TYPES OF ROOTS

TYPES OF 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. There are three primary functions of roots: (1) to anchor the plant to a substrate, (2) to absorb water and dissolved minerals, and (3) to store food reserves. Typically we see roots in soil, but there are specialized types of aerial roots (air roots) that enable climbing plants and epiphytes to become attached to rocks, bark, and other nonsoil substrates. In addition, parasitic plants may form specialized haustorial roots that form an attachment disc to the host during the first stage of colonization. To absorb water and dissolved minerals, a young sector of a root commonly possesses numerous single-celled projections called root hairs, which greatly increase the absorbing surface of the root and achieve much greater contact with soil particles. Water uptake into the young root is rapid because there is little resistance through the outer cell walls, and in general these walls contain virtually no water-repellent wax (cutin). Both young and old roots can be important repositories for carbohydrates, usually in the form of starch grains located in root cortex, but in addition older roots may store massive quantities of starch and even become specialized below-ground storage organs. Storage of carbohydrates in roots and other below-ground plant organs is an important plant strategy for surviving stress and dormancy, just as certain mammals store extra fuel as fat for winter. 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. The radicle (note spelling) is the initial root of a plant, the one that is generally present on the embryo within the seed. This forms the primary root of a young plant. In certain

lineages, the embryo is so tiny and immature, such as in microseeds of orchids (Family Orchidaceae), that a radicle is not present. 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 belowground 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 Variations of Roots • •

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Nodal roots: adventitious roots that form characteristically in rings from stem tissues around a node. Aerial roots: roots that are formed in and exposed to air, e.g., by epiphytes and hemiepiphytes; in some species, aerial roots grow downward from the tropical tree canopy toward the ground as extremely long, unbranched roots. 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

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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.

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Root system: It is part Plate 1: Stem and which Root Types 323 × 402 - 19k - jpg 700 × 1055 depssa.ignou.ac.in 262k - jpg vplants.org

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Root systems The roots of a plant are connected differently in different plant species and this is known as the root system. There are two main types of root systems :

Tap root system

Fibrous root system

The first root produced from a seed is called the radicle. In many dicotyledonous plants this root greatly enlarges to become the most prominent root of the plant and is A tap root system known as a tap root. Many smaller branch roots may grow from the tap root.

Fibrous root system In monocotyledonous plants, the radicle is short lived and is replaced by numerous roots of more or less equal size. These roots are adventitious which means they can grow from plant organs other than roots e.g. stems.

Different types of roots A tree or plant's roots can have functions apart from anchoring it and absorbing water and nutrients from the soil. There are many different types of specialised roots which have evolved in both trees and plants. The image below shows some interesting examples. Click on it when it indicates a specialised root type about which you want to learn more.

Sometimes it is not so easy to know whether a plant structure is a root or a stem. There are some stems which grow under the ground and there are some roots which grow above the ground. See if you know which of these common vegetables are roots. You may be in for a surprise !

The Root Veggie Game