Trees
To many, the word trees evokes images of such ancient, powerful, and majestic structures as the redwood and the giant sequoia (红杉), among the most massive and longest living organisms in the world．Although the majority of the Earth's biomass is represented by trees, the fundamental importance of these seemingly ubiquitous plants for the very existence and diversity of life on Earth is perhaps not fully appreciated．Our very biosphere is dependent on the metabolism, death, and recycling of plants, especially trees．Their vast trunks and root systems store carbon dioxide and water and respire oxygen into the atmosphere．The organic matter of the soil develops primarily from plant residues (that is, from decayed leaves, twigs, branches, roots, and fallen trees), which release important nutrients, such as nitrogen, carbon, and oxygen.
The tree is not an immutable biological category but rather a human concept based on visual criteria．Perhaps a general definition would describe a tree as a perennial(多年生的) woody plant that develops along a single main trunk to a height of at least 4.5 metres at maturity．This may be contrasted with a shrub, which might be loosely defined as a woody plant with multiple stems; in most cases, a shrub is less than 3 metres tall．However, a species fitting the description of either in one area of the world might not necessarily do so in other regions, since a variety of stresses shape the habit of the mature plant．Thus, a given woody species may be a tree in one set of habitats within its range and a shrub elsewhere．For example, the spruce and fir may thrive in the tree form．at the base of a mountain but assume a shrub form．near the mountain-top, the variation due principally to stresses exerted by such environmental conditions as altitude, temperature, and oxygen tension.
In the early stages of the development of terrestrial life, land plants were rootless and leafless, since they had their origins in aqueous environments, they did not require the specialized conducting and supporting tissues afforded by roots and stems, nor did they require localized regions of carbohydrate synthesis, since each cell was involved in metabolism, water and nutrient absorption, and respiration．Habitats farther from the water as well as aerial habitats represented available uninhabited environments.
One key to exploiting these habitats is large size．This, however, requires physiological and morphological (形态的) complexity．If all the tissues of massive tree trucks were alive, for example, the physiological cost of maintaining these structures in the living state would be enormous, and probably unattainable．The elegant solution came in the form．of tremendous structural adaptations, new tissues and organs permitted localization of the functions of the plant body.
The evolution of vascular tissues and localized regions of cell division permitted the strengthening and conducting tissue, called wood, to be dead, hollow, thick-walled tubes at functional maturity．Roots provided anchorage and absorption of sufficient amounts of water and nutrients to support the huge biomass of the tree．Stems were not only strong enough to support the tree and project it into ever higher habitats but conductive enough to transport the water and nutrients to the leaves at the very top of the tree.
The shape of a tree is an ecological construct as well, since its form．is dependent on the habitat and the stresses of the environment．Open-grown trees, such as those in gardens and parks, generally have foliage extending along the length of the trunk for a considerable distance．Forest trees, on the other hand, compete for growing space and generally have an expanse of foliage-free bole below a more limited tree crown．The aggregate of the tree crowns constitutes the canopy of the forest, and this may be displayed in a single layer or stratified (