THEIR FORMS AND VENATION. 161 



The numerous entirely unconnected technical terms which have 

 gradually accumulated from the infancy of the science, and have 

 multiplied with its increasing wants, are mostly quite arbitrary, 

 or have been suggested by real or fancied resemblances of their 

 shapes to natural or other objects. This arbitrary nomencla- 

 ture, which formerly severely tasked the memory of the student, 

 was reduced by De Candolle to a clear and consistent system, 

 based upon scientific principles, and of easy application. The 

 fundamental idea of the plan is, that the almost infinite varieties in 

 the form and outline of leaves may be deduced from the different 

 modes and degrees in which the woody skeleton or framework of 

 the leaf is expanded or ramified in the parenchyma. Upon this 

 conception our following sketch is based ; in which we endeavour 

 to introduce and define the more important terms of the nomencla- 

 ture of leaves. It should be kept in mind, however, that this sys- 

 tem is partly if not altogether empirical, and is not to be taken as 

 an explanation of the actual formation of the leaf; but rather as 

 an account of the mutual adaptation and correspondence of the 

 outlines and the framework of leaves. For the parenchyma is de- 

 veloped, and the form of the leaf is often fixed, before the frame- 

 work has an existence. The latter, therefore, cannot have deter- 

 mined the. outline or shape of the organ. The distribution of the 

 veins or fibrous framework of the leaf in J,he blade is termed its 



276. Venation, The veins are distributed throughout the lamina 

 in two principal modes. Either the vessels of the petiole divide at 

 once, where they enter the blade, into several veins, which run 

 parallel with each other to the apex, connected only by simple 

 transverse veinlets (as in Fig. 201) ; or the petiole is continued 

 into the blade in the form of one or more principal or coarser 

 veins, which send off branches on both sides, the smaller branch- 

 lets uniting with one another (anastomosing) and forming a kind 

 of network ; as in Fig. 191, 199. The former are termed parallel- 

 veined, or commonly nerved leaves ; the veins in this case having 

 been called nerves by the older botanists, a name which it is 

 found convenient to retain, although of course they are in no re- 

 spect analogous to the nerves of animals. The latter are termed 

 reticulated or netted-veined leaves. 



277. Parallel-veined or nerved leaves are characteristic of En- 

 dogenous plants ; while reticulated leaves are almost universal in 

 Exogenous plants. We are thus furnished with a very obvious, al- 



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