The plant root system constitutes the major part of the plant body, both in terms of function and bulk. In terrestrial plants, the root system is the subterranean or underground part of the plant body while the shoot is the aboveground part. Roots are branching organs which grow vertically into the soil, a manifestation of positive geotropism. Branching occurs irregularly and not from nodes as in stems.
In contrast to shoot, the plant root has no leaves, nodes, internodes and buds. With rare exception, roots also lack stomata. Other morphological and anatomical features which are distinct to this plant structure are: (1) a hard, protective root cap at the tip of the root; (2) absence of the pith; (3) presence of endodermis; and (4) presence of pericycle next to the endodermis. These features are found in the root apex which is divided into three regions: (1) a region of cell division which includes the apical meristem protected by the root cap, (2) a short region of cell elongation where individual cells elongate and force the root tip to move forward through the soil, and (3) a region of cell differentiation and maturation.
In general, the plant root system consists of a taproot (primary root) or fibrous roots (adventitious roots) with attached branch roots and finer rootlets having root hairs close to the tip.
Roots are so massive that their total dry weight may exceed that of the entire plant body. Quantitative investigation revealed that a single rye plant (Secale cereale) that was 4 months old had a total root length of 387 miles (623 km) or an average root growth of about 3 miles (4.83 km) per day. It consisted of some 14 million separate branch roots, with more than 14 billion root hairs. All the roots and root hairs convert to an equivalent total absorptive surface area in contact with the soil of almost 640 sq meters, all contained within a limited volume of about 2 cu ft (0.057 cu. meter) of soil. (Went and The Editors of Life 1963; Feininger 1968; Mader 1993).
Root length may also exceed shoot length many fold. The roots of corn (maize), sugar beets, and cotton may extend downward up to a depth of 5-6 feet (~1.5-1.8 meter) (Chevron Chemical Company 1984). A five-year old breadfruit (Artocarpus altilis) with a height of 7 meters can have lateral roots up to 200 meters in length. However, coconut trees which can grow to heights of 20-25 meters have roots which are only 8-10 meters (Farm forestry News 1992, 5(2):1 in Agroforestry Seeds Circular, March 1993. p. 30).
As reviewed by Gomez and Prado (2007), coconut roots usually grow to a depth of close to 0.80 m with 60-90% found in the top 0.5 m of the soil. In 10-year old dwarf coconut grown under rainfed conditions, the effective root zone of absorption was at 1.4 m from the trunk. Coconut has no root hairs but it produces plenty of roots with a large quantity of rootlets (biodiversity.org, accessed December 7, 2010).
Functions of the Plant Root System
Despite being inconspicuous because they are normally hidden underground, the plant root system performs various functions which are essential to growth and development. The extent of underground expansion of this plant structure serves as limitation in the growth of the plant. Thus potted plants usually exhibit slow growth but once the roots leak out from the bottom of the pot and penetrate into the ground, growth rate accelerates.
The functions of the plant root system include:
1. Anchorage and support. The plant root system anchors the plant in the soil and provides physical support. Redwood trees (a gymnosperm) about 100 meters tall have stood erect for thousand years only because millions of individual fibrous roots dig into the ground, even though the depth of penetration is only up to about 5 meters. In general, however, taproot system provides more effective anchorage such that they are more resistant to toppling during storms.
2. Absorption and conduction. The plant root system absorbs water, oxygen and nutrients from the soil in mineral solution, mainly through the root hairs. They are capable of absorbing inorganic nutrients in solution even against concentration gradient. From the root, these are moved upward. Plants with a fibrous root system are more efficient in absorption from shallow sources.
In the desert plants called phreatophytes like the mesquite, the roots seek permanent underground water reserves. These plants are water indicators and knowledge of such plants has been put to use by digging wells where they grow (Went and The Editors of Life 1963).
3. Storage. The root serves as storage organ for water and carbohydrates as in the modified, swollen roots of carrot, sweet potato (camote) and yam bean (sinkamas). Fibrous roots generally store less starch than taproots. Some roots are capable of storing large amounts of water; the taproots of some desert plants store more than 70 kg of water (Moore et al. 2003). (Click to read Starchy Root Crops, Tuber Crops and Corm Crops)
4. Photosynthesis. Some roots are capable of performing photosynthesis, as in the epiphytic orchids and aerial roots of mangrove.
5. Aeration. Plants that grow in stagnant water or other watery places have modified roots called pneumatophores to which oxygen from the air diffuses.
6. Movement. In many bulb- and corm-forming plants, contractile roots pull the plant downward into the soil where the environment is more stable.
7. Reproduction. The plant root system also serves as a natural means of perpetuating a species. In mature Norfolk Island pine and certain plants, clonal seedlings or offshoots are commonly seen growing profusely around the trunk from horizontally growing roots. Likewise, new plants emerge from left-over tuberous roots after harvest in fields grown to sweet potato and yam bean (Pachyrhizus erosus). As a rule, plants with a fibrous root system are easier to transplant than those with tap roots.
(Ben G. Bareja April 2011)