102 Water 



the plant gives up, in a regular, systematic manner, 

 the moisture with which it would else be overcharged. 



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

 the form of vapour ; 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, 

 therefore, 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 

 moisture coming from the leaf-pores, as it escapes 

 faster than the air can evaporate it. This is the case 

 with many grasses, especially the maize, which may 

 be seen studded with actual drops of water. 



A grass-plant gives up its own weight of water in 

 the course of twenty-four hours, in hot, dry weather; 

 and a square foot of turf will yield more than i£ pints 

 of water in this time. But a square foot of long 

 pasture-grass gives off nearly 4I pints, or as much as 

 106 tons of water to the acre ! 



The larger the surface, the larger of course the 

 amount of water which passes off from it ; and there- 

 fore the extent of surface exposed is a matter of great 

 importance, though it is also one which we are very 

 likely to overlook, at least in many cases. Of course 

 we can all see that a tropical plantain with its broad, 



