150 



ORGANOGRAPHY. 



divides so as to separate the blade into two or more portions, 

 each of which bears the same relation to the petiole, as the petiole 

 itself does to the stem from whence it arises {fig. 249). The 

 separated portions of a compound leaf are then called leafl^ets or 

 foiioles; and these may be either sessile {figs. 338-340), or 

 have stalks {figs. 352 and 354), each of which is then termed a 

 fetiolule or partial -petiole, and the main axis which supports 

 them, the racMs or common petiole. The leaflets of a compound 

 leaf may be at once distinguished from the separate leaves of 

 a branch, from the fact of their being all situated in the same 

 plane, and also generally, although to this there are niimerous 

 exceptions, from the whole leaf separating as one piece from 

 the stem when it dies. 



A simple leaf has never more than one articidation, which is 

 placed at the point where it joins the stem ; but a compoimd leaf 

 frequently presents two or more articulations ; thus, besides the 

 common articulation to the stem, each of the separate leaflets 

 may be also articulated to the common petiole. This character 

 frequently forms a good mark of distinction between simple 

 and compound leaves, for although it is quite true that 

 many compound leaves only present one articulation, and 

 can then only be distinguished from those simple leaves which 

 are divided „ to their midribs by the greater 

 breadth of attachment of the divisions in the 

 latter instances ; yet, if such leaflets are articu- 

 lated to the common petiole, their compound 

 nature is at once evident. The presence 

 of more than one articulation is, therefore, 

 positive proof as to the compound nature of a 

 leaf, but the absence of such articulation does 

 not necessarily prove it to be simple, as is 

 sometimes stated. We thus look upon the leaf 

 of the common Orange, which consists of only 

 a single blade {fig. 294, V) as a compound leaf, 

 because its petiole p, is not only articulated 

 to the stem, but the blade is also articulated 

 to the petiole. There are, however, numerous 

 instances of leaves in a transitional state be- 

 tween simple and compound, so that it is im- 

 possible in all cases to draw a distinct line 

 of demarcation between them. "We shall now 

 treat in detail of simple and compound leaves. 

 1. Simple Leaves. — The modifications 

 which leaves present as regards the form, 

 general outline, and other variations of their 

 blades, are extremely numerous ; hence we require a correspond- 

 ing number of terms to define them. These terms are also ap- 

 plied in a similar sense to describe like modifications of the other 



Fig. 294. Leaf of 

 Orange {Citrus 

 Aurantium ). p. 

 Winged petiole 

 articulated to the 

 lamina, I. 



