THE LEAVES OF PLANTS. 



commonly marked by a number of ridges which are called veins, and which consist of 

 woody tissue, spiral vessels, and cellular tissue ; and they are retained in their position, 

 and the intervening spaces filled up, by cellular tissue. The tissues of the veins are 

 brought in closer proximity in the petiole, which is a small stem, and having passed 

 through it into the stem, one part enters the bark, whilst the other traverses the wood 

 &nd penetrates to the medullary sheath at the centre of the stem. Thus every leaf is 

 in direct communication with the stem, and not only so but it is a prolongation of the 

 very pith, spinal vessels, and wood of the stem. The similarity between tho leaf and 

 the stem may be carried yet further, for not only do the same structures enter into the 

 composition of both, but in both there is a double set of vessels, one of which conveys 

 the fluid from the root, and the other back again towards the root. The only difference 

 between a leaf and a stem is, that the parts of the stem are more widely distributed in 

 the leaf, and there is an increased quantity of cellular tissue to fit them for their pecu- 

 liar functions. 



We have already, in a previous page, described the cuticle of leaves, with its appendages, 

 and may, therefore, at once proceed to consider the internal structure of these organs. 



The veins of leaves are distributed on an uniform plan, and not as a matter of acci- 

 dent, and may be arranged under two heads viz., the venation of exogens and the 

 venation of endogens. The leaf of an exogen is said to be reticulated, and that of an 

 endogen straight or parallel veined. 



The venation of an exogen, as an oak or the holly, consists of a central midrib and 

 a series of festoons, arranged on either side of it 

 (Fig. 150). The large branches proceeding from 

 either side of the midrib are termed primary veins 

 (2) ; and after they have proceeded for some 

 distance towards the edge of the leaf they form 

 a series of curves by which they communicate 

 with each other, and which are termed curved 

 veins (3). The curved veins in their turn be- 

 come trunks, from which other and lesser veins 

 are given off (4), which from their relative positions 

 are known as the external veins ; while others, 

 of a still smaller size, distributed to the mar- 

 gin of the leaf, are termed marginal veinlets (5). 

 Thus far all the veins have proceeded from one 

 source, and clearly belong to one system, being a 

 series of arches placed upon each other, and all 

 resting upon the midrib. But besides these veins 

 there are others, which may be said to belong to an 

 inner system. Thus the costal veins (6) are 

 small branches which proceed from the midrib, 

 at points intermediate to the primary veins ; whilst 

 the branches of the primary veins themselves are 

 termed proper veinlets (7), and their anastomoses 



common veinlets (8). Such is Dr. Lindley's arrangement, and it is one which merits 

 approbation. 



it. must not be supposed that these systems of veins can be traced in all leaves; for 

 in the leaves of mosses, and other plants of the lowest class, the-e are no veins ; and in 



Fig. 150. representing the complete 

 venation of an exogenous leaf, as in 

 the ilex or holly. 



1. The midrib. 



2. The primary veins. 



3. The curved veins. 



4. The external veins. 



5. The marginal veinlets. 



6. The cortal veins. 



7. The proper veinlets. 



8. The common veinlets. 



