156 



ORGANOGRAPHY. 



298), however much this may be divided, so that the divisions 

 do not extend to the midrib, or petiole; or in some cases the divi- 

 sions even extend to the midrib, or petiole, but the leaf is 

 still called simple when those are attached to these points by a 

 broad base, as in fig. .307. A leaf is termed compound, when 

 the midrib, or petiole 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 (Jig. 256). The separated portions of a compound leaf 

 are called leaflets or folioles ; and these may be either sessile 

 (figs. 344—346), or have stalks (figs. 347 and 348), each of 

 which is then termed a petiolule or partial petiole, and the main 

 axis which supports them, the rachis 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 from the whole leaf commonly 

 separating as one piece from the stem when it dies. 



A simple leaf never has more than one articulation, which Is 

 placed at the point Avhere it joins the stem; but a compound 

 leaf frequently presents two or more, thus besides the common 

 articulation to the stem, each of its separate leaflets may be also 

 articulated to the common petiole upon which they are placed. 

 This character frequently forms a good mark of distinction be- 

 tween simple and compound leaves, for although it is quite 



Fig. 300. 



true that many compound leaves only present 

 one articulation, and can then only be distin- 

 guished from those simple leaves which are 

 divided to their midribs by the greater breadth 

 of attachment of the divisions in the latter in- 

 stances ; yet, if such leaflets are articulated 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 ab- 

 sence of such 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. 300) 

 as a compound leaf, because its petiole is not 

 only articulated to the stem, but the blade is 

 also articulated to the petiole. Tiierc are 

 however numerous instances of leaves in a 

 transitional state between simple and com- 

 pound, so that it is impossible in all cases to 

 draw a distinct line of demarcation between 

 them. We shall now treat in detail of simple 

 and compound leaves. 

 Simple Leaves. — Tlie modifications which leaves present as 



Fio. 300. Leaf of 

 Orange ( Citru.t 

 A iirunliiim ). jt. 

 Winged petiole 

 articulated to the 

 lamina, /. 



