70 The Great World's Farm 



in damp weather, or when the leaves are wet. But both 

 go on more briskly in the sun, in dry air, and more espe- 

 cially in a drying wind. 



The leaf-pores by which transpiration proceeds are 

 usually more abundant on the under surface — the shady 

 side — of the leaves, and are few or altogether wanting on 

 the upper surface, where they would be exposed to the 

 sun, and water might pass off too rapidly. In moist, 

 shady situations there is no danger of too much trans- 

 piration, and plants growing in these not only have more 

 leaf-pores than others, but can also have them without 

 risk, both on the under and upper surface of the leaves, 

 for here transpiration goes on more slowly, and the loss 

 of water is also easily made up. 



Thick, fleshy leaves have the fewest leaf-pores, and 

 thick, fleshy leaves are particularly characteristic of hot 

 countries, where plants can afford to lose but little of the 

 scanty supply of water which comes to them. 



Many leaves which are alike on both sides have about 

 an equal number of pores above and below; but when 

 there is any difference, as, for instance, where one side is 

 dull and the other glossy, the dull side, which is also the 

 under side, has the larger number of 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 one hundred and seventy thousand 

 pores to the square inch, but this seems to be the largest 

 number. An apple-leaf of ordinary size has about one 

 hundred thousand leaf-pores altogether. 



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



