THE LEAVES. 79 



is simply an expansion of the wood and bark of its stem, the 

 wood issuing from the side of the shoot whilst in its green 

 young state in fibrous bundles, which carry with them at the 

 same time the green cellular bark of the shoot, and then by 

 their expansion spread it out to the air and light of heaven. 

 There must, therefore, be a natural adaptation and corres- 

 pondence between the spread of the woody fibre which 

 constitutes the framework of the leaf, and the peculiarities 

 of its form. This idea was first suggested by Decandolle. 

 According to him, the shape of leaves depends on the mode 

 in which the fibres diverge when they leave the side of the 

 shoot, and upon the quantity of parenchyma or bark which 

 they carry with them ; and by him this arbitrary nomenclature 

 of form was rendered intelligible and reduced to something 

 like system based on scientific principles. Decandolle distin- 

 guishes three principal modes in the venation of leaves, viz. : 

 the net-veined, the parallel-veined, and the fork-veined. 



1. The reticulated or net-veined leaves are characteristic of 

 exogens, which are justly regarded as the most highly organ- 

 ized plants in the vegetable world. Two modifications of net- 

 veined structure have been observed, the feather-veined and 

 radiate-veined ; the leaves of the chestnut are good examples 

 of the former, and those of the garden nasturtium of the latter. 

 The margins of net-veined or exogenous leaves are very seldom 

 entire, but most frequently notched in various ways, described 

 in books as dentate or toothed, crenate or scolloped, serrate or 

 having teeth like a saw, of which last we have a good example 

 in the leaf of the rose. The cause of these incisions has not 

 been clearly ascertained. 



2. The parallel-veined leaves are the distinguishing feature 

 of endogens, which are considered humbler in their organic 



