Unit Seven. Plant Life
Roots have a simpler pattern of organization and development than do stems, and we will examine them first. Although different patterns exist, the kind of root shown here is found in many dicots.
The outer layer of the root is the epidermis. The mass of parenchyma in which the root’s vascular tissue is located is the cortex. In the photographs above, you can also see that the dicot root contains both xylem and phloem. Focusing on the core, you can see a central column of xylem with radiating arms. Alternating with the radiating arms of xylem are strands of primary phloem. Surrounding the column of vascular tissue, and forming its outer boundary, is a cylinder of cells one or more cell layers thick called the pericycle. Branch, or lateral, roots are formed from cells of the pericycle. Just outside the pericycle is the endodermis, a single layer of specialized cells that regulate the flow of water between the vascular tissues and the root’s outer portion.
Endodermal cells are encircled by a thickened, waxy band called the Casparian strip. Here you see a drawing of endodermal cells showing how the wax substance that makes up the Casparian strip surrounds each cell. As the black arrows indicate, the Casparian strip blocks the movement of water between cells and instead directs the movement of water through the plasma membrane of the endodermal cells. In this way, the Casparian strip controls the passage of minerals into the xylem because transport through the endodermal cells is regulated by special channels embedded in the plasma membrane.
The apical meristem of the root (shown as a group of cells at the base of the root in figure 33.2a,b) divides and produces cells both inwardly, back toward the body of the plant, and outwardly. The three primary meristems, shown in the “cutaway” portion of the dicot root in figure 33.2a, are the protoderm, which becomes the epidermis; the procambium, which produces primary vascular tissues (primary xylem and primary phloem); and the ground meristem, which differentiates further into ground tissue that is composed of parenchyma cells. Outward cell division results in the formation of a thimblelike mass of relatively unorganized cells, called the root cap, which you can clearly see in the photo above. The root cap covers and protects the root’s apical meristem as it grows through the soil.
Figure 33.2. Root structure of dicots and monocots.
(a) Diagram of primary meristems in a dicot root, showing their relation to the apical meristem. The three primary meristems are the protoderm, which differentiates further into epidermis; the procambium, which differentiates further into primary vascular strands; and the ground meristem, which differentiates further into ground tissue. (b) Median longitudinal section of a monocot root tip in corn, Zea mays, showing the differentiation of protoderm, procambium, and ground meristem.
The root elongates relatively rapidly just behind its tip in the area called the zone of elongation. Abundant root hairs (see figure 33.14), extensions of single epidermal cells, form above that zone, in the area called the zone of differentiation. Virtually all water and minerals are absorbed from the soil through the root hairs, which greatly increase the root’s surface area and absorptive powers.
One of the fundamental differences between roots and shoots has to do with the nature of their branching. In stems, branching occurs from buds on the stem surface; in roots, branching is initiated well back of the root tip as a result of cell divisions in the pericycle. The developing lateral roots (the red-stained mass of cells in figure 33.3) grow out through the cortex to ward the surface of the root, eventually breaking through and becoming established as lateral roots. In some plants, roots may arise along a stem, or in some place other than the root of the plants. These roots are called adventitious roots. Adventitious roots occur in ivy, bulb plants such as onions, perennial grasses, and other plants that produce rhizomes, which are horizontal stems that grow underground.
Figure 33.3. Lateral roots.
A lateral root growing out through the cortex of the black willow, Salix nigra. Lateral roots originate beneath the surface of the main root, whereas lateral stems originate at the surface.
Key Learning Outcome 33.3. Roots, the belowground portion of the plant body, are adapted to absorb water and minerals from the soil.