253. Leaves, simple and compound. A leaf is simple when its blade consists of a single piece, however cut, cleft or divided; and compound when it consists of several distinct blades, supported by as many branches of a compound petiole.
254. Nature of veins. The blade of the leaf consists of, (1) the frame-work, and (2) the tissue commonly called the parenchyma. The frame-work is made up of the branching vessels of the foot-stalk, which arc woody tubes pervading the parenchyma, and conveying nourishment to every part. Collectively, these vessels are called veins, from the analogy of their functions.
255. Venation is a term denoting the manner in which the veins are divided and distributed. The several organs of venation, differing from each other only in size and position, may be termed the midvein, veins, veinlets and veinulcts. (The old terms, midrib and nerves, being anatomically absurd, are here discarded).
256. The midvein is the principal axis of the venation, or prolongation of the petiole, running directly through the lamina, from base to apex, as seen in the leaf of the oak or birch. If there be several similar divisions of the petiole, radiating from the base of the leaf, they are appropriately termed veins; and the leaf is said to be three-veined, five-veined, etc. Ex. maple.
257. The primary branches sent off from the midvein, or the veins we may term the veinlets, and the secondary branches, or those sent off from the veinlets, are the veinulets. These also branch and subdivide until they become too small for vision.
Varieties of venation. 106, feather-veined, - leaf of Betula populifolia (white birch), lying upon a leaf of plum -tree; same venation with different outlines. 107, Palmate-veined. - leaf of white maple, contrasted with leaf ofCercis Canadensis. 10S, Parallel venation, - plant of "three-leaved Solomons-seal," (Asteranthemum trifoliatum Kunth) 109, Forked venation, - climbing fern (Lygodium).
258. Mooes of venation. Botanists distinguish three principa. modes of venation, which are in general characteristic of the three grand divisions of the vegetable kingdom already noticed.
Reticulate, or net-veined, as in the Exogens: this kind of venation is characterized by the frequent reunion or inosculation of its numerously branching veins, so as to form a kind of irregular net-work.
Parallel-veined, as in the Endogens. The veins, whether straight or curved, run parallel, or side by side, to the apex of the leaf, or to the margin, and are always connected by simple transverse veinlets.
Fork-veined, as in the ferns (and other Cryptogamia, where veins are present at all). Here the veins divide and subdivide in a furcate manner, and do not re-unite.
259. Of the reticulate venation, the student should carefully note three leading forms, the feather-veined, the palmate-veined, and the tripli -veined.
The feather-veined (pinni-veined) leaf is that in which the venation consists of a midvein giving off at intervals lateral veinlets and branching veinulets. Ex. beech, chestnut.
260. In the radiate-veined (palmi-veincd) leaf the venation con-sists of several veins of nearly equal size, radiating from the base towards the circumference, each with its own system of veinlets. Ex. maple, crow-foot.
261. The tripli-veined seems to be a form intermediate between the two others when the lowest pair of veinlets are conspicuously stronger than the others above them towards the apex, extending with the midvein towards the summit.
262. In parallel-veined venation the veins are either straight, as in the linear leaf of the grasses, curved, as in the oval leaf of the orchis, or transverse as in the Canna, Calla, etc.