Evergreens are those that bear foliage throughout each year. Deciduous trees are broad-leaved and lose their foliage each year, usually at the approach of the coldest and darkest season. Angiosperms are further divided into monocots and dicots, depending on seed structure. Of the 60,000 to 70,000 species of trees, all are dicotyledonous except a few hundred monocotyledonous species and less than 1000 gymnospermous species. The most important gymnosperm orders are the Pinales and Taxales, which make up the conifers.
Trees grow wherever adequate groundwater is available for the major portion of the year.
They do not grow profusely in desert areas or in areas in which the groundwater table is sufficient only for grassland vegetation. Under optimum conditions, however, trees grow in large aggregations called forests. The climatic and soil requirements of trees are different for each species. Most tree species grow over large areas, of which only a small proportion permits optimum growth of the plant. The growth of trees requires the successive addition of many layers of woody tissue to the stem.