance, cabbage leaves ; and in both cases it is the 

 waxy substance in or upon the skin of the leaf, which 

 not only prevents water from soaking in, but also 

 prevents all but a very small quantity of moisture from 

 being drawn out. 



Then, if water is constantly passing off, and that in 

 considerable quantities, how does it escape ? A little, 

 as we have said, passes off through the whole surface, 

 but the bulk finds its way out through special openings, 

 pores, or mouths, to which the name of ' stomata ' has 

 been given. These pores are extremely minute open- 

 ings in the outer skin of leaf and stem, and vary very 

 greatly in size and number in different plants. It is 

 through them that used-up air and water in the form 

 of vapour are allowed to escape. 



The process by which vapour is given off through 

 the leaf pores is called * transpiration,' and is not the 

 same thing as evaporation, tliough like it, it proceeds 

 more quickly in hot dry weather. But evaporation 

 goes on — or, in other words, the air sucks moisture — 

 from the whole surface of a plant — from trunk, stem 

 and leaves more or less, and would suck much more 

 than it does if it were not prevented. 



Transpiration, on the other hand, is confined to the 

 leaf-pores, and is the process by which the plant parts 

 by its own action, so to say, with its superfluous 

 moisture. In evaporation the plant is merely acted 

 upon by the air ; the moisture is sucked out as it is 

 sucked from a wet sheet hung out to dry, or a piece of 

 dead wood. In transpiration the moisture passes out 

 through the proper openings, and the plant itself acts, 

 or at least discharges one of the natural functions of 



