48 DEPARTMENT OF AGRICULTURE GEORGIA. [334] 



Leaves perform an essential function in plant nutrition, 

 as one of the organs of vegetation, since nearly the whole 

 of the organic structure of all plants is primarily received 

 into the organism through them. 



Their structure is very complex, their offices manifold, 

 and absolutely necessary for the normal growth and devel- 

 opment of the plant. 



"In a general, mechanical way, it may be said, leaves 

 are definite protrusions of the green layer of the bark, 

 expanded horizontally into a thin lamina, and stiffened by 

 tough, woody fibres (connected both with the liber, or 

 inner bark, and the wood), which form its framework, ribs 

 or veins. Like the stem, therefore, the leaf is made up of 

 two distinct parts, the cellular and the woody. The cel- 

 lular portion is the green, pulp, or parenchyma ; the woody 

 is the skeleton, or framework, which ramifies among and 

 strengthens the former. The woody or fibrous portion 

 fulfils the same purposes in the leaf as in the stem, not 

 only giviqg firmness and support to the delicate cellular 

 apparatus, but also serving for the conveyance and distri- 

 bution of sap. 



"The subdivision of these ribs or veins of the leaf, as 

 they are not inappropriately called, continues beyond the 

 limits of unassisted vision, until the bundles, or threads, 

 of woody tissue are reduced to very delicate fibres, rami- 

 fied throughout the green pulp." 



"The cellular portion of the leaf consists of thin-walled 

 cells of parenchyma, containing grains of chlorophyll, to 

 which the green color of the foliage is entirely owing." 

 GRAY. 



The upper surface of the leaf seems to be connected 

 with the alburium, or sap-wood, and the lower with the 

 liber, or inner bark. 



Assimilation of the food of the plant is carried on in the 

 expanded leaf in the presence of sun-light, which is essen- 



