Читать книгу Collins Complete Guide to British Trees: A Photographic Guide to every common species - Paul Sterry - Страница 13

The first part of a tree to emerge from a seed is a tiny root whose first function is to draw up water and dissolved minerals from the soil. In the case of most of our tree species, successful germination is dependent upon this first root making contact with a species-specific symbiotic fungal partner, a relationship that continues for the rest of the tree’s life. This relationship is discussed in more detail on p. 34. From this simple start the tiny root will grow and divide, eventually forming a large network of powerful roots, side-branches and fine root hairs that spread out in all directions around the base of the trunk. The main roots will be woody and very strong, but their many branches terminate in fine root hairs that are only a few cells long; these have thin, permeable walls through which will pass all the water and minerals needed for the survival and growth of the tree. Although the sturdy roots strengthened with woody tissue help anchor the tree in the ground, it is the millions of fine root hairs that keep the tree alive by supplying it with water and nutrients. These fine root hairs are very short-lived, being constantly replaced as the main roots grow through the soil.

The root system of a large, mature tree does not penetrate far down into the soil. The most useful supply of dissolved nutrients for the tree lies in the shallow layer of topsoil and the adjacent sub-soil, so it is more beneficial if the roots spread outwards through this layer rather than penetrate to a great depth into a rather sterile and hostile layer. A 50mtall tree will probably have a spread of smaller branching roots all around the bole, the extent approximately equal to the spread of the branches or, sometimes, to the height of the tree. The proximity of other trees, the nature of the soil, and the presence of obstacles like rocks or river banks will all influence the final extent of the root system, however. This knowledge of the spread of the roots is useful when planning where to plant large trees that may damage drains or the foundations of buildings when they reach maturity, and it should also be borne in mind when digging ditches or ponds near large trees.

Spreading Beech roots.

In order to be able to function at all, roots require a supply of nutrients from the leaves, so within the root system there is a two-way traffic of water and minerals up from the soil to the leaves, and dissolved sugars and other nutrients down from the leaves.

The root hairs are living cells that require oxygen in order to be able to carry out respiration. They give off carbon dioxide as a waste gas, so they need access to air in the soil to allow these gases to circulate. Most trees, and most land plants, cannot grow in completely waterlogged soils and those that do have special adaptations for survival.

A number of trees, especially members of the Fabaceae, such as the Honey Locust, have many rounded nodules on the roots that contain colonies of nitrogen-fixing bacteria. These are able to use gaseous nitrogen and turn it into compounds vital to the growth of the plants.