External Features, Origin, and Internal Structure
Taxonomists use an inordinate number of terms as a means to separate and name plants. The terminology applied to the way leaves are attached to the stem, for example, includes alternate—the arrangement shown in Figure —as well as opposite and whorled and is based on the number of leaves attached at each node: one (alternate), two (opposite), and three or more (whorled). If a single blade is attached to a petiole, as in Figure , the leaf is simple; if the blade is divided into two or more individual parts, the leaf is compound and may be pinnately or palmately so depending upon how the leaflets (the individual separate units of the blade) are attached to the extension of the petiole (the rachis). Other standard terms are used for venation, overall shape, shape of the tip, condition of the edge of the blade (toothed, smooth, lobed), hairy (what kind of hairs) or smooth (on both upper and lower surfaces or just on one) and more.
Leaves arise in the shoot apex of stems in cells immediately below the protoderm. Division and expansion of the cells in this area result in a leaf primordium in which meristematic regions soon become identifiable in the upper and lower regions of the tissue destined to become the blade. A strand of procambium from the shoot, the leaf trace, makes connection with differentiating vascular tissues of the primordium thus assuring the continuity of the conducting tissues throughout the plant. The area on the vascular cylinder of the stem where the leaf trace diverges into the leaf primordium is called a leaf gap, a confusing name; it is not a hole but an area filled with parenchyma cells. “Gap” refers to the absence of xylem and phloem cells at this point in the vascular cylinder.
The tissues of the evolving blade develop faster on the lower ( abaxial surface) than those on the upper ( adaxial surface) with the result that the primordium bends inward towards the shoot apex. The elongating primordia arch over and protect the apical meristem of the shoot. Cells divide and elongate in the primordium, differentiating downward from the tip and the intercellular spaces characteristic of the mature leaf soon appear among the young blade tissues. Cell divisions cease when the leaf is less than full size, and subsequent enlargement consists of elongation and expansion of cells and intercellular spaces. Leaves thus have determinate growth, whereas the apical meristem, with its cells that continue to divide indefinitely, has indeterminate growth.
The standard leaf has three tissue regions: the epidermis, the mesophyll, and the vascular bundles or veins (Figure ).