BOTANY. 



117 



Tlio veins which carry the latex, or nutricious fluid of 

 the leaf, "having reached the edge of the leaf, double back 

 upon themselves," spread through the lower surface, and are 

 again collected, and returned through the leaf-stalk into the 

 bark. 



Fig. 35. 



[Fig. 35 shows a magnified section of the leaf of the lily: the upper 

 surface, a, consists of flattened cells of the epidermis, arranged in a 

 single layer; beneath this, b, is the more compact part of the paren- 

 chyma, consisting of a layer of oblong cells placed in such a position 

 that their longer axis is perpendicular to the leaf's surface. Next 

 below is the parenchyma of the lower surface, c, composed of oblong 

 cells arranged longitudinally, and so loosely compacted as to leave 

 larger spaces between. Lastly, d, is the epidermis of the lower sur- 

 face, with stomata, e, e, opening into air chambers, f.] 



FUNCTIONS OF THE LEAF. 



The functions of the leaf are, exhalation, absorption, respi- 

 ration and digestion. The ultimate end of these functions is 

 to produce the necessary changes on the crude sap brought 

 up from the roots, and to convert it into the latex, which is the 

 proper nutrition of the growing plant : this ftuid is to the plant 

 what the arterial blood is to the animal system. 



Exhalation in plants is the throwing off of the excess of 

 water in the sap, so as to leave it in a more concentrated form, 

 and consequently better adapted to nutrition : exhalation takes 

 place through the stomata, and is different from mere evapo- 

 ration, which depends upon the state of temperature and air. 

 Exhalation is supposed to cease during darkness. 



Absorption is performed mainly by the roots, in nearly all 

 plants : when, however, these are defective or wanting, the leaf 



