Delphinidin

Most of the cells mature, or differentiate, into the various distinctive cell types of the primary tissues in this region, which is sometimes called the region of differentiation, or roothair zone. The large numbers of hairlike, delicate protuberances that develop from many of the epidermal cells give the root-hair zone its name. The protuberances, called root hairs, which absorb water and minerals, adhere tightly to soil particles with the aid of microscopic fibers they produce and greatly increase the total absorptive surface of the root.

The root hairs are not separate cells; rather, they are tubular extensions of specialized epidermal cells. In fact, the nucleus of the epidermal cell to which each is attached often moves out into the protuberance. They are so numerous that they appear as a fluffy mass to the naked eye, typically numbering more than 38,000 per square centimeter (250,000 per square inch) of surface area in roots of plants such as corn; they seldom exceed 1 centimeter (0.4 inch) in length. A single ryegrass plant occupying less than 0.6 cubic meter (20 cubic feet) of soil was found to have more than 14 billion root hairs, with a total surface area almost the size of a football field.

When a seedling or plant is moved, many of the delicate root hairs are torn off or die within seconds if exposed to the sun, thereby greatly reducing the plant’s capacity to absorb water and minerals in solution. This is why plants should be watered, shaded, and pruned after transplanting until new root hairs have formed. In any growing root, the extent of the root-hair zone remains fairly constant, with new root hairs being formed toward the root cap and older root hairs dying back in the more mature regions. The life of the average root hair is usually not more than a few days, although a few live for a maximum of perhaps 3 weeks.

The cuticle, which may be relatively thick on the epidermal cells of stems and leaves, is thin enough on the root hairs and epidermal cells of roots in the region of maturation  to allow water to be absorbed but still sufficient to protect against invasion by bacteria and fungi. The cells of the cortex, a tissue composed of parenchyma cells lying between the epidermis and inner tissues, mostly store food. This tissue, which may be many cells thick, is similar to the cortex of stems except for the presence of an endodermis at its inner boundary.

The endodermis consists of a single-layered cylinder of compactly arranged cells whose primary walls are impregnated with suberin. The suberin bands, called Casparian strips, are on the radial and transverse walls. The plasma membranes of the endodermal cells are fused to the Casparian strips, which are perpendicular to the root’s surface; they prevent water from passing through the otherwise permeable (porous) cell walls. The Casparian strip barrier forces water and dissolved substances entering and leaving the central core of tissues to pass through the plasma membranes of the endodermal cells or their plasmo desmata. This regulates the types of minerals absorbed and transported by the root to the stems and leaves.

The plasma membranes tend to exclude harmful minerals while generally retaining useful ones. An endodermis is rare in stems but so universal in roots that only three species of plants are known to lack a root endodermis. In some roots, the epidermis, cortex, and endodermis are sloughed off as their girth increases, but in those roots where the endodermis is retained, the inner walls of the endodermal cells eventually become thickened by the addition of alternating layers of suberin and wax. Later, cellulose and sometimes lignin are also deposited. Some endodermal cells, called passage cells, may remain thin-walled and retain their Casparian strips for a while, but they, too, eventually tend to become suberized.

A core of tissues, referred to collectively as the vascular cylinder, lies to the inside of the endodermis. Most of the cells of the vascular cylinder conduct water or food in solution, but lying directly against the inner boundary of the endodermis is an important layer of parenchyma tissue known as the pericycle. This tissue, which is usually one cell wide, may in some plants be a little wider. The cells of the pericycle may continue to divide even after they have matured. Lateral (branch) roots and part of the vascular cambium of dicots arise within the pericycle.

In most dicot and conifer roots, the primary xylem consists of a solid central core of water-conducting cells (e.g., tracheids; vessels). In cross section, this first root xylem usually loosely resembles the rear view of a rocket with fins. The fins, generally referred to as arms, tend to taper toward their tips and terminate just inside of the thin cylindrical pericycle layer. There are usually four of these arms, with some plants having two, three, or several. Branch roots begin to grow and develop in the pericycle opposite the xylem arms and push their way out to the surface through the endodermis, cortex, and epidermis.

The primary xylem surrounds pith parenchyma cells in monocot roots and those of a few dicots; in such plants, the arms may not be well defined. Primary phloem, which conducts food, forms in discrete patches between the xylem arms of both dicot and monocot roots. A vascular cambium develops from parts of the peri cycle and other parenchyma cells between the xylem arms and phloem patches in most dicots and conifers. This cambium at first follows the starlike outline of the primary xylem as it starts producing secondary phloem to the outside and secondary xylem to the inside. Eventually, however, the position of the cambium gradually shifts so that instead of appearing as patches and arms, the secondary conducting tissues appear as concentric cylinders. The primary phloem, in particular, may be sloughed off and lost as secondary tissues are added.

In woody plants, a second cambium, the cork cambium, normally arises in the pericycle outside of the vascular cambium and gives rise to cork tissue (periderm). Cork cells, which are dead at maturity, are impregnated with suberin and are impervious to moisture; similar tissues are produced in stems. Although there are exceptions, monocot roots generally have no secondary meristems and therefore no secondary growth.

In both roots and stems, growth may be determinate or indeterminate. Determinate growth is growth that stops after an organ such as a flower or a leaf is fully expanded or after a plant has reached a certain size. Indeterminate growth occurs in trees and other perennials where new tissues are added indefinitely, season after season.

Natural grafting can take place between roots of different trees of the same species, especially in the tropics. The roots unite through secondary growth when they come in contact with one another, but the details of the uniting process are not yet known. One unfortunate aspect of this grafting is that if one tree becomes diseased, the disease can be transmitted through the grafts to all the other trees connected to it.