External Structure of Roots

ORGANIZATION OF ROOT SYSTEMS

Roots must have an enormous absorptive surface; in order for a single root to have sufficient surface area, it would have to be hundreds of meters long, which would make conduction impossible (Table 1). Instead, plants have a highly branched root system (Fig. 1). Most dicots have a single prominent taproot that is much larger than all the rest and numerous small lateral roots or branch roots coming out of it (Fig. 2). This taproot develops from the embryonic root, called the radicle, that was present in the seed; after germination it grows extensively and usually becomes the largest root in the system. Carrots, beets, turnips, and other taproots sold in stores have dozens of fine lateral roots while growing, but these are removed before the products are shipped to market.

FIGURE 1:some taproots, such as carrot (a), become extremely swollen and are much larger tan the numerous lateral roots, whereas in other species, such as sunflower (b), the taproot is about the same size as the laterals. The important criterion is that the taproot develops from the root of the embryo, the radicle. (c) A fibrous root system, as in this winter wheat, consists of many roots, none of whrch is the radicle. Instead, al HE main roots are adventitious roots that originated in stem tissue.

Lateral roots may also produce more lateral roots, resulting in a highly ramified set of roots analogous to the highly branched shoot system of most plants. Lateral roots can become prominently swollen like a taproot, as in sweet potatoes and the tropical vegetable manioc (cassava). If the plant is perennial and woody, the roots also undergo secondary growth, increasing the amount of wood and bark.

FIGURE 2: (a) The taproot of this tobacco seedling (Nicotiana tabacum) is definitely larger than the lateral roots, but more importantly, its development can be traced directly to the embryo. Relative size is not the critical factor; in many species lateral roots are the ones that become enlarged. (b) The roots of radishes are obviously taproots.

Most monocots and some dicots have a mass of many similarly sized roots constituting a fibrous root system (Fig. 3). This arises because the radicle dies during or immediately after germination; root primordia at the base of the radicle grow out and form the first stages of the fibrous root system. As the plant ages, more root primordia are initiated in the stem tissue. Because these roots do not arise on pre-existing roots and because they are not radicles, they are known as adventitious roots. Adventitious roots increase the absorptive and transport capacities of the root system.

FIGURE 3: Onions (Allium), like other bulbs and monocots, have a fibrious root system. The radicle died shortly after germination; no root here has developed from the radicle.

The functional significance of taproots versus fibrous root systems becomes apparent when the general growth forms of dicots and monocots are considered. Many dicots are perennial and undergo secondary growth, resulting in an increased quantity of healthy, functional wood (xylem) in both the trunk and the roots (Fig. 4a and b). This enlarging conduction capacity permits an increase in the number of leaves and fine, absorptive roots.

Most monocots cannot undergo secondary growth; once their stem is formed, the number of vascular bundles, tracheary elements, and sieve tubes is set, and their conducting capacity cannot be increased. Extra leaves could not be supplied with water, nor could their sugar be transported (Fig. 4c and d). Such a shoot could not supply sugars to an ever-increasing taproot system. However, some monocots do increase their size by means of stolons or rhizomes: Their horizontal shoots branch and then produce adventitious roots (Fig. 4e). Because these roots are initiated in the new stem tissues, they transport water directly into the new portions of the shoot, unhindered by the limited capacity of the older portions of the shoot. By this mechanism, monocot shoots can branch and grow larger, as long as they remain close enough to the substrate to produce new adventitious roots. For them, a fibrous root system is functional, whereas a taproot system is not.

FIGURE 4: (a) A young dicot has a few leaves and a small root system; the narrow trunk with a few vascular bundles can conduct water and nutrients between them. (b) An older dicot has more leaves and a larger root system; the stem has more wood and bark, which increases the capacity to conduct water and sugar. Because most monocots do not undergo secondary growth, the stem of an older plant is not wider (d) than that of a young plant (c), and it has no increased conducting capacity. Consequently, the old plant has no more leaves or roots than the young plant. (e) If a plant can produce adventitious roots, the bottleneck of the monocot stem does not matter. New roots originate near the aerial shoots and conduct directly into them, and little or no long-distance conduction occurs in the rhizome. No part of the monocot stem needs to conduct all the water from all the roots to all the leaves and flowers, as does the trunk of a dicot.

The ability to form adventitious roots is not limited to monocots; many rhizomatous and stoloniferous dicots also grow this way naturally. Furthermore, many dicots that never produce adventitious roots in nature do so if they are cut; this is important in the process of asexual propagation by cuttings.

STRUCTURE OF INDIVIDUAL ROOTS

An individual root is fairly simple; because it has no leaves or leaf scars, it has neither leaf axils nor axillary buds (Figs. 5 and 6). The tip of the root, like that of the shoot, is the region where growth in length occurs. In roots, growth by discrete apical meristems is the only feasible type of longitudinal growth. In most animals, all parts of the body grow simultaneously (diffuse growth), whereas the roots and stems of plants elongate only at small meristematic regions (localized growth). Because the root is embedded in a solid matrix, it is impossible for all parts to extend at once; the entire root would have to slide through the soil. With apical growth, only the extreme tip must push through the soil.

FIGURE 5: The tip of a root consists of a root cap, the root apical meristem, a zone of elongation growth, and a region where root hairs are formed. 

FIGURE 6: Although the exterior of a root appears rather uniform, several distinct zones of differentiation are present internally. Root hairs form only above the elongation zone, and the endodermis and first vascular tissues appear earlier than do root hairs. 

Whereas the shoot apical meristem is protected by either bud scales or young, unex-panded foliage leaves, the root apical meristem is protected by a thick layer of cells, the root cap (Figs. 6 and 7). Although some soils may appear soft and easily penetrable, on a microscopic scale, all contain sand grains, crystals, and other components that can easily damage the delicate apical meristem and root cap. Because the cap is forced through the soil ahead of the root body, it is constantly being worn away and must be renewed by cell multiplication.

The dictyosomes of root cap cells secrete a complex polysaccharide called slime or mucigel, which helps to lubricate the passage of the root through the soil. It causes the soil to release its nutrient ions and permits the ions to diffuse more rapidly toward the root. Mucigel is rich in carbohydrates and amino acids, which foster rapid growth of soil bacteria around the root tip. The metabolism of these microbes is believed to help release nutrients from the soil matrix.

 FIGURE 7: (a) The root cap of this corn root is distinct from the root proper. The root apical meristem is located at the apex of the root proper but is buried under the root cap. Because no leaves, leaf traces, or branches are present, cells develop in an extremely orderly fashion in regular files. It is easy to see that these cells of the primary body of the root are derived directly from the meristem (X 150). (Bruce Iverson) (b) Scanning electron micrograph showing the root cap margin. As cells are damaged by being pushed through the soil, they die and break off the cap (X 1000). (Photo Rearchers)

Just behind the root cap and root apical meristem is a zone of elongation only a few millimeters long within which the cells undergo division and expansion (see Fig, 6). Behind it is the root hair zone, a region in which many of the epidermal cells extend out as narrow trichomes. Root hairs can form only in a part of the root that is not elongating or they would be shorn off.

Root hairs greatly increase the root's surface area. In a study of rye, a single plant was found to have 13 million lateral roots with 500 km of root length and a surface area of 200 m². Because of the abundant production of root hairs, however, the total surface area was doubled. The presence of root hairs has other effects that should not be overlooked. Most pores in soil are too narrow for a root (usually at least 100 µm in diameter) to penetrate (Fig. 8). But root hairs, being only about 10 µm in diameter, can enter any crevice and extract water and minerals from it. Furthermore, carbon dioxide given off by the respiration of root hairs combines with soil water to form carbonic acid, which helps to release ions from the soil matrix. Without the acid, ions would be too firmly bound to soil particles for the root to absorb them Root hairs are unicellular, never have thick walls, and are extremely transitory. They die and degenerate within 4 or 5 days after forming.

FIGURE 8: Much of a soil is composed of spaces between soil particles; in the uppermost 20 cm of a soil composed of sandy loam, 54% of the soil is pore space. Of this, 9% consists of extremely fine spaces less than 0.2 µm wide, 33% spaces ranging between .0.2 and 6.0 µm, and 12% large spaces wider than 60 µm. At deeper levels, the soil is more compacted, containing only about 42% pore space; the decrease is due mostly to a compaction of the intermediate-size spaces. 

Behind the root hair zone is a region where new lateral roots in rows or may appear to be randomly distributed on the parent lateral roots often depends on the microenvironment of the soil. into a zone of rich, moist soil, numerous lateral roots form and the If the soil is poor, hard, or dry, few lateral roots emerge.