Roots

Core Concepts

Root functions and the two root systems

Roots perform four jobs: anchorage (holding the plant against wind and pull), absorption of water and minerals, transport of those materials to the shoot, and storage of carbohydrates. The two architectures reflect different strategies. A taproot — one dominant root growing straight down, with smaller lateral roots — reaches deep water tables and stores food (carrot, beet, dandelion). A fibrous system — many roughly equal thin roots spreading horizontally near the surface — captures water from light rains and binds topsoil, which is why grasses and grain crops prevent erosion. Dicots tend toward taproots; monocots toward fibrous roots.

The root tip: zones of growth

A root lengthens only near its tip, in a series of zones:

• Root cap — a thimble of cells at the very tip that protects the dividing cells behind it as the root pushes through abrasive soil. Its cells are constantly sloughed off and replaced, and it senses gravity (via starch-filled statoliths that settle to the lowest side of the cell), steering the root downward.
• Zone of cell division (meristematic zone) — the apical meristem; small, rapidly dividing cells that produce all new root cells.
• Zone of elongation — cells stop dividing and stretch lengthwise, often many times their original size; this expansion pushes the root tip forward through the soil.
• Zone of maturation (differentiation) — cells finish differentiating into mature tissues — epidermis, cortex, endodermis, xylem, phloem — and root hairs appear: single epidermal cells with long extensions that enormously increase the surface area for absorption.

Internal anatomy: the path from soil to xylem

Working inward from the surface: the epidermis (bearing root hairs) surrounds the cortex, a thick zone of parenchyma that stores starch and through which water must pass. Inside the cortex lies the endodermis — the single layer of cells that rings the vascular cylinder. The endodermis is the gatekeeper because of the Casparian strip: a band of waterproof suberin embedded in the radial walls of each endodermal cell. Water and dissolved ions moving between cells (the apoplast pathway, through cell walls and intercellular spaces) are physically blocked at the endodermis; they are forced into the cells themselves (the symplast pathway, through the cytoplasm, which is continuous between cells via plasmodesmata). This forces selective, membrane-controlled uptake: the plant chooses what enters the vascular system. Inside the endodermis lie the pericycle (a meristematic layer from which lateral roots arise — that is why branch roots emerge from deep inside, not from the surface) and the central vascular cylinder (stele) with xylem (often X-shaped) and phloem between the arms.

Water and mineral uptake

Water enters root hairs by osmosis whenever the soil is more dilute than the root cells; minerals are taken up by active transport, which requires energy and carrier proteins. Because active transport concentrates ions inside the root, water follows by osmosis — the osmotic gradient is what drives water into the xylem. Mycorrhizae make absorption far more efficient: fungal hyphae extend far beyond the root hairs, giving the plant a much larger absorbing surface, and the fungus delivers phosphorus and other minerals in exchange for sugars. Most land plants form mycorrhizae — it is the rule, not the exception.

Modified roots

Roots have been modified for storage and special habitats: storage roots (carrot, beet, sweet potato — a root, unlike the potato, which is a stem); adventitious roots that arise from stems or leaves rather than the primary root; prop roots (corn) that brace tall shoots; aerial roots (orchids, epiphytes) that absorb moisture from the air; and pneumatophores (mangroves) — vertical breathing roots that stick up out of waterlogged, oxygen-poor mud to supply the submerged roots with oxygen. Each modification solves a specific environmental problem.

Root symbioses: mycorrhizae and nitrogen-fixing nodules

Two partnerships dominate. Mycorrhizae (fungus + root) improve mineral uptake, especially phosphorus; ectomycorrhizae form a sheath around the root, while arbuscular (endomycorrhizae) hyphae penetrate between or into root cells. Nitrogen-fixing nodules are swellings on the roots of legumes (and a few other plants) containing Rhizobium bacteria that convert atmospheric N₂ into ammonia the plant can use, in exchange for sugars — the same mutualism covered in the prokaryote chapter, now seen from the plant's side.

Common Confusions