154 



THE LEAVES. 



characterizing species. Their principal modifications are therefore 

 classified, minutely defined, and embodied in a system of nomen- 

 clature which is equally applicable to other parts of the plant, and 

 which as an instrument is indispensable to the systematic botanist. 

 The numerous technical terms which have gradually accumulated 

 from the infancy of the science, and have multiplied with its increas- 

 ing wants, are mostly quite arbitrary, or have been suggested by 

 real or fancied resemblances of their shapes to various natural or 

 other objects. This arbitrary nomenclature, which formerly severe- 

 ly tasked the memory of the student, was reduced by De CandoUe 

 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 frar&ework of the leaf is expanded or ramified 

 in the parenchyma. Upon this conception the following sketch is 

 based ; in which all the more important terms of the nomenclature 

 of leaves are mentioned and defined. It should be kept in mind, 

 however, that tliis is not to be taken as an explanation of the actual 

 formation of leaves ; but rather as an account of the mutual adap- 

 tation and correspondence of their outlines and framework. For 

 the parenchyma is developed, and the form of the leaf more or less 

 determined, before the framework has an existence. The latter, 

 therefore, cannot have given rise to the outline or shape of the 

 organ. The distribution of the veins or fibrous framework of the 

 leaf in the 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, wliich run 

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

 transverse \-einlets (as in Fig. 230) ; 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. 229. 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 respect 

 analogous to the nerves of animals. The latter are termed reticu- 

 lated or netfed-veined leaves. 



