STRUCTURE.] FORMS OF LEAVES. + 2 til 



It is described by two opposite arcs, whose points of inter- 

 section are the apex and base. 



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 the Box tree, the serrated blade of the 

 Apple leaf, the toothed blade of Coltsfoot, the runcinate 

 blade of Taraxacum, the 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 compound, whether the divisions 

 be two, as in the conjugate leaf of Zygophyllum, or indefinite 

 in number, as in the many varieties of pinnated leaves. 

 Another notion of a compound leaf consists in its divisions 

 being articulated with the petiole, by which it is better dis- 

 tinguished from the simple leaf than by the number of its 

 divisions. Thus, the pedate leaf of a Hellebore or an Arum, 

 both in this sense belong to the class of simple leaves ; while 

 the solitary blade of the Orange, the common Barberry, &c. 

 are referable to the class of compound leaves. This distinc- 

 tion has been thought to be of some importance to the 

 student of natural affinities; for, while division of whatever 

 degree may be expected to occur in different species of the 

 same genus or order (provided there is no articulation), it 

 rarely happens that such compound leaves, as are articulated 

 with their petiole, are found in the same natural assemblage 

 with those in which no articulation exists. Alphonse De 

 Candolle remarks, however, that in Gleditschia, whose leaves 

 are mostly articulated, we find some leaves with their leaflets 

 united, and therefore not articulated with their midrib ; and 

 this, and other similar instances, diminishes the value of 

 articulation as the test of a compound leaf; moreover, in 

 such apparently simple leaves as those of Zamia, the leaflets 

 are, in fact, articulated with their midrib, as is proved by 

 macerating them, when they spontaneously disarticulate. 



