Size and stratification. Each of the life-forms described previously may be classified according to size. The words, tall, medium, and low may be given definite limits for each life-form. For example, a tree higher than 25 meters (82 feet) is tall; from 10 to 25 meters (33 to 82 feet) is medium; 8 to 10 meters (26 to 33 feet), low. For the smaller life-forms different limits are set. Such standardization of size enables the plant geographer to make precise descriptions of vegetation. Further standardization is achieved by setting height limits for a series of layer, numbered successively from the ground up.
3. Coverage. The degree to which the foliage of individual plants of a given life-form cover the ground beneath them is designated the coverage. We may use four terms descriptive of the coverage: barren or very sparse; discontinuous; in tufts or groups; and continuous. For example, the trees may be of discontinuous converge whereas the herb layer is continuous, or vice versa.
4. Function, or periodicity. Of primary importance in the classification of forms of natural vegetation is the response of the plant foliage to the annual climatic cycle. Deciduous plants shed their leaves and become dormant in an unfavorable season, which is either too cold or too dry to permit growth. Evergreen plants retain green foliage year-around although in some cases becoming almost dormant in a cold or dry season. Where the climate is equable (that is, moist and not cold throughout the year) evergreen plants grow continuously. A third class, the semi deciduous plants, is those, which shed their leaves at intervals not in phase with a season. Thus a semi deciduous forest would not at any one time have all of its individuals devoid of foliage. As a fourth class we recognize evergreen-succulent and evergreen-leafless plant, those with very thick fleshy leaves, which retain their foliage year-around, and those with fleshy stems but no functional leaves, such as the cacti.
5. Leaf shape and size. Recognition of the leaf form of a plant constitutes an essential part of the structural description. One form is the broadleaf, familiar to us in such common trees as the maple, beech, and rhododendron. In contrast is the needleleaf, also familiar in the pine, spruce, fir, and hemlock. A similar form is the spine, which in some plants represents the transformation of the entire leaf. The slender, tapering leaves of grass are referred to as graminoid in form. We may also recognize the small leaf-form, as in the birch or holly; and the compound leaf, as in the hickory and ash.
6. Leaf texture. Leaf textures range widely according to the climate and habitat, because of the different degrees to which the water loss from the leaf into the air must be controlled. Leaves of average thinness are described as membranous; those which are thin and delicate (as in the maiden-hair fern) are described as filmy. Leaves which are hard, thick, and leathery are sclerophyllous; a, forest dominated by trees and shrubs having such leaves is termed a sclerophyll forest. Very greatly thickened leaves, capable of holding much water in their spongy structure, are described as succulent.
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