The root system is greatly influenced and modified by the environment. In shallow soils over rock, heavy clay soils, or on sites with a high level of underground water, even species whose roots normally penetrate to great depths may have shallow anchorage.
The outer live part is called sapwood and this serves as the tree’s pipeline for conducting water and other important substances up to the crown. In broad-leaved trees this function is performed by the broad tubular cells, called tracheae, visible in the cross-section as small pores. In conifers these tubular cells are narrower and shorter and are called tracheids.
In the cells and cell walls of heartwood various organic and inorganic substances are stored, e.g. tannins, resins, silicon dioxide, etc. In some trees such as the yew, larch, pine and oak the heartwood is further distinguished from the sapwood by a darker coloration. Heartwood is generally much more durable and of higher quality than sapwood and, in the case of some tropical trees in which the wood is subject to rapid decay and damage by pests, the soft sapwood is hacked off on the spot and only the heartwood is shipped for further processing.
The xylem is surrounded by a thin layer of phloem which conducts the organic substances manufactured by the leaves down to the trunk and roots. It consists of long tubular cells with perforated partitions placed end to end. The surface of the tree trunk is covered with bark which protects it from excessive evaporation, sudden changes in temperature and mechanical damage. Protection against undue evaporation is very important, for great quantities of water are conducted through the outer woody layers.
Most woody plants live in an association, or symbiosis, with fungi whose filamentous growths (hyphae) either encircle the rootlets of the plant (ectomycorrhiza) or penetrate the cells of the root surface (endomycorrhiza). The fungus makes the nutriments in the soil more easily accessible to the tree, besides supplying it with other complex compounds. It receives from the tree substances in return, mainly sugars. Chief ectomycorrhizal associates in the case of forest trees are mushrooms or other mushroom-like fungi. It is well known that certain species of mushroom grow in association with certain species of trees, e.g. the species Leccinum is generally found in the company of the birch, aspen and hornbeam, Boletus beside the pine, oak, beech and spruce, .Cuilltis is a companion of the larch, etc.
The symbiosis of mushrooms and woody plants occurs primarily in soils rich in organic substances and raw humus; in soils with insufficient organic matter such a symbiotic association may change to a parasitic one.
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