80 FABRIC OP PLANTS. 



with a raised border, which contracts when water or 

 moisture is applied, and expands in dry air or when 

 exposed to sun-light. At the bottom of these pores are 

 spaces full of air mutually communicating. The same 

 view is taken by M. Adolphe Brongniart in his splendid 

 paper on the anatomy of leaves. 



No pores have been hitherto detected, even by 

 Amici, in roots, old stems, seeds, fleshy fruits, petals, 

 or blossom leaves in general, nor in any water-plants. 

 Some leaves again have pores on both surfaces, others 

 only on one, and that chiefly the under surface. 



In the water crowfoot there are pores only on the 

 upper surface of the floating leaves ; none in the leaves 

 under water. A leaf of green mint has no fewer than 

 1800 pores on its under side: but after it is placed so 

 as to be under water for a month, it falls off, and the 

 new leaf that succeeds has no pores. When water- 

 plants again are forced to grow in a dry place, they 

 acquire pores. Blanched plants have no pores. 



Rudolphi found the largest pores on the leaf of the 

 white lily; the smallest on the leaf of the French 

 bean. 



Richard thinks it probable that the pores are in- 

 tended to afford a passage to the air, either outward 

 or inward, or both ; though, from their being always 

 shut at night, when the leaves absorb the carbonic acid 

 gas dissolved in dew, it is probable their chief office is 

 to give off oxygen by day an opinion corroborated by 

 the petals of flowers, which do not give off oxygen, 

 being without pores. 



