1130. The younger leaves have been originally inclosed in the older, as in their sheaths.
1131. Every perfect leaf, i. e. every leaf-vesicle, must be regarded as the terminal extremity of an entire twig, from or out of the angle of which a new twig grows forth, that again as a bud ruptures, and from which again a twig grows forth.
1132. All leaves therefore range directly opposite to each other. A branch with many leaves is a system of branches, which grow out of each other, like the articular pieces of the grass-culm or straw.
1133. A leaf is a whole plant with all its tissues and systems; with cells, ducts, tracheæ; bark, liber, wood, stalk and branches. The leaf is a tree of special form, a tree, whose branches or tracheal fascicles all lie in one plane and are held together by parenchyma. It is the bodily expression of the position of the tracheal circle in the stem, only ruptured and to the greatest degree attenuated.
1134. In the division of the ribs of the leaf the internal arrangement of the woody fasciculi in the stem has been placed before our eyes, as by the scalpel of an anatomist.
1135. From the arrangement of the ribs of the leaf the structure of the whole plant can be recognized and its character determined. The leaf is the table of contents or index of the stem.
1136. Plants, which have no tracheæ, have also no leaf-ribs—Mosses.
1137. Plants, which have only isolated or non-ramifying fascicles of tracheæ, have parallel leaf-ribs that do not ramify—Monocotyledons.
1138. Plants, which have a circle of tracheæ, or rings of wood, have leaves with ramified ribs—reticular leaves or true foliage—Dicotyledons.
1139. The stronger indeed the ramification of the leaf-ribs, by so much the higher is the perfection of the leaf. The lowest leaf is that devoid of ribs, the higher that with parallel ribs, the highest being the reticular-veined leaf.