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 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.
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