THE LEAVES. 75 



amount of surface to the sun ; whilst, in the lower stratum of 

 parenchyma, the cells are arranged horizontally, having amongst 

 them numerous intercellular passages, or cavities filled with air, 

 which communicate freely with each other through the sub- 

 stance of the leaf, and with the external air by means of the 

 stomata or pores in the epidermis. 



The dense parenchyma of the upper surface of the leaf 

 accounts for its deeper tint, and is well adapted to restrain 

 the excessive evaporation to which the fluids in the upper 

 stratum of cells are liable, by their direct exposure to the sun ; 

 whilst the loose parenchyma of the lower surface is the cause 

 of the lighter tint of the underside of the leaf, which, together 

 with the pores of the cuticle, is well calculated to give the air 

 free access to all parts of the leaf, from which source plants 

 derive the greater part of their nutriment. Leaves growing 

 erect possess uniformity of structure in both strata of paren- 

 chyma. 



The vegetable membrane which forms the walls of the 

 cells of the parenchyma, is perfectly white and colorless. 

 The green color of the leaf is found to be caused by the forma- 

 tion of granules of green matter in the cells, which either float 

 free in the sap contained in their cavities, or else collect into 

 grains and adhere to the walls or sides of the cells. This sub- 

 stance is called chlorophyl (^wpo$, green, and <J>VM.OV, a leaf), in 

 contradistinction to chromule (^pc^ua, a color), which is the 

 term employed by botanists to designate the colored substances 

 with which the cells of flowers are filled. 



Chlorophyl appears to belong to the class of waxy bodies. 

 It is soluble in alcohol or ether, but not in water, and is 

 developed only in those cells which are exposed to the action 

 of light. It is therefore only formed in the superficial strata 



