104 ORGANOGRAPHY, 



BOOK 1. 



succulent, or from the non-developenient of it when they 

 become membranous, or from the total suppression of it, and 

 even of the veins also in great part, as in those which are 

 called ramentaceous, such as the primordial leaves of the 

 genus Pin us. 



I have dwelt thus much at length upon the structure of the 

 leaf, because it is by far the most important part of a plant, 

 and that of which the functions are the best ascertained. Let 

 us next turn our attention to the modifications of the leaf. 



It has already been seen that a leaf may consist of two 

 distinct parts ; the petiole, or stalk, and the lamina, or blade : 

 both of these demand separate consideration. 



The BLADE, lamina, or llmhus, as it is called by some, is 

 subject to many diversities of figure and division ; most com- 

 monly it forms an approach to oval, being longer than broad. 

 That extremity of the blade which is next the stem is 

 called its base ; the opposite extremity, its apex ; and the line 

 representing its two edges, the margin or circumscription. 



If the blade consists of one piece only, the leaf is said to 

 be simple, whatever may be the depth of its divisions : thus, 

 the entire blade of Box, the serrated blade of the Apple, the 

 toothed blade of Coltsfoot, the runcinate blade of Taraxacum, 

 tiie pinnatifid blade of Hawthorn (which is often divided 

 almost to its very midrib), are all considered to belong 

 to the class of simple leaves. But if the petiole branches 

 out, separating the cellular tissue into more than one distinct 

 portion, each forming a perfect blade by itself, such a leaf is 

 often said to be compowul, whether the divisions be two, as in 

 the conjugate leaf of Zygophyllum, or indefinite in number, 

 as in the many varieties of pinnated leaves. Nevertheless, a 

 more accurate notion of a compound leaf is found to consist 

 in its divisions being articulated with the petiole, by which it 

 is much better distinguislied from the simple leaf than by the 

 number of its divisions. Thus, the pinnated leaf of a Zamia, 

 and the pedate leaf of an Arum, both in this sense belong to 

 the class of simple leaves ; while the solitary blade of the 

 Orange, the common Barberry, 8cc. are referable to the class 

 of compound leaves. This distinction is of some importance 

 to the student of natural affinities ; for, while division, of 

 whatever degree it may be, may be expected to occur in 



