142 INTERNAL STRUCTURE OF LEAVES. 
such is frequently the case. Thus, the leaves of the Water 
Pepper (fig. 273), and of the Trailing Sallow (fig. 274), may be 
taken as illustrations of the most highly developed leaves, 
namely, those in which all the parts are 
found ; but in many plants one of these 
parts is absent, and in some two, so that 
the leaf is in such cases reduced to but two, 
or one of its portions only. The petiole 
and the sheath or stipules are those parts 
which are more commonly absent. When 
the petiole is absent, the leaf is said to be 
sessile (fig. 286); when the stipules are 
absent, it is exstipulate (fig. 290). The 
lamina or blade is that part which is most 
generally present. The leaf is called 
simple if there is but one blade (figs. 275 
and 274), or compound if this is divided 
into two or more separate parts (fig. 
275). The lamina of the leaf is usually 
that part also which is most developed, 
which performs the most important func- 
Fig. 275. Compound leaf of tions of the leaf, and which is also in ordi- 
Robinia Pseud - acacia, nary language known under the name of 
eh iigas: inystipules atits Jeaf tis the part, therefore, which will 
come more particularly under our notice ; 
but before we proceed to describe it and the other parts of the 
leaf separately, it will be necessary for us to treat of the internal 
structure of leaves, and of their insertion and arrangement. 
Pre. 210s 
2. THE INTERNAL STRUCTURE OF LEAVES. 
Leaves with reference to their structure are divided into 
aerial and submersed ; by the former is to be understood those 
that are developed and live entirely or partially in the air ; by 
the latter, those that are formed and dwell wholly immersed in 
water. 
1. Arrtat Leaves.—In the lowest leaf-bearing plants, such 
as Mosses, the leaves consist, as we have seen, simply of paren- 
chymatous tissue, formed by the growing outwards of the 
parenchyma of the circumference of the stem or branch ; while 
in the majority of the higher plants they contain, in addition 
to this parenchyma, a framework or skeleton formed of wood- 
cells or liber-cells, or of both, and vessels of different kinds, all 
of which are in direct connexion with corresponding parts of 
the fibro-vascular system of the stem or branch. We distinguish 
therefore, in such leaves, as in the stem and branch, both a 
parenchymatous and a fibro-vascular system, the former consti- 
tuting the soft parts or the parenchyma of the leaf; the latter 
