SECT. I. INTRO-SUSCEPTION. Q5 



which his experiments were made ; which are on 

 this account the less satisfactory, as well as the less 

 valuable. And hence it is now impracticable to as- 

 certain what particular species of Mallow are in- 

 tended by the Great and Little Mallows ; or what 

 particular species of Poplar it is that differs so much 

 in its capacity of absorption from the Populus tre- 

 mula or Aspen. But the inference deducible from, 

 the whole, and deduced accordingly by Bonnet, is 

 that the leaves of herbs absorb moisture chiefly by 

 the upper surface, and the leaves of trees chiefly by 

 the under surface. 



But what is the cause of this direct opposition Their dif- 

 between the absorbing surface of the leaf of the herb pYchiesac- 

 and of the tree ? The immediate cause must be, counted 

 that there exists a greater number of absorbents in 

 the upper surface of the one, and in the under sur- 

 face of the other. But what is the cause in the 

 economy of the vegetable subject, or state of sur- 

 rounding bodies, that requires this arrangement ? 



Du Hamel thought the lower surface of the leaf By Du 



TT 1 



of the tree was endowed with the greater capacity 

 of absorbing moisture, chiefly for the purpose of 

 catching the ascending dews and exhalations that 

 must necessarily come into contact with it as they 

 rise, but which might possibly still escape if ab- 

 sorbabie only by the upper surface, as being now 

 considerably rarefied, as well as more rapid in their 

 ascent ; * presuming, as it appears, that absorption 

 * Phys. des Arbres, liv. ii. chap. iii. 



