In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the surface of the soil, but roots can also be aerial or aerating, that is, growing up above the ground or especially above water.
Function Roots perform several essential and specialised roles that support plant growth, development and survival. Their primary functions are anchorage, uptake (absorption) of water and dissolved minerals, and conduction of these resources to the shoot. Beyond these, roots carry out a range of important secondary and adaptive functions — storage of reserves, synthesis of growth regulators, gas exchange in waterlogged environments, facilitation of symbiotic nutrient acquisition, and vegetative propagation.
Primary functions
Anchorage and mechanical support. Roots fix the plant in the substrate and resist mechanical forces (wind, waterflow); vigorous root systems are essential for crop stability and prevention of lodging.
Absorption of water and mineral nutrients. Root epidermal cells and root hairs take up soil water and dissolved ions; these are then conducted to the shoot via the xylem. Root architecture (depth, spread) determines access to water and nutrients in different soil horizons.
Secondary and specialised functions
Storage of reserve food and water. Many roots (fleshy taproots, tuberous roots) accumulate carbohydrates and water that support perennial growth or regrowth after dormancy. Examples include sweet potato and carrot.
Conduction. Roots conduct absorbed water, minerals and some synthesised compounds to the shoot via vascular tissues (xylem and phloem connections).
Synthesis of growth regulators. Roots synthesise and modulate plant hormones (e.g., cytokinins, some auxin metabolism), influencing shoot development and whole-plant physiology.
Aeration and gas exchange (in aquatic/waterlogged soils). Specialised structures (pneumatophores, aerial roots) permit gas exchange where soil oxygen is limited, enabling respiration in mangroves and other hydrophytes.
Symbiotic interactions and enhanced nutrient uptake. Roots form mutualisms (e.g., mycorrhizae, nitrogen-fixing nodules) that greatly increase nutrient acquisition and stress tolerance. These associations remodel root architecture and function.
Mechanical adaptations and additional support. Adventitious and brace/prop roots (e.g., maize, some mangroves, banyan) provide extra support, anchorage and stability in particular environments.
Vegetative propagation. Adventitious rooting on stems, nodes or cuttings allows many species to reproduce asexually and to regenerate after damage; this is widely exploited in horticulture.
Notes
Many functions overlap: a given root can simultaneously anchor, absorb, store reserves and engage in symbiosis. Root form and function are plastic and shaped by species genetics and environmental context (soil texture, water availability, aeration and mechanical stresses).
Types of roots (major rooting system) Plants produce a variety of root systems that differ in origin, structure and function (anchorage, absorption, storage, aeration and vegetative propagation). The two classical, broad categories are taproot and fibrous systems, but several specialised root types — notably adventitious, aerial, prop/stilt, climbing/adhesive, buttress, tuberous (storage) and floating roots — are biologically and ecologically important.
Taproot system A taproot system is dominated by a single, vertically growing primary root (the radicle) from which lateral roots arise. Taproots often function in deep anchorage and in storage of carbohydrates and water (common in many dicotyledons and some biennials/perennials). Examples include carrot (Daucus carota), dandelion (Taraxacum) and many true dicots.
Fibrous root system A fibrous root system consists of numerous, similarly sized roots that form a dense network near the soil surface. In many species this network is composed largely of adventitious roots that arise from the stem base rather than the primary radicle. Fibrous systems are effective at soil binding, rapid uptake of surface nutrients and erosion control. Typical examples are grasses (Poaceae), wheat and rice.
Adventitious roots Adventitious roots arise from non-root organs (stems, nodes, leaves or callus tissue) and play multiple roles: replacing or supplementing the primary root, providing mechanical support, enabling vegetative propagation, and forming specialized root types (aerial roots, prop roots, pneumatophores). They are especially important in monocots and many cultivated plants, and are a common response to wounding, flooding or other stressors.
Examples and notes: in many monocots the functional root system is adventitious (forming a fibrous habit); maize (Zea mays) produces nodal brace/prop roots that stabilise the stem; banyan (Ficus spp.) develops aerial adventitious roots that may become supportive trunks; many cuttings root adventitiously during vegetative propagation.
Anatomy
Root morphology is divided into four zones: the root cap, the apical meristem, the elongation zone, and the hair. The root cap of new roots helps the root penetrate the soil. These root caps are sloughed off as the root goes deeper creating a slimy surface that provides lubrication. The apical meristem behind the root cap produces new root cells that elongate. Then, root hairs form that absorb water and mineral nutrients from the soil. The first root in seed producing plants is the radicle, which expands from the plant embryo after seed germination.
When dissected, the arrangement of the cells in a root is root hair, epidermis, epiblem, cortex, endodermis, pericycle and, lastly, the vascular tissue in the centre of a root to transport the water absorbed by the root to other places of the plant.
Perhaps the most striking characteristic of roots that distinguishes them from other plant organs such as stem-branches and leaves is that roots have an endogenous origin, i.e., they originate and develop from an inner layer of the mother axis, such as pericycle. In contrast, stem-branches and leaves are exogenous, i.e., they start to develop from the cortex, an outer layer.
In response to the concentration of nutrients, roots also synthesize cytokinin, which acts as a signal as to how fast the shoots can grow. Roots often function in storage of food and nutrients. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizae, and a large range of other organisms including bacteria also closely associate with roots.
Root system architecture (RSA)
Definition In its simplest form, the term root system architecture (RSA) refers to the spatial configuration of a plant's root system. This system can be extremely complex and is dependent upon multiple factors such as the species of the plant itself, the composition of the soil and the availability of nutrients. Root architecture plays the important role of providing a secure supply of nutrients and water as well as anchorage and support.
The configuration of root systems serves to structurally support the plant and compete with other plants for uptake of nutrients within the soil. Roots grow in response to specific ecological conditions, which, if changed, can impede a plant's growth. For example, a root system that has developed in dry soil may not be as efficient in flooded soil, yet plants are able to adapt to other changes in the environment, such as seasonal changes.
Terms and components The main terms used to classify the architecture of a root system are:
Branch magnitude Number of links (exterior or interior) Topology Pattern of branching (Herringbone, Dichotomous, Radial) Link length Distance between branches Root angle Radial angle of a lateral root's base around the parent root's circumference, the angle of a lateral root from its parent root, and the angle an entire system spreads. Link radius Diameter of root
All components of the root architecture are regulated through a complex interaction between genetic responses and responses due to environmental stimuli. These developmental stimuli are categorized as intrinsic, the genetic and nutritional influences, or extrinsic, the environmental influences, and are interpreted by signal transduction pathways.
Extrinsic factors affecting root architecture include gravity, light exposure, water and oxygen, as well as the availability or lack of nitrogen, phosphorus, sulphur, aluminium and sodium chloride. The main hormones (intrinsic stimuli) and respective pathways responsible for root architecture development include: Auxin Lateral root formation, maintenance of apical dominance and adventitious root formation. Cytokinins Cytokinins regulate root apical meristem size and promote lateral root elongation. Ethylene Promotes crown root formation. Gibberellins Together with ethylene, they promote crown primordia growth and elongation. Together with auxin, they promote root elongation. Gibberellins also inhibit lateral root primordia initiation.
Growth
Early root growth is one of the functions of the apical meristem located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these are sacrificed to protect the meristem), and undifferentiated root cells. The latter become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.
Growth from apical meristems is known as primary growth, which encompasses all elongation. Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of sweet potato have secondary growth but are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm.
In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The vascular cambium forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem including the epidermis and cortex, in many cases tend to be pushed outward and are eventually "sloughed off" (shed).
At this point, the cork cambium begins to form the periderm, consisting of protective cork cells. The walls of cork cells contains suberin thickenings, which is an extra cellular complex biopolymer. The suberin thickenings functions by providing a physical barrier, protection against pathogens and by preventing water loss from the surrounding tissues. In addition, it also aids the process of wound healing in plants. It is also postulated that suberin could be a component of the apoplastic barrier (present at the outer cell layers of roots) which prevents toxic compounds from entering the root and reduces radial oxygen loss (ROL) from the aerenchyma during waterlogging. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity (in some) but are responsible for most water transport through the vascular tissue in stems and roots.