Water 95 



pores. The leaves of the laurustinus have no leaf- 

 pores at all on their shiny, upper surface, neither have 

 those of the lilac ; while those of the carnation, which 

 show no such difference as these do, have about an 

 equal number on each side. Some leaves have as 

 many as 170,000 pores to the square inch, but this 

 seems to be the largest number. An apple leaf 

 of ordinary size has about 100,000 leaf-pores alto- 

 gether. 



The size of the pores varies very much, but at their 

 largest they are so minute as entirely to exclude the 

 very finest dust. Those of the white lily, for instance, 

 which are called ' remarkably large,' measure only one 

 4,25oth part of an inch across. 



Hard, evergreen leaves, such as those of the pine, 

 are like the thick, fleshy ones in this, that they have 

 but few leaf-pores, and lose but little water except 

 through these openings. For pines grow in very dry, 

 sandy soils, and often in elevated situations, where the 

 air, though cold, is exceedingly dry and drying, and 

 they therefore need as much protection as plants 

 which grow in hot, dry climates. 



Many and various are the devices by which evapora- 

 tion is checked and controlled, even in temperate 

 latitudes, lest the plant's need of water should exceed 

 the supply. For it must be remembered that air has 

 an immense appetite for water; the drier it is the 

 more it takes up, but it goes on sucking, if allowed, as 

 long as it is in contact with anything containing 

 moisture until it can hold no more. 



It is this which makes the misery of an east wind, 

 which is a very dry wind, as well as a cold one, and 

 sucks up moisture Wherever it can, not only from vege- 



