Root
- For other meanings of root, see Root (disambiguation).
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In vascular plants, the root is that organ of a plant body that typically lies below the surface of the soil (compare with stem). However, this is not always the case, since a root can also be aerial (that is, growing above the ground) or aerating (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground is not exceptional either (see rhizome). So, it is better to define root as a part of a plant body that bears no leaves, and therefore also lacks nodes. There are also important internal structural differences between stems and roots. The two major functions of roots are 1) absorption of water and inorganic nutrients and 2) anchoring the plant body to the ground.
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Root structure
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At the tip of every growing root is a conical covering of tissue called the root cap. It usually is not visible to the naked eye. It consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the apical meristem. The root cap provides mechanical protection to the meristem cells as the root advances through the soil, its cells worn away but quickly replaced by new cells generated by cell division within the meristem. The root cap is also involved in the production of mucigel, a sticky mucilage that coats the new formed cells. These cells contain statoliths, starch grains that move in response to gravity and thus control root orientation.
The outside surface of a root is the epidermis. Recently produced epidermal cells absorb water from the surrounding environment and produce outgrowths called root hairs that greatly increase the cell's absorptive surface. Root-hairs are very delicate and generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as osmosis. For this reason, water that is saline is more difficult for most plant species to absorb.
Beneath the epidermis is the cortex, which comprises the bulk of the root. Its main function is storage of starch. Intercellular spaces in the cortex aerate cells for respiration. An endodermis is a thin layer of small cells forming the innermost part of the cortex and surrounding the vascular tisues deeper in the root. The tightly packed cells of the endodermis contain a substance known as suberin and create an impermeable barrier of sorts. Water can only flow in one direction through the endodermis: in towards the center of the root, rather than outward from the stele to the cortex.
The vascular cylinder or stele consists of the cells inside the endodermis. The outer part, known as the pericycle, surrounds the actual vascular tissue. In monocotyledonous plants, the xylem and phloem cells are arranged in a circle around a pith or center, whereas in dicotyledons, the xylem cells form a central "hub" with lobes, and phloem cells fill in the spaces between the lobes.
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Root growth
Early root growth is a function 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 sacrificed to protect the meristem), and undifferentiated root cells. The latter will 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.
Roots will generally grow in any direction where the correct environment of air, nutrients and water exists to meet the plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At germination, roots grow downward due to gravitropism, the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as ivy), the "root" actually clings to walls and structures; this is known as thigmotropism, or response to touch.
Most plants experience growth only along the apical meristems; this is known as primary growth, which encompasses all vertical growth. On the other hand, secondary growth encompasses all lateral growth, a major component of woody plant tissues. 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, found only in woody plants.
In woody plants, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The cambium layer 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 (noting that this process only occurs in woody plants) begins to form the periderm, consisting of protective cork cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
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The vascular cambium produces new layers of secondary xylem annually. This dead tissue is responsible for most water transport through the vascular tissue (systems and roots).
Types of roots
A true root system consists of a primary root and secondary roots (or lateral roots).
The primary root originates in the radicle of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized:
- the taproot: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward.
- the primary root is not dominant: the whole root system is fibrous and branches in all directions.
Adventitous roots arise from the stem and not from another root. They usually occur in monocots and pteridophytes, but also in a few dicots, such as strawberry (Fragaria vesca) and white clover (Trifolium repens).
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Specialized roots
The roots, or parts oof roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.
- Aerating roots (or pneumatophores): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia)
- Aerial roots: roots entirely above the ground, such as in ivy (Hedera helix) or in epiphytic orchids. They function as prop roots or anchor roots.
- Contractile roots: they pull bulbs or corms of monocots deeper in the soil through expanding radially and contracting longitudinally. They show a wrinkled surface.
- Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and Rafflesia.
- Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in plants from the following families Betulaceae, Casuarinaceae, Eleagnaceae, 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 nutrients, such as carrots and beets
- Tubiferous roots: A portion of a root forms into a roundish knob called a (tuber) for food storage.
Rooting depths
The distribution of roots within the soil depends on plant life form, and the spatial and temporal availability of water and nutrients in 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.
See also
- Rhizophilous - organisms which thrive in a proximity or in a symbiotic relationship with plant roots.
- Mycorrhiza - root symbiosis in which individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant.
- fibrous root system
- stolon
References
- Brundrett, M. C. 2002. Coevolution of roots and mycorrhizas of land plants. New phytologist 154(2): 275-304. (Available online: DOI | Abstract | Full text (HTML) | Full text (PDF))
- Chen, R., E. Rosen, P. H. Masson. 1999. Gravitropism in Higher Plants. Plant Physiology 120 (2): 343-350. (Available online: Full text (HTML) | Full text (PDF)) - article about how the roots sense gravity.
- Clark, Lynn. 2004. Primary Root Structure and Development - lecture notes
- Raven, J. A., D. Edwards. 2001. Roots: evolutionary origins and biogeochemical significance. Journal of Experimental Botany 52 (Suppl 1): 381-401. (Available online: Abstract | Full text (HTML) | Full text (PDF))
- Schenk, H.J., and R.B. Jackson. 2002. The global biogeography of roots. Ecological Monographs 72 (3): 311-328.
- Phillips, W.S. 1963. Depth of roots in soil. Ecology 44 (2): 424.