Root

The root is the descending axis of the plant body. It is positively geotropic (grows towards gravity) and positively hydrotropic (grows towards water), while it is negatively phototropic (grows away from light) and negatively aerotropic (grows away from air). Because of this, the root grows beneath the soil surface. Roots are generally non-green, more or less cylindrical, and — unlike the stem — do not show nodes and internodes.

Regions of a typical root. Starting from the tip and moving towards the base, a root shows the following distinct regions:

• Region of the root cap — a parenchymatous, multicellular structure shaped like a cap that covers the young, growing root apex. Root cap cells secrete mucilage that lubricates the passage of the root through the soil, and they contain starch granules that help the root sense gravity (graviperception) and carry out geotropic movement. Usually a single root cap is present, but in plants such as Pandanus (screw pine) multiple root caps occur, while in hydrophytes like Pistia and Eichhornia the root cap is replaced by a root pocket. Because of the root cap, the actual growing apex of the root lies just beneath the surface (a subterminal position).
• Meristematic region (region of cell division) — the growing point of the root, about 1 mm long, protected by the root cap. It is made up of compactly arranged, thin-walled, actively dividing cells that bring about the lengthwise growth of the root.
• Region of elongation — lies just above the meristematic region. Its cells are newly formed and elongate rapidly, which is what increases the length of the root; these cells also help in the absorption of mineral salts.
• Region of root hairs (piliferous/absorption zone) — made up of numerous hair-like outgrowths. The epiblema (piliferous layer) produces tubular, elongated, unicellular root hairs that stay in close contact with soil particles, greatly increasing the surface area available for water absorption. Root hairs are short-lived (ephemeral) and are replaced roughly every 10–15 days. …