The products of the roots primary meristems become root tissues

What are the products of the three primary meristems? The protoderm gives rise to the outer layer of cells—the epidermis—which is adapted for protection of the root and for the absorption of mineral ions and water (Figure 35.16). In the zone of maturation, many of the epidermal cells produce amazingly long, delicate root hairs, which vastly increase the surface area of the root (Figure 35.16b). It has been estimated that the root system of a mature rye plant has a total absorptive surface of more than 600 square meters (almost half again the area of a basketball court). Root hairs grow out among the soil particles, probing nooks and crannies and taking up water and minerals.

Internal to the epidermis, the ground meristem gives rise to a region of ground tissue that is many cells thick, called the cortex. The cells of the cortex are relatively unspecialized and often function in nutrient storage.

In the great majority of plants, especially in trees, a fungus is closely associated with the root tips. This association, called a mycorrhiza, increases the plant's absorption of minerals and water (see Figure 31.16). Such roots have poorly developed

Epidermis

Root hairs

Epidermis

Root hairs

Primary Meristems Plants

Root cap or no root hairs. These plants cannot survive without the my-corrhizae that help them absorb minerals.

Proceeding inward, we come to the endodermis of the root, a single cylindrical layer of cells that is the innermost cell layer of the cortex. Unlike those of other cortical cells, the cell walls of the endodermal cells contain suberin. The placement of this waterproofing substance in only certain parts of the cell wall enables the cylindrical ring of endodermal cells to control the access of water and dissolved ions to the vascular tissues.

Moving inward past the endodermis, we enter the vascular cylinder, or stele, produced by the procambium. The stele consists of three tissues: pericycle, xylem, and phloem (Figure 35.17).

The pericycle consists of one or more layers of relatively undifferentiated cells. It has three important functions:

► It is the tissue within which lateral roots arise (see Figure 35.16a).

► It can contribute to secondary growth by giving rise to lateral meristems that thicken the root.

► Its cells contain membrane transport proteins that export nutrient ions into the cells of the xylem.

35.16 Root Anatomy The drawing at the left shows a generalized root structure. (a) Cross section through the tip of a lateral root. Cells in the pericycle divide and the products differentiate, forming the tissues of a lateral root. (b) Root hairs, seen with a scanning electron microscope, (c, d] Cross sections showing the primary root tissues of (c) a eudicot and (d) a monocot.The monocot has a central pith region; the eudicot does not.

Endodermis Pericycle

Endodermis Pericycle

Eudicot Root

(c) Eudicot root Endodermis Phloem Xylem

Pith

(c) Eudicot root Endodermis Phloem Xylem

Pith

Ground Tissue Monocot Stem Image
(d) Monocot root

Root cap

Stele

Stele

Pith Xylem Phloem Pericycle Endodermes Cortex

- Epidermis

Eudicot root

Eudicot root

Pith Xylem Phloem Pericycle Endodermes Cortex

- Epidermis

Stele

Stele

Eudicot Roots

35.17 The Stele The arrangement of tissues in the stele—the region internal to the endodermis—differs in the roots of eudicots and monocots.

(see Figure 35.16). The vascular tissue of a young stem, however, is divided into discrete vascular bundles. Each vascular bundle contains both xylem and phloem. In eudicots, the vascular bundles generally form a cylinder, but in monocots, they are seemingly scattered throughout the stem (Figure 35.18).

In addition to the vascular tissues, the stem contains other important storage and supportive tissues. Internal to the ring of vascular bundles in eudicots is a storage tissue, the pith, and to the outside lies a similar storage tissue, the cortex. The cortex may contain supportive collenchyma cells with thickened walls. The pith, the cortex, and the regions between the vascular bundles in eudicots—called pith rays— constitute the ground tissue system of the stem. The outermost cell layer of the young stem is the epidermis, the primary function of which is to minimize the loss of water from the tissues within.

At the very center of the root of a eu-dicot lies the xylem—seen in cross section in the shape of a star with a variable number of points. Between the points are bundles of phloem. In mono-cots, a region of parenchyma cells, called the pith, lies in the center of the root. The pith often stores carbohydrate reserves.

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Responses

  • Edward
    How do plant roots absorb minerals?
    8 years ago
  • konsta
    What is primary root of plant?
    8 years ago
  • esa
    What primary meristem tissues give rise to the endodermis and pericycle in a root?
    7 years ago
  • christopher
    What is the sequence of the tissue in a young root , from outside to the center?
    7 years ago

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