Plant Roots

Roots are the principal water-absorbing organs of a plant. They are present on essentially all vascular plants, although roots are never formed on the primitive-looking whisk fern (Psilotum) and its closest relatives (Order Psilotales), on Wolfiella (the tiniest duckweed), and on the plant body of certain atmospheric epiphytes, such as Spanish moss (Tillandsia). In fact, a root, by definition, must have vascular tissues, i.e., water conduits in xylem and sugar conduits in phloem, arranged in a particular way ("exarch"). Much thinner, threadlike rhizoids (means "root-like") are present on the nonvascular plants, such as mosses and liverworts, and on gametophytes of vascular plants without seeds, such as ferns, horsetails, and club mosses. Rhizoids also absorb water but totally lack vascular tissues.

The first root that comes from a plant is called the radicle. The four major functions of roots are 1) absorption of water and inorganic nutrients, 2) anchoring of the plant body to the ground, and supporting it, 3) storage of food and nutrients, 4) vegetative reproduction. Roots may be assisted in their function by other organisms living in the substrate. Many plants, including the majority of vascular plants and even the free-living gamatophytes, are involved in symbiotic relationships with fungi, called mycorrhizae.

Particular soil fungi grow either on the outside or on the inside of a root. This mycorrhizal association improves water absorption and the uptake of certain minerals from the soil. Certain genera of plants have roots that are inoculated with colonies of nitrogen-fixing microorganisms, especially legumes and their associated nitrogen-fixing bacteria (rhizobial bacteria). Living in tumor-like root nodules, nitrogen-fixing bacteria are able to convert atmosphere nitrogen gas to ammonia, under anaerobic conditions produced by the plant cells, and then use this fixed nitrogen to make amino acids. So, it this regard, root physiology may be involved in a very special way to deliver nutrients to the shoot.

TYPES OF ROOTS

A true root system consists of a primary root and secondary roots.

There are several possible fates of the primary root. In gymnosperms and dicotyledons, the primary root commonly grows to become a thick central root, the taproot, which may or may not have thick lateral roots (branches). This structural organization is frequently termed a taproot system, although in many old woody plants there may be many roots that are essentially the same diameter. The easiest designation of taproot is for something like a carrot (Daucus carota), where the lateral (secondary) roots are very thin, so that plant indeed has a single, thick central root. What may appear to be a taproot can also include enlarged portions of the hypocotyl (of the seedling) or even tissues of the lower stem.

 In monocotyledons, the radicle is very short-lived, and before it dies other adventitious roots have already originated from shoot or mesocotyl tissue to become the new root system, called a fibrous root system. Fibrous roots are typically thought of as slender, often with few or no lateral roots. However, many monocotyledons have below-ground adventitious roots that are thicker than a pencil, and in some the fibrous roots above-ground, such as the prop or stilt roots of screwpines (Pandanus) and certain palms (Family Arecaceae), can be as thick as an arm.

Adventitious roots are the ones that form from shoot tissues, not from another (parent) root. Most commonly, adventitious roots arise out of stems, originating via cell divisions of the stem cortex or less often from axillary buds hidden in the bark. In some plants leaves can also be encouraged to form adventitious roots. The field of horticulture is based in large part on cloning plants from cuttings of stems or leaves that form adventitious roots. [More examples: adventitious roots of a palm; of a Canary Island date palm; specialized adventitious roots of an epiphytic orchid; of an aquatic plant that has unattached roots in moving water] 

Certain "root crops" that botanically are below-ground shoots, such as tubers, bulbs, rhizomes, and corms, form adventitious roots when planted in soil. Vegetative reproduction (apomixis) of cacti and other succulent plants is also achieved largely by rooting either stems or leaves using methods to stimulate adventitious root formation.

Specialized roots

The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.

Aerating roots (or knee root or knee or pneumatophores or Cypress knee): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia). In some plants like Avicennia the erect roots have a large number of breathing pores for exchange of gases.

Aerial roots: roots entirely above the ground, such as in ivy (Hedera) or in epiphytic orchids. They function as prop roots, as in maize or anchor roots or as the trunk in strangler fig.

ontractile roots: they pull bulbs or corms of monocots, such as hyacinth and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.

Coarse roots: Roots that have undergone secondary thickening and have a woody structure. These roots have some ability to absorb water and nutrients, but their main function is transport and to provide a structure to connect the smaller diameter, fine roots to the rest of the plant.

Fine roots: Primary roots usually <2 mm diameter that have the function of water and nutrient uptake. They are often heavily branched and support mycorrhizas. These roots may be short lived, but are replaced by the plant in an ongoing process of root 'turnover'.

Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and dodder.

Propagative roots: roots that form adventitious buds that develop into aboveground shoots, termed suckers, which form new plants, as in Canada thistle, cherry and many others.

Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in Proteaceae and some plants from the following families Betulaceae, Casuarinaceae, Elaeagnaceae, Moraceae, Fabaceae and Myricaceae.

Stilt roots: these are adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil.

Storage roots: these roots are modified for storage of food or water, such as carrots and beets. They include some taproots and tuberous roots.

Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees.

Surface roots: These proliferate close below the soil surface, exploiting water and easily available nutrients. Where conditions are close to optimum in the surface layers of soil, the growth of surface roots is encouraged and they commonly become the dominant roots.

Tuberous roots: A portion of a root swells for food or water storage, e.g. sweet potato. A type of storage root distinct from taproot.

Rooting Depths

The distribution of vascular plant roots within soil depends on plant form, the spatial and temporal availability of water and nutrients, and the physical properties of the soil. The deepest roots are generally found in deserts and temperate coniferous forests; the shallowest in tundra, boreal forest and temperate grasslands.

                                           

The deepest observed living root, at least 60 m below the ground surface, was observed during the excavation of an open-pit mine in Arizona, USA. Some roots can grow as deep as the tree is high. The majority of roots on most plants are however found relatively close to the surface where nutrient availability and aeration are more favourable for growth. Rooting depth may be physically restricted by rock or compacted soil close below the surface, or by anaerobic soil conditions.

Specialized Variations of Roots

Nodal roots: adventitious roots that form characteristically in rings from stem tissues around a node.

Prop or stilt roots: adventitious roots that develop on a trunk or lower branch that begin as aerial roots (another example; reaching for the water) but eventually grow into a substrate of some type; these roots in some cases seem to provide mechanical support, having either good compression or tensile properties to help support trees at their bases.

Buttress or tabular roots: vertically flattened roots that project out of the ground and lower trunk at the base of large trees. Models have suggested how these buttresses provide additional tensile forces to resist uprooting of large tropical trees.

Contractile roots: roots that become shortened in length (shrivel or shrink in length) and thereby draw the plant or plant part downward into the soil profile; many examples can be found among bulbous plants.

Pneumatophores: spongy, aerial roots of marsh or swamps, such as in mangal (mangroves), where roots are present in waterlogged soils and cannot obtain enough oxygen for maintaining healthy tissues. Here, pneumatophores are "breathing roots" that are emergent, and they have special air channels (lenticels) for gas exchange in the atmosphere (air enters at zones called "pneumathodes") and there is an internal pathway for getting O2 into the root and to supply submerged roots. The aerial loop of a mangrove root is sometimes called a "knee" or "peg root," but it is not clear that knees are necessarily breathing roots.

Caudex or lignotuber: a taproot that has fused with the stem may become woody. Lignotubers often occur in seasonally dry or fire-prone habitats, and the plants appear to use this strategy to recover from dormancy or fire.

Haustorial root: the root of particular parasitic plants that become cemented to the host axis via a sticky attachment disc before the root or sinker intrudes into the tissues of the host.

Strangling roots: the special name for roots of strangling figs (Ficus), which are primary hemiepiphytes that begin life as tropical epiphytes in trees and send down adventitious roots that become rooted in the soil. The roots surround the host trunk, eventually strangling the bark and killing the host tree.

Root tubers: swollen portions of a root that can have buds to produce new shoots; when broken off, these can grow into a new plant, so this is a form of cloning. In the older literature, these were sometimes referred to as fascicled roots.