Roots account for more than 80 percent of plants’ biomass in ecosystems such as tundra and short grass prairies. In many plants, roots are longer and spread wider than the shoots. The extensive root systems of plants are effective collectors of water and minerals necessary for the life of the plant.
Root Cap and Quiescent Center
The root’s structure facilitates each of its functions. Tips of roots are covered by a thimble-shaped root cap. At the base of the root cap is a meristem that produces cells that form the cap.
The meristem pushes cells forward into the cap, which protects the tip of the growing root as it forces its way through the soil. As these cells move through the cap, they differentiate into rows of columella cells. Columella cells contain plastids that sediment, in response to gravity, to the lower side of the cell. This sedimentation is how roots perceive gravity.
Surrounding the outside of the root cap are peripheral cells. Peripheral cells produce and secrete large amounts of a slimy, water-soluble substance called mucigel. Mucigel has several important functions.
It protects roots from desiccation and contains compounds that diffuse into the soil and inhibit growth of other roots. Mucigel also lubricates roots as they force their way between soil particles. Soil particles cling to mucigel, thus increasing the root’s contact with the soil, which helps roots absorb water.
Just behind the root cap is the quiescent center, which is made up of five hundred to one thousand seemingly inactive cells. Cells of the quiescent center divide only about once every twenty days, while those of the adjacent meristem divide as often as twice per day.
Cells of the quiescent center become active when the tip of the root is damaged. When this occurs, the quiescent cells divide rapidly to form cells to repair the damaged root tip. The quiescent center also organizes the patterns of primary growth in roots.
The subapical region of roots consists of three zones: the zone of cellular division, the zone of cellular elongation, and the zone of cellular maturation.
These regions of the root intergrade and are not sharply defined. The zone of cellular division surrounds the quiescent center and is a dome shaped meristem 0.5 to 1.5 millimeters behind the root tip.
Thus, the meristem of a root is subterminal and is made of small, multisided cells. Meristematic cells divide between one and two times per day. In some plants, the meristem produces almost twenty thousand new cells per day. The rate of these divisions is influenced by hormones such as ethylene.
The zone of cellular elongation is 4 to 15 millimeters behind the root tip. Cells in this zone elongate rapidly by filling their vacuoles with water. As a result, the elongating zone is easily distinguished fromthe root cap and zone of cellular division by its long, vacuolate cells.
Cellular elongation in the elongating zone pushes the root cap and apical meristem through the soil as fast as 2 to 4 centimeters per day. Cellular elongation is typically inhibited by the hormone auxin and stimulated by low concentrations of ethylene.
Cells behind the elongating zone do not elongate. Elongation begins the process of cellular differentiation, or specialization. Differentiation is completed in the zone of cellular maturation, which is 1 to 5 centimeters behind the root tip. The maturation zone is distinguished by the presence of numerous root hairs—as many as forty thousand per square centimeter.
Root hairs increase the surface area of the root by a factor of several thousands and are usually less than 1 millimeter long. They live only a few days and form only in the mature, nonelongating region of the root. Because root hairs are fragile extensions of epidermal cells, they usually break off when plants are transplanted.
All mature tissues of roots form behind the zone of cellular maturation. The root is surrounded by an epidermis, which is usually only one cell thick. Epidermal cells usually lack a cuticle. The epidermis covers all of the root except the root cap and typically has no openings.
Immediately inside the epidermis is the cortex. The cortex occupies most of a root’s volume and consists of three concentric layers: the hypodermis, storage parenchyma cells, and the endodermis. The hypodermis is a waxy, protective layer that slows outward movement of water. Thus, the hypodermis helps roots retainwater and nutrients that have been absorbed.
Most of the cortex consists of thin-walled parenchyma cells that store carbohydrates. These cells are separated by large intercellular spaces occupying asmuch as 30 percent of the root’s volume. The innermost layer of the cortex is the endodermis.
Unlike other cortical cells, endodermal cells are packed tightly together and lack intercellular spaces. Their radial and transverse walls, further more, are impregnated with a Casparian strip of lignin and suberin.
If endodermal cells are compared to bricks in a brick wall, then the Casparian strip is analogous to the mortar surrounding each brick. The Casparian strip prevents inward movement of water and nutrients through the cell wall and intercellular spaces. The endodermis functions somewhat like a valve that regulates movement of nutrients.
Collectively, the tissues inside the cortex are called the stele, which consists of the pericycle, vascular tissues, and sometimes a pith. The pericycle is the outermost layer of the stele and is a meristematic layer of cells one to several cells thick; it produces secondary, or lateral, roots.
Inside the pericycle is the root’s vascular tissue, which consists of xylemand phloem. Vascular tissues transport water, minerals, and sugars throughout the plant and differentiate in response to auxin, a plant hormone, coming from the shoot.
Roots of most dicotyledons (dicots) and gymnosperms have a lobed, solid core of primary xylemin the center of the root. Roots of monocots and a few dicots have a ring of vascular tissue that surrounds a pith.
Bundles of primary phloem differentiate between lobes of xylem. In dicots and gymnosperms, a vascular cambium later forms between the xylem and phloem and produces secondary growth that thickens the root.
Types of Root Systems
|Types of Root Systems|
Different kinds of plants often have different kinds of root systems. Most gymnosperms and dicots have a taproot system consisting of a large primary root and smaller branch roots. In plants such as the carrot, fleshy taproots store large amounts of carbohydrates. Not all taproots store food.
Long taproots of plants such as poison ivy and mesquite are modified for reaching water deep in the ground rather than storing food. Many plants have very long taproots. Engineers digging a mine in the southeastern United States uncovered the taproot of a mesquite tree more than 50 meters down.
Most monocots such as corn and other grasses have a fibrous root system that consists of an extensive mass of similarly sized roots. Most of these roots are adventitious roots, which form on organs other than roots themselves.
Fibrous roots of some plants are edible—for example, sweet potatoes are fleshy parts of fibrous root systems of ipomoea (morning glory) plants. Plants with fibrous root systems reduce erosion because their root systems are extensive and cling tightly to soil particles.
Adventitious roots are common in ferns, club mosses, and horsetails. In plants such as tree ferns, adventitious roots form in stems, grow down through the cortex, and finally emerge at the base of the stem.
Adventitious roots are a primary means of asexual reproduction in many plants. For example, prairie grasses and forests of quaking aspen trees are often derived from a single individual propagated by adventitious roots.
Humans use adventitious roots to propagate plants such as raspberries, apples, and brussels sprouts. Formation of adventitious roots is controlled by hormones such as auxin, which is often an ingredient in "rooting" compounds sold commercially.
The structure of a root relates directly to its four primary functions: absorption, anchorage, conduction, and storage. For example,most water and nutrients are absorbed by root hairs in the zone of maturation of the root.
The water there after moves through the root either inside cells or in spaces between cells. Water seeping between cells finally encounters the endodermis, which is the primary barrier to absorption.
The Casparian strip in the endodermis ensures that water and nutrients enter the stele via the plasmodesmata (narrow strands of cytoplasm that connect the cytoplasms of adjacent cells). Most nutrients are absorbed and accumulate in the apical 0.3 to 0.5 meter of the root.
Few nutrients are absorbed past a few centimeters beyond the root tip because these parts of the root lack root hairs and have a waxy endodermis.
These non absorptive regions of roots anchor plants andmay later produce branch roots. Water and dissolved nutrients absorbed by roots move to the shoot in xylem. Roots receive nutrients from the shoot via the phloem. These nutrients either are used for growth or are stored in cortical cells for future use.
Roots of many plants are modified for special functions. For example, roots of plants such as beets, radishes, dandelions, and cassava store large amounts of starch. Sweet potato roots store carbohydrates as sugars. Roots of other plants are used for asexual reproduction.
Roots of cherry, apple, and teak possess adventitious buds that form shoots called suckers. When separated from the parent plant, suckers become new individuals. Adventitious buds are a common means of propagating many other plants.
For example, most groups of creosote bushes are clones derived from a single plant. Some of these clones are more than twelve thousand years old—meaning that the first seed germinated approximately four thousand years before humans began writing.
The roots of many plants minimize competition for water and nutrients by growing in different parts of the soil. For example, mesquite trees growing in deserts often have tap roots more than 20 meters long that obtain water from the underground water table.
Nearby cacti, however, survive in the same environment by producing a shallow root system that spreads as far as 30 meters. Cactus roots do not reach the water table; rather, they quickly absorb water after the infrequent and often heavy rains that occur in the desert.
Roots can protect the plant from other organisms. Most root defenses against soil pathogens are chemical rather than structural. Roots often secrete noxious chemicals that inhibit the growth of pathogens and other organisms, including other plants, in some cases.
Prop roots are aerial roots that grow into the soil. They are common in plants such as corn and banyan trees. Banyan trees produce thousands of prop roots that grow down from horizontally oriented stems and form pillar like supports.
The growth and distribution of roots are controlled by several environmental factors. For example, the short days and cooler temperatures of winter typically cause roots to become inactive.
Microbes living in the soil also affect how roots grow. Mostmicrobes in soil live within 10 micrometers of the root and secrete compounds that significantly affect growth and distribution of roots. These compounds can affect the anatomy, morphology, number of root hairs, and branching patterns of root systems.
Microbes also affect how plants absorb and transport minerals from the soil. Plants growing in sterile soil absorb fewer minerals than those growing in soil containing microbes. Beneficial fungi called mycorrhizae live in and on roots of almost all plants in a form of mutualism, meaning that both the plant and the fungus benefit from the association.
Themycorrhizae absorb nutrients fromthe environment, while the host plant provides the fungus with carbohydrates, amino acids, vitamins, and other organic substances. Plants with mycorrhizae tolerate drought and other types of stress better than uninfected plants.
Roots of legumes such as beans are often infected with Rhizobium (from rhiza, the Greek word for "root"), a genus of the nitrogen-fixing bacteria. Swellings in response to these infections are called nodules. Bacteria receive carbohydrates and other substances from the host, while the host plants receive large amounts of usable nitrogen from the bacteria.
Many organisms compete with roots to collect nutrients and water in the soil. For example, 1 gram of fertile soil contains approximately 109 bacteria, 106 actinomycetes (a group of fungilike bacteria), 105 fungi, and 103 algae.
Plants have evolved several different strategies for competing with these organisms.One is to produce an extensive root systemconsisting ofmany roots that permeate the soil. Most roots grow in the upper 3 meters of soil, however, where nutrients are most abundant.
The narrow zone of soil surrounding a root and subject to its influence is called the rhizosphere. Roots modify the rhizosphere by secreting organic matter, compressing the soil, and absorbing nutrients. As a result of these effects, the rhizosphere is significantly different from bulk soil. It usually contains large amounts of energy-rich molecules. These molecules are eaten by fungi and bacteria.
Roots tend to grow best in moist, loosely packed soil. Roots of many plants grow two to four times faster in loose, sandy soil than in tightly packed clay. The slow growth of roots in poorly aerated soil is probably caused by the accumulation of ethylene, a plant hormone that slows root growth.
Roots of plants that grow in wet areas are usually small and modified for gas exchange. They possess small amounts of xylem, lack root hairs, and contain large intercellular spaces, thereby improving ventilation.
Roots of aquatic plants are also modified for gas exchange. For example, the black mangrove produces specialized roots called pneumatophores, which grow up into the air, where they function like snorkels through which oxygen diffuses to submerged roots.
Epiphytes, including some bromeliads and orchids, are plants that grow on other plants but are not parasites. Adaptations among these "air plants" include a thickened root epidermis which protects the cortex and retards water loss.
The "flower pot plant" (Dischidia rafflesiana) grows a "pot" that collects water and debris; the plant’s aerial roots grow into the pot, where they can absorb the minerals collected there.
Roots of plants that grow in dry areas are often extensive and modified for rapid transport of water. They contain large amounts of xylem, which allows them to move water rapidly to the shoot after rainfall.
Plants such as witch weed and broomrape use their roots to parasitize other plants. Witch weed is a red-flowered plant that infects grains such as corn and sorghum; it is the second leading cause of cereal famine in Africa.