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.

Conversely, in humus-rich sandy soils the roots of woody plants which normally spread out may penetrate to greater depths. Roots anchor the tree in the ground, absorb water and the mineral elements dissolved in it, and serve as storage for reserve food supplies. The structure of the roots is very similar to that of the trunk, only somewhat simpler.

Water from the soil is absorbed through the root hairs, which are filamentous outgrowths near the tip of each rootlet. The movement of water in the trunk is governed both by the osmotic function of the roots and by the rate of transpiration in the leaves.

Roots aerate the soil and promote the disintegration of the rock substrata. In this they are aided by the carbon dioxide they eliminate as well as by certain chemical substances which break up mineral particles. The nutriments in the soil arc also made available by various bacteria and fungi growing in association with the roots. Nitrogen-fixing bacteria that increase the nitrogen content of the soil live in nodules attached to the roots of the alder, black locust, honey locust and other woody plants of the pea family (Leguminosae).

On the bark we can often see small round or slit-like patches that are slightly raised, and different in colour, from the surrounding bark. These are called lenticels and serve as a path for the exchange of gases between the atmosphere and the living cells inside the trunk and branches.

The bark thickens every year by the addition of a very thin layer of corky tissue. The thickness differs in various trees. Trees growing in the shade usually have thin bark, whereas those exposed to the sun often have thick bark which serves as protection against heat. Old, surface bark layers cannot adapt to the continuous thickening of the trunk and split - usually in furrows or scales. In some trees the bark remains permanently on the trunk (oak, elm), in others it peels off in scales (plane, sycamore), in lengthwise strips (cypress, eastern arbor-vitae) or crosswise strips (birch, cherry).