(Redirected from Adventitious roots)
Primary and secondary roots in a cotton plant
In
vascular plants, the 'root' is the 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. Roots also function in
cytokinin synthesis, which supplies some of shoot needs. They often function in storage of food.
Root structure
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 the primary 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 primary 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 tissues deeper in the root. The tightly packed cells of the endodermis contain a substance known as
suberin in their cell walls. This suberin layer is the
Casparian strip, which creates an impermeable barrier of sorts. Mineral nutrients can only move passively within root cell walls until they reach the endodermis. At that point, they must be actively transported across a cell membrane to continue further into the root. This allows the plant to accumulate mineral nutrients in the
stele.
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.
Secondary growth
All roots have primary growth or growth in length. Roots of many vascular plants, especially
dicots and gymnosperms, often undergo secondary growth, which is an increase in diameter. A vascular cambium forms in the stele to produce secondary phloem and secondary
xylem. The epidermis is replaced by a periderm. As the stele increases in diameter, the cortex, pericycle and endodermis are lost. Even nonwoody roots often undergo secondary growth, including those of tomato and alfalfa.
Root 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 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, mineral
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.
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 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 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.
Stilt roots in the
Amazon Rainforest support a tree in very soft, wet soil conditions
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems 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 system: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. Most common in
dicots. The main function of the
taproot is to store food.
★ the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common in
monocots. The main function of the fibrous root is to anchor the plant.
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.
★ 'Adventitious roots' arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in
monocots and pteridophytes, but also in many
dicots, such as
clover (''Trifolium''),
ivy (''Hedera''),
strawberry (''Fragaria'') and
willow (''Salix''). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system.
★ 'Aerating roots' (or 'pneumatophores'): 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.
★ 'Contractile 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,
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 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 and
dahlia. 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.
Root architecture
The pattern of development of a root system is termed 'root architecture', and is important in providing a plant with a secure supply of nutrients and water as well as anchorage and support. The architecture of a root system can be considered in a similar way to above-ground architecture of a plant - i.e. in terms of the size, branching and distribution of the component parts. In roots, the architecture of fine roots and coarse roots can both be described by variation in topology and distribution of biomass within and between roots. Having a balanced architecture allows fine roots to exploit soil efficiently around a plant, but the 'plastic' nature of root growth allows the plant to then concentrate its resources where nutrients and water are more easily available. A balanced coarse root architecture, with roots distributed relatively evenly around the stem base, is necessary to provide support to larger plants and trees.
Economic importance
The term
root crops refers to any edible underground plant structure, but many root crops are actually stems, such as
potato tubers. Edible roots include
cassava,
sweet potato,
beet,
carrot,
rutabaga,
turnip,
parsnip,
radish,
yam and
horseradish. Spices obtained from roots include
sassafras,
angelica,
sasparilla and
licorice.
Sugar beet is an important source of sugar.
Yam roots are a source of estrogen compounds used in birth control pills. The fish poison and insecticide
rotenone is obtained from roots of ''Lonchocarpus'' spp. Important medicines from roots are
ginseng,
aconite,
ipecac,
gentian and
reserpine. Several legumes that have nitrogen-fixing root nodules are used as green manure crops, which provide nitrogen fertilizer for other crops when plowed under. Specialized
bald cypress roots, termed knees, are sold as souvenirs, lamp bases and carved into folk art. Native Americans used the flexible roots of
white spruce for basketry.
Tree roots can heave and destroy concrete sidewalks and crush or clog buried pipes. The aerial roots of
strangler fig have damaged ancient
Mayan temples in
Central America and the temple of
Angkor Wat in
Cambodia.
Vegetative propagation of plants via cuttings depends on adventitious root formation. Hundreds of millions of plants are propagated via cuttings annually including
chrysanthemum,
poinsettia,
carnation, ornamental
shrubs and many
houseplants.
Roots can also protect the environment by holding the soil to prevent soil erosion.
See also
★
Rooting Powder
★
Root cutting
★
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.
★ Sutton, R.F., and R.W. Tinus. 1983. Root and root system terminology. ''Forest Science Monograph 24'' pp 137.
★ Phillips, W.S. 1963. Depth of roots in soil. ''Ecology'' '44' (2): 424.
External links
★
Introduction to Botany - University of Arkansas
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