Water y'7 



And this brings us to another part of the subject, the 

 question, namely, as to the amount of water given off by 

 trees and other plants, notwithstanding the various ways 

 in which, as we have seen, they are protected. 



We have distinguished hitherto between the two pro- 

 cesses of evaporation and transpiration, because they are 

 distinct ; the one being due to the action of the air, and 

 the other to the action, so to say, of the plant. Evapo- 

 ration takes place whenever air comes in contact with 

 anything moister than itself; whether it be animal or 

 vegetable, whether it be wet earth or damp clothes, from 

 all it draws water, and by its own heat converts this water 

 into vapor. The other process, transpiration, is that by 

 which, through the pores — the openings left in the skin of 

 stem and leaf — the plant gives up, in a regular, systematic 

 manner, the moisture with which it would else be over- 

 charged. 



But in both cases the water passes off into the air in 

 the form of vapor; and in both cases it passes off as 

 nearly pure water, all mineral matter being left behind; in 

 both cases also, the amount given off varies with the 

 weather, there being more loss on a hot, dry, sunny, or 

 windy day, than on a damp, dull, still one. When, there- 

 fore, we consider the amount of water which passes off 

 into the air from a plant in a certain time, it is generally 

 impossible to distinguish between that which comes 

 through the whole surface and that which comes through 

 the pores; and both processes are frequently spoken of 

 together as transpiration or evaporation. The quantity 

 transpired is, however, usually very much larger than the 

 quantity evaporated. 



In some plants it is occasionally possible to see the 



