38 VITAL ACTIOISS. 



lying for some time in water. Of these layers, one is 

 superior and arises from the neighbourhood of the 

 pith, the other inferior and arises from the liber ; the 

 former maintains a connexion between the wood and 

 leaf; the latter establishes a communication with the 

 bark. As sap, or ascending fluid, rises through the 

 wood, and principally the alburnum, afterwards de- 

 scending through the liber, it follows from what has 

 been stated, that a leaf is an organ of which the upper 

 system of veins is in communication with the as- 

 cending, and the lower system with the descending, 

 current of sap. 



61. A leaf has moreover a skin, or epidermis, 

 drawn all over it. This epidermis is often separable, 

 and is composed of an infinite number of minute cavi- 

 ties, originally filled with fluid, but eventually dry 

 and filled with air. In plants growing naturally in 

 damp or shady places it is very thin ; in others, in- 

 habiting hot, dry, exposed situations, it is very hard 

 and thick ; and its texture varies between the two 

 extremes, according to the nature of the species. The 

 epidermis is pierced by numeroug invisible pores, 

 called stomates, through which the plant breathes 

 and perspires. Such stomates are generally largest 

 and most abundant in plants which inhabit damp and 

 shady places, and which are able to procure at all 

 times an abundance of liquid food ; they are fewest 

 and least active under the opposite conditions. It 

 will be obvious, that, in both these cases, the struc- 

 ture of a leaf is adapted to the peculiar circumstan- 

 ces under which the plant to which it belongs natu 



