Root Hairs. It has been estimated that there may be a total length of a mile in the roots of a corn plant, and alfalfa roots have been found to extend twenty feet deep in dry soil.
For the purpose of absorbing as much as possible, the surface of the active parts of all roots is covered with root hairs. These are outgrowths of the epidermal cell walls and increase the total absorbing surface enormously. They also enable the osmotic membrane to almost touch the film of water, which, even in the driest soils, clings close to the soil grains.
So important are these root hairs that their injury or loss might mean death to the plant, hence they are never borne at the extreme tip of the root, where its growth through the soil would strip them off, but are found a little back from the tip and extending various distances along the younger roots.
As the root grows, new hairs are produced near the tip, to gather moisture from new areas; the upper ones die away; the cortex and epidermis thicken, cease active absorption, and become protective in use. In frequent cases, the root hairs secrete a weak acid which helps in dissolving soil substances and in penetrating hard earth.
The adaptations of root hairs may be summarized as follows:
- Extent of surface.
- Thinness of walls.
- Protection from injury.
- Location.
- Close contact with soil grains.
- Acid secretion.
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