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:Roots

Plants have three organs: roots, stems, and leaves. Growth, flowering, food production, and storage all depend on the activities of these three organs. The combination of stems and leaves makes up the shoot system that is usually visible because it grows above the ground and gives rise to flowers, fruits, and seeds. Roots, on the other hand, are often underground, but the root system can be every bit as massive as the aboveground shoot system. An active root system makes it possible for the plant to carry out growth, photosynthesis and other chemical reactions, branching, flowering, fruiting, and seed production

Roots are so critical to plant survival that a sprouting seedling devotes its first days to root growth before allowing leaves to pop out of the seed. During this time of early germination, the root anchors the plant in the soil and begins to deliver a reliable water supply to the growing seedling. In dry climates, the time taken by roots to find reliable water can take years. The two-year-old sprouts of some California oaks can have a three-foot long root, while their shoots are restricted to only two leaves. In the Namib Desert of Southwest Africa, Welwitschia plants spend their entire one-hundred-year life growing underground roots, leaving only two leaves on the plant even after a century of growth. These examples illustrate the priority placed on root growth over shoot growth in specialized (dry) environments. In moister climates, roots will still be the first organs to emerge from seeds, but leaves will follow them within about ten days

The primary root, or radicle, is the first root to emerge from the seed. From there, the plant can develop a taproot system or a fibrous root system. In dicotyledons, including most trees (except palms), the radicle grows into a strong taproot that sends small branch roots out to the side. In monocotyledons, especially grasses and palms, the radicle stops growing early on and many roots emerge from the seed, each forming similar-sized branch roots that combine into a fibrous root system. Long taproots allow plants to reach deep reliable water supplies, while fibrous roots absorb surface water before it evaporates. Fibrous roots also stabilize soil, reducing erosion

:Root Functions

Most root functions take place in the youngest part of roots, usually 10 centimeters from the growing root tip. Four activities are accomplished in that area

:Roots Stabilize the Plant

Roots anchor plants in soil by pressing between soil particles. Loose or wet sand or mud makes it much harder for roots to secure mechanical anchorage. Trees topple when high winds sweep sandy areas, wetlands, or rainsoaked slopes. In free-floating aquatic plants, such as duckweed and water hyacinth, roots stabilize the plant by dangling into the water, acting as a keel to keep the leaves upright and preventing plants from toppling over. When roots are experimentally removed from water hyacinth, the plants tip over and die

:Roots Absorb Water and Minerals.

Roots absorb water and minerals and transport them to the shoot. In a root system, each root elongates from its tip, entering new soil that can be exploited as an undepleted source of minerals and water. Delicate single-celled root hairs grow from the epidermis and greatly increase the root surface area devoted to absorption. Conditions that harm root hairs in the soil will impede nutrient uptake and harm the entire plant as leaves turn yellow and plant growth slows. Soil compaction and salt accumulation are two environmental factors that harm plants by killing root hairs. In 1999, decline in fruit quality from date orchards in Southern California was slowed by reducing tractor traffic near the trees. This decreased the likelihood of crushing root hairs on the shallow fibrous root systems of date palms

Roots of an elephant ficus tree spread out on the surface from the base of its trunk.

There are more than fifteen elements needed by plants to grow. Of these, roots absorb nitrogen (N), phosphorus (P), and potassium (K) in the largest amounts. The soil minerals most commonly available for root up-take are those that are stuck onto tiny colloid particles in the soil. Colloids are abundant in clay and in broken-down compost. They enhance soil fertility by anchoring minerals in the soil and preventing them from being washed (leached) down and away from plant roots. Colloids are negatively charged particles, allowing minerals with positive charges (potassium, calcium, iron, etc.) to stick to them. Roots can free these stuck minerals by releasing protons (H+) from inside the root and exchanging them for positively charged minerals on the colloid surface in a process called cation exchange. The cation exchange capacity of a soil is a strong indicator of soil fertility, the ability of a soil to provide roots with essential minerals over long periods of time. Acid rain (high in H+) in Europe and Northeastern North America (Canada and the United States) decreases soil fertility by displacing minerals from soil colloids, thereby lowering the soil's exchange capacity

:Roots Attract Microbes.

The roots of many species attract beneficial soil microbes by secreting a paste (mucigel) rich in sugar. The sugars support large populations of soil bacteria and fungi that help roots absorb minerals, especially nitrogen and phosphorus. The bacteria involved are nitrogen fixing bacteria. The fungi are mycorrhizae, long threadlike fungi that attach to plant roots and form a bridge connecting roots to minerals that would have been out of reach of the shorter root hairs. Almost all major agricultural crops have fungi or bacteria associated with their roots. Plants invest up to 10 percent of all food made by leaves to the paste they secrete out of roots. In some cases, the mucigel can feed more than just microbes. In lava tubes such as the Kaumana caves of Hawaii, entire populations of insects live in total darkness, fed only by mucigel from roots pushing through the cave ceiling

:Roots Store Food

Since roots are underground and away from light, making their own food by photosynthesis is impossible. Instead, sugar made by leaves is transported to roots for storage as starch. The stored food can power root growth, or it can enlarge roots in some cases, turning them into economically important commodities such as cassava (Manihot) from West Africa and the Caribbean, sweet potato (Ipomoea), and ginseng (Panax) from China and North Carolina

:Root Anatomy

Four features help distinguish roots from shoots. In roots: 1) a protective cap covers the growing tip, 2) single-celled root hairs are present, 3) branching starts deep within the root, and 4) xylem and phloem alternate around the vascular cylinder.

Comparisons of roots: tap (I and II) and fibrous (III). Redrawn from Kutschera, 1960.

The root cap protects the tender growing tip against abrasion by soil particles. By secreting mucilage, the cap may help lubricate passage of the growing root through the soil. The cap helps attract nitrogen fixing bacteria and mycorrhizal fungi by secreting sugars that feed them. Finally, the root cap detects gravity and directs most roots to grow downward in a process called gravitropism. Corn roots with caps removed by microsurgery grow in random directions until the cap regenerates, after which time they return to growing downward

When viewed from the outside in, root cross sections show six tissues. The epidermis is the interface between the soil environment and the living root. There is no water-repellent cuticle over the epidermis of roots growing in moist media such as soil or water. Everything absorbed by the root must pass through the epidermis including water, minerals, and pollutants such as heavy metals and pesticides. The surface area devoted to uptake of water and minerals is greatly increased by single-celled root hairs extending from thousands of epidermal cells. Root hairs are especially important for water uptake from soil. Aquatic plants with ample supplies of water, such as Elodea and water chestnut (Trapa), produce no root hairs

The root cortex contains food-storage cells usually filled with starch. The cortex is especially large in storage roots such as those of cassava, sweet potato, and tropical yam (Dioscorea). The cortex spans from the epidermis to the endodermis, which is the innermost layer of cortex. The endodermis helps regulate which compounds spread throughout the plant. Cell walls of mature endodermal cells contain a ribbon of wax called the Casparian band, which prevents water from passing from the cortex into the root vein through the cell walls. Instead, water (and the compounds dissolved in it) must pass through the cytoplasm of endodermal cells before it can spread throughout the plant. This allows the plasma membrane of endodermal cells to regulate which compounds pass deeper into the root. Calcium absorption is regulated this way, as is the uptake of pollutants, including soil-borne lead, which are known to accumulate at the endodermis and to pass no farther

The root vein, or vascular cylinder, is at the center of the root. In di-cotyledons, xylem with two to six armlike lobes in a starlike configuration is at the core of the root. In monocotyledons, there can be twenty or more xylem arms in a circle around a central pith. Patches of phloem are tucked between each of the xylem arms. Water and minerals absorbed by the root move upward through the root xylem to the above-ground shoot system. At the same time, in the phloem, the solution of sugars and other carbon compounds made by leaves moves down into the roots

The pericycle occupies the space between the outmost reaches of xylem arms and the endodermis. It ranges from one to six cell layers wide. Branch roots develop from the pericycle. By combining mechanical force with parent cell breakdown, the young branch root pushes through the endodermis, cortex, and epidermis of the parent root before reaching the soil. Developing from the pericycle allows branch roots to connect to xylem of the parent root at the earliest stages of their development. Root branching increases in response to pockets of enhanced resources in soil, leading to root proliferation around fertilizer pellets, dissolving rock, decaying animals, buried reptile eggs, or water drops.

Parts of root cap and quiescent center. Redrawn from Moore et al., 1998, Figure 15.6.

:Root Growth

Roots grow 1 to 6 centimeters each day by cell division and cell elongation near their tip. Growth depends on oxygen and temperature. Air space takes up 30 to 50 percent of productive soil volume, and depletion of that space by soil compaction or by flooding will reduce or stop root growth. The growing root tip is organized into three distinct zones, with an inactive zone sandwiched between two actively dividing meristems. The cap meristem is a layer of rapidly dividing cells between the root cap and the root body. It maintains the root cap, whose tip cells are abraded by soil particles. Behind the cap meristem is a lens- or bowl-shaped group of cells that seldom, if ever, divide. This is called the quiescent center, and its roles are to produce growth regulators (including cytokinin) that regulate root growth, to provide a reserve of cells that can replace injured cells of the cap and body meristems, and to physically regulate the size of those meristems. Behind the quiescent center is the body meristem responsible for creating all root tissues except the cap

:Modified Roots and Their Economic Importance

In warm climates where freezing is not a problem, roots can grow above ground where theyfig trees, and assume diverse functions. Above-ground roots are well developed in orchids, mangroves growing throughout the tropics and subtropics

The orchid family includes climbing orchids and epiphytes that live on tree branches, close to sunlight but away from soil. Roots of the vanilla orchid emerge from the stem and support this climbing vine by twining around sticks and branches of trees. Roots of epiphytic orchids are out in the open where they have access to sunlight and rainwater, but not to soil. Long-term studies at Hummingbird Cay Tropical Field Station in the Bahamas show these orchids grow very slowly but live for decades. Their aerial roots have a spongy multilayered epidermis called velamen that enables roots to store water from rain and bark runoff. To the inside of the velamen the cortex is modified for photosynthesis, performing a function usually restricted to leaves. Photosynthesis by the elaborate green roots of epiphytic orchids in the tropics and Japan compensates for the reduced leaves in these plants

Ficus trees such as figs and banyans use buttress roots and stilt roots to support large canopies atop shallow tropical soil. Buttress roots resemble rocket fins at the bottom of large tree trunks. They develop from the fusion of the upper side of a horizontal root with the vertical tree trunk in response to tension as the tree leans away from the developing buttress. Trees growing on hillsides show larger buttresses on the uphill side of the trunk than on the downhill side. Buttresses are prominent on Brazilian rubber tree (Hevea), and kapok (Ceiba). Stilt roots develop from horizontal branches and grow down to the soil where they thicken and become long-lived supports for the tree canopy

Mangroves are trees living in tropical coastal areas. Their roots are of enormous value in stabilizing tropical coast lines against typhoons, hurricanes, and wave action, and they give refuge to young stages of commercially important fish. The global value of such ecosystem services provided by mangroves was estimated in 1997 to exceed $600 billion. To help restore damaged environments, mangroves are being replanted in Vietnam, the Philippines, and Mexico. Mangrove roots allow these coastal trees to grow in shifting sand and oxygen-poor soil. The stilt roots of red mangrove (Rhizophora) spread down into sand from dozens of canopy branches, thereby stabilizing the tree. Pores, called lenticels, on the root surface allow oxygen to enter the aerial part of the root and diffuse down to submerged tissues in oxygen-depleted soil. Massive intertwined root systems of red mangrove forests prevent hurricanes from removing acres of land from south Florida and the Caribbean. The root system of black mangrove (Avicennia) has at least three root types. Underground cable roots radiate horizontally from the central tree trunk and stabilize the tree. They produce upward-growing roots (pneumatophores) that grow out of the soil and act as snorkels to bring oxygen into belowground roots. Feeder roots branch from the base of each pneumatophore where their large surface area absorbs minerals from the soil

:Bibliography

Kutschera, L. Wurzelatals. Frankfurt: DLG-Verlag, 1960

McCully, Margaret E. "Roots in Soil: Unearthing the Complexities of Roots and Their

Rhizospheres." Annual Review of Plant Physiology and Plant Molecular Biology 50 (1999): 695-71

Moore, Randy, W. Dennis Clark, and Darrell S. Vodopich. Botany, 2nd ed. New York: McGraw

Hill, 1998

Pimm, Stuart L. "The Value of Everything." Nature 387 (1997): 231-32

Russell, R. Scott. Plant Root Systems: Their Function and Interaction with the Soil. London: McGraw-Hill, 1977

Stegmann, Edwin W., Richard B. Primack, and George. S. Ellmore. "Absorption of Nutrient Exudates from Terrapin Eggs by Roots of Ammophila breviligulata (Gramineae)." Canadian Journal of Botany 66 (1988): 714-18

Waisel, Yoav, and Eshel Amram, eds. Plant Roots: The Hidden Half, 2nd ed. New

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