Root: the underground organ of a plant that anchors it, absorbs water and minerals from the soil, and stores food. Root systems take two principal forms — taproots, in which a single dominant root grows vertically downward (as in carrots or oaks), and fibrous root systems, in which a mass of similarly sized roots spreads laterally through the soil (as in grasses). Root hairs — tiny extensions of epidermal cells near the root tip — vastly increase the absorptive surface area, allowing the plant to extract water and dissolved nutrients from the soil matrix.
Roots do not operate in isolation. They interact extensively with soil microorganisms, most notably mycorrhizal fungi, which form symbiotic associations with the root surface or penetrate root cells. These fungal partners extend the root’s effective absorptive reach by orders of magnitude, accessing water and phosphorus that the root alone could not obtain. In return, the plant supplies the fungus with sugars produced by photosynthesis. This partnership is ancient and nearly universal among land plants — it is one of the foundational instances of symbiosis in plant biology.
Roots also sense and respond to their environment. Gravitropism — the growth of roots downward in response to gravity — orients the root system into the soil. Hydrotropism — growth toward moisture gradients — guides roots toward water sources. These responses are mediated by hormone redistribution, particularly auxin, in the root tip and are coordinated with signals from the shoot via the phloem. The root is the plant’s primary interface with the soil environment, and its behavior is shaped by continuous feedback between the organism and its substrate.
Related terms
- Xylem — the vascular tissue that carries water and minerals absorbed by roots upward through the plant
- Phloem — transports sugars from leaves to roots and carries hormonal signals coordinating root growth
- Meristem — the root apical meristem generates new root tissue through cell division
- Symbiosis — mycorrhizal associations are a central example
- Stomata — transpiration through stomata drives the upward pull of water from roots