30 LEAVES. 



leaf. Cellular tissue of this nature is called 

 cavernous. 



The skin covering the leaf called cuticle, is 

 formed of one or more layers of depressed 

 cellular tissue, which is generally hardened, 

 and always dry and filled with air. 



Between many of the cells of the cuticle are 

 placed apertures or pores called stomata, which 

 have the power of opening and closing as cir- 

 cumstances may require. 



It is by means of this apparatus that leaves 

 prepare the sap which they absorb from the 

 alburnum, or new wood, converting it into the 

 secretions peculiar to the species. 



Their cavernous structure enables them to 

 expose the greatest possible surface of their 

 cellular tissue to the action of the atmosphere. 



Their cuticle is a non-conducting skin, which 

 protects them from great variations in tempera- 

 ture, and through which gaseous matter will pass 

 readily. 



Their stomata are pores that are chiefly in- 

 tended to facilitate evaporation ; for which they 

 are well adapted by the power they possess of 

 opening or closing as circumstances may re- 

 quire. 



They arc also intended for facilitatmg the 



