4.2 Read and translate the text 1 using the active vocabulary and a dictionary. Text 1 Root structure

Figure 4.1

Roots are usually found at the lowest end of the tree and spread in a vast and intricate network (figure 4.1), like underground branches. They usually extend as far underground as the twigs spread in the crown of the tree. If laid end to end, the roots of some giant oaks would stretch more than 160 km in length. A tree’s roots absorb water and nutrients from the soil, store sugar and anchor the tree upright in the ground. The tree uses these to manufacture food and grow.

The first root to appear when a seed germinates is a primary root. In dicots this root grows down into the soil and becomes both strong and thick. This mature root in a dicot is called a taproot (figure 4.2). A carrot is an edible taproot. Some plants in dry areas have a taproot that reaches down as far as 20 feet. In monocots such as grass and corn, the primary root is usually short-lived, and the root system doesn’t develop from this root. Instead numerous long, thin roots grow from the stem and spread out through the soil. These roots and their side branches are fibrous roots. No single root is more prominent than the others in a fibrous root system.

T he type of roots formed initially is specific to a given species; with age the initial root form is often modified by the growing environment. Such thing as soil hard-pans, water tables, texture, structure, and degree of compaction all influence the mature root form.

There are three basic classes of tree root systems (figure 4.2):

1. Tap root (hickory, walnut, butternut, white oak, hornbeam);

2. Heart root (red oak, honey locust, basswood, sycamore, pines);

3. Flat root (birch, fir, spruce, sugar maple, cottonwood, silver maple, hackberry).

All trees have lateral roots that branch into smaller and smaller roots and usually extend horizontally beyond the branch tips. Each root is covered with thousands of root hairs that make it easier to soak up water and dissolved minerals from the soil. The majority of the root system is located in the upper 12 to 18 inches of soil because the oxygen that roots require to function properly is most abundant there. Roots branch into smaller and smaller structures, where new growth occurs. This newest area of growth is lined with roots hairs. It is these structures which penetrate into the soil and absorb the nutrients required by the tree.

Each root hair is a thin-walled extension of an epidermal cell. Root hairs absorb necessary gases, minerals and water through diffusion and active transport. These substances are then passed through adjacent cells and into the conducting tissues of the root and eventually to the stem and leaves of the tree where required. The terminal portion of a root contains a variety of cells. The root cap is found at the tip of the root and consists of cells which protect the meristem or area of actively dividing cells.

Roots of most species of trees are invaded by soil fungi to form root-fungus structures called mycorrhizae. The mycorrhizal association (figure 4.2) is beneficial to both the tree and the fungus. The tree supplies carbohydrates and other growth requirements to the fungus, and the fungus increases water and mineral uptake (particularly phosphorus) of the host tree by increasing the total absorptive area of the root system. There are more than 2500 different fungi which form mycorrhizal relationships with trees; often there are several different fungi associated with an individual tree. The presence of this association is necessary for establishment and growth of many trees; its absence has often reduced the success of new tree plantings, especially on old field sites. Nurseries are now careful to maintain the mycorrhizae populations in the nursery beds.