If the plant geographer is to describe vegetation according to its structure, or physical properties and outward forms, he must set up a number of categories of information, each designed to contribute and essential element to the description of the vegetation. Six categories are used in a system set up by Dr. Pierre Dansereau. These six categories of information tell us about the growth-form of the plants, their size and stratification, the degree to which they cover the ground, their time functions, and their leaf forms.
1. Life-form. Plants can be classified according to their life-forms. The first forms, trees and shrubs are erect, woody plants. The word tree means a perennial plant having a single upright main trunk, often with few branches in the lower part, but branching in the upper part to a crown, which, in the mature individual, contributes to one of the upper layers of vegetation. The word shrub generally refers to a woody plant having several stems branching near the ground, so as to place the foliage in a mass starting close to ground level. Next are recognized woody vines, whic
h climb up on trees: the lianas. These may climb upon mature plants, or may be lifted during the growth of young trees to which they have become attached. Although the term "Liana" is usually associated with woody vines of tropical forests. Thus it is apparent that the life-form classes cut across the taxonomic categories of the plant world.
Fourth of the life-forms is the plant group known as the herbs. These are usually small tender plants, lacking woody stems. They occur in a wide variety of forms and leaf structures; including annuals and perennials, and broad-leaved plants as well as the grasses. Broad-leaved herbs are termed forbs in distinction with grasses. The adjective herbaceous is applied to this life-form. The herbaceous layer will normally occupy a low position in a layered or stratified plant community. Smaller, and laying in close contact with the ground or attached to tree trunks are the bryoids, a life-form named for plant phylum, which includes the mosses and liverworts.
Finally, among the life-forms are the epiphytes, plant, which use other plants as supporting structures and thus live above the ground level, out of contact with soil. Familiar to all are the tropical orchids that live high upon tree limbs and are sometimes referred to as air plants. Ferns also commonly exist as epiphytes.
Not included in our list of life-forms are lower forms of plants, grouped under the name of thallophytes. These include bacteria, algae, molds, and fungi--all of which are plants lacking true roots, stems and leaves. Lichens, plant forms in which algae and fungi live together to form a single plant structure, are included within the life-form classification of bryoids. Lichens occur as tough, leathery coatings or crusts and as leaflike masses attached to rocks and tree trunks. In arctic and alpine environments lichens may grow in profusion and dominate the vegetation in the near absence of more conspicuous plant forms.
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