44 PRACTICAL BOTANY. 



87. The structure of the leaf presents (at least in the 

 higher organized plants) two parts — icoody tissue and pa- 

 renchyma. While the former, derived from the liber, and 

 composing the framework, exists in small quantity, the lat- 

 ter, the green pulp, is predominant, and is nearly identical 

 with the green layer of the bark. The cells of tlie green 

 pulp exist in two layers, beneath the epidermis. The 

 upper layer consists of oblong cells, placed perpendi- 

 cularly to the surface of the leaf ; the lower is made up of 

 similar cells placed parallel to the same, and traversed by 

 intercellular passages, so as to be less compact than tlie 

 upper layer. 



The green color of the leaf is owing to a green pigment, 

 lying loose in its cells, and Called chlorophyll. 



Not only the Phcenogams^ but also the higher-grade 

 Cryptogams, have leaves, consisting of both w^ood-fibres 

 and parenchyma. The leaves of the cellular Cryptogams, 

 however, consist of parenchyma alone. 



88. As regards their forms, we divide the leaves into 

 ORDiNAiiY LEAVES ou the ouc hand, and modified, trans- 

 formed, and DEGENERATED LEAVES OU the otlicr, such as 

 stipules, hracts, involucral leaves, spathes, scales, pales ^ 

 cupides, hollow leaves, etc. Modified, etc., leaves exhibit 

 essentially the same structure as the ordinary sort. 



* ORDINAEY LEAYES. 



89. Ordinary leaves — that is, those which serve par- 

 ticularly as foliage — are divided into, 1, simple, sm^ 2, com- 

 pound leaves. 



