Water 69 



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 openings 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 vapor are allowed to 

 escape. 



The process by which vapor is given off through the 

 leaf-pores is called ''transpiration," and is not the same 

 thing as evaporation, though 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 its being. Evapora- 

 tion may continue in a dead plant, but only a living plant 

 transpires. 



Both processes are affected by the weather, however, 

 and both in a similar way. 



Nothing, we know, dries on a very damp day, because 

 the more moisture the air contains, the less it can take 

 up; or, in other words, evaporation proceeds slowly in 

 moist air. So, too, transpiration almost or quite ceases 



