Conceptual Overview

The root is a primary vegetative organ of vascular plants and is typically underground. The two principal functions of roots are absorption of water and mineral salts (mineral nutrition), and anchorage. The root is an axial organ with radial symmetry. In contrast to the stem it never produces leaves and its apical meristem is always covered by a rootcap rather than by leaf primordia of the bud. On the whole, roots possess a more uniform and simpler organization than stems.

The delicate cells of the root apical meristem growing in the soil are always protected by the rootcap (almost like a living thimble). The rootcap is continuously renewed and, as its old cells are sloughed off from the surface, new cells are formed from the inside by the apical meristem. The outer rootcap cells produce slime which lowers friction between root and soil particles, and thereby facilitates root movement.

The axial portion of the rootcap is a columella, a cell complex thought in part to be responsible for root growth towards the pull of gravity, or gravitropism. In experiments when the orientation of a root is changed from vertical to horizontal, gravitropic bending of the root tip occurs. This bending results from different growth rates on the upper and lower halves of a horizontally oriented root due to different auxin concentrations in them. The columella is believed to be capable of perceiving gravity and initiating a chemical signal, which is transmitted to the zone of elongation of roots. A number of studies have shown starch and mineral deposits (called statoliths) present in the cells of the columella. These inclusions have been considered triggering devices for gravitational orientation. However, some mutant strains of plants have been isolated that lack statoliths; yet the roots still convey a signal for positive gravitropism. In any event, that signal brings about different concentrations of auxin in the upper and lower sides of the root in the zone of elongation. Increased auxin concentration on the lower side inhibits growth of cells, and the root tip curves downward.

In the root apical meristem (or promeristem) a central complex of rarely dividing cells (the quiescent center) and a surrounding zone of actively dividing cells are distinguished. Dividing cells contribute distally to the rootcap, and proximally to the protoderm, ground meristem and procambium which subsequently give rise to rhizodermis, cortex and vascular cylinder. The region of dividing cells in a root tip extends for a considerable distance basipetally from the apical meristem, i.e. towards the older part of the root. These divisions are combined with cell elongation and may also overlap with cell differentiation. The planes of cell divisions are strictly ordered, and are directed mainly across (anticlinal) and, less frequently, along the root axis (periclinal) ensuring both longitudinal and circumferential root growth. The resulting regular arrangement of cells organized as columns enables one to follow accurately the developmental changes and fate of each of the tissues and cells. Regular periclinal (transverse) cell divisions also results in a radial pattern of roots, which is made up of concentric rings of cells.

In many plants these columns of cells can be traced back to specific sets of cells in the apical meristem, termed apical initials, which are usually vertically positioned one on top of another, thus forming cell tiers, or histogens. The upper tier, plerome, generates procambium, from which vascular tissues are differentiated. The medium cell tier, periblem, gives rise to the ground meristem, the source of the root cortex. In most monocots, the lower tier of cells, dermatogen, is a precursor of a protoderm that is finally differentiated into rhizodermis. The root cap originates from calyptrogen. In most gymnosperms both protoderm and root cap are the derivatives of a common histogen, the dermatocalyptrogen. By contrast, most angiosperms have a type of root meristem, where all tissues of the root proper have independent origin from three tiers, and the rootcap and rhizodermis share a common histogen. Grasses among monocots, however, have four tiers of histogens which includes the calyptrogen that gives rise to the rootcap.

Cellular differentiation in roots actually begins within the meristem. Within the procambium, the developing metaxylem cells can be recognized by their enlargement and vacuolation. However, the fully mature (dead and with secondary walls) elements appear first in the protoxylem, at a level of the root where the protoderm differentiates into rhizodermis and where root hairs begin to grow. The first sieve elements of protophloem become functional even closer to the apical meristem than the first tracheary elements of the protoxylem. The first sieve elements begin to function as the part of food-conducting channels in which cells in the other columns of the root tip are still dividing.

The following tissues are distinguished in the transverse section of the root in the primary state of growth: rhizodermis, possibly exodermis, cortex, endodermis, pericycle, primary xylem and primary phloem. The presence of pith in the root is found in some monocots but not dicots. The thin-walled rhizodermis (also rarely identified as the epiblem) with root hairs and without cuticle and stomata, being homologous to the epidermis of the above ground organs carries out a water-absorbing function as opposed to the thick-walled and cuticularized epidermal cells of stems and leaves. The rhizodermis and rootcap are capable of generating a secretion of slime that not only lubricates the root for growth in soil, but also provides nutrition for associated soil microorganisms.

The exodermis is the outer layer (or layers) of the root cortex in some plants; its cells initially have a Casparian strip and later, in some cases, thickened walls with suberin lamellae. The exodermis may have a protective function, and may replace the rhizodermis in a root zone above the absorption zone. The cortex, a ground parenchyma of the primary root, is characterized by thin-walled cells and well-developed intercellular spaces. In some plants it contains sclerenchyma. The innermost cell layer of cortex is differentiated as the endodermis. In the primary state, endodermal cell walls are thin except for a band-like thickening extending completely around the cell on radial and transverse walls. This thickened strip, the Casparian strip, is impregnated with lignin and suberin. The plasmalemma in the strip region is tightly attached to the strip creating a localized seal in the apoplast. The Casparian strip blocks the passage of water and mineral ions from one side of the endodermis to the other via cell walls, i.e. apoplast. Therefore, ions can leave or enter the vascular cylinder only by passing through the symplast via the plasmodesmata or by crossing the plasmalemma of the endodermis, and thus their movements are under the control of the endodermal protoplast.

In gymnosperms and dicotyledons, the endodermis usually remains thin-walled, and eventually sheds off together with the cortex when the root advances to a secondary state of growth in which a periderm develops.

In monocotyledons in which there is no secondary thickening, the walls of the endodermal cells change with age. Suberin lamellae are deposited on the inner sides of the wall, especially on the inner tangential and radial walls including the Casparian strip. Then the lamellae are covered from the inside by a thick secondary wall which soon becomes lignified. Formation of secondary walls may be delayed in endodermal cells opposite the xylem. Such cells (with Casparian strips) are termed passage cells.

There is only one central vascular bundle in the root. Typically, it forms a solid vascular cylinder in which no pith is developed. The outermost layer of the cylinder consists of non-vascular living thin-walled cells known as the pericycle. It is commonly one cell layer in thickness. The pericycle retains meristematic potential, initiates lateral root primordia and cork cambia, and gives rise to vascular cambium in the regions of the vascular cylinder located opposite xylem groups.

The vascular tissues of roots in the primary state are so arranged that in cross-section we can see the xylem as several ridge-like projections, or arms extending outward up to the pericycle from a solid core, and phloem as the strands located between the arms. At the ends of the xylem arms no phloem lies exterior to the xylem as it always does in the stem. Such radial (not collateral as in stems) arrangement of vascular tissues makes it possible for water movement inward from the rhizodermis to enter the tracheary elements without having to cross the phloem. The number of xylem "arms" varies among different plants and different roots of the same plant, and is expressed by the terms diarch, triarch, tetrarch and polyarch. In contrast to the stem, the differentiation of both primary xylem and primary phloem is centripetal, or exarch, i.e. the first mature conducting elements of protoxylem and protophloem are seen adjacent to the pericycle. The tracheary elements of the protoxylem are the narrowest and have helical or annular secondary thickenings. Closer to the center are the increasingly wider and pitted vessels of the metaxylem.

In contrast to the shoot where branches originate exogenously from the apical meristem, lateral (or branch) roots are initiated endogenously in the pericycle without any relation to the apical meristem. Typically, the point of origin of a lateral root is opposite to the xylem. Here, the pericyclic cells become densely cytoplasmic and resume meristematic activity. The lateral root primordium must penetrate the endodermis, cortex, exodermis and the rhizodermis in order to appear on the surface of parental root. These tissues are stretched and finally ruptured.