BSC 314 Lecture Notes - Lecture 7: Phloem, Cork Cambium, Vascular Cambium
26 Jun 2018
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Primary Root Tissues and Structure
The organization of tissues in the primary root is simpler than in the primary stem
because no leaves are produced on the roots and, consequently, there is no need to
connect the vascular system laterally to offshoots. The primary body, produced by the
three primary meristems, consists of a central cylinder of vascular tissue, the stele,
surrounded by large storage parenchyma cells—the cortex—on the outside of which
lies a protective layer of cells—the epidermis.
Epidermis
The root hairs of the young epidermal cells vastly increase the surface area through
which movement of materials can occur. The thread-like hairs are simply enlargements
of the protoplast that extend outward into the soil. They have little wall material and are
extremely fragile and easily broken. The root epidermis of some plants is covered by a
thin, waxy cuticle, which apparently isn't thick enough to impede movement of
substances through the epidermis.
Cortex
The cortex, composed primarily of parenchyma cells, is the largest part of the primary
root, but in most dicots (eudicots) and in gymnosperms that undergo extensive
secondary growth, it is soon crushed, and its storage function assumed by other
tissues. Three layers of cortex are recognized: the hypodermis (also called exodermis),
the endodermis and, between them, the storage parenchyma. The outer and inner
layers of the cortex, the hypodermis and endodermis, are cylinders of tightly packed
cells with heavily suberized walls and no intercellular spaces. (Suberin is the fatty
substance that gives cork its distinctive attributes.) In contrast, the storage parenchyma
cells are thin-walled and loosely packed with many intercellular spaces among them.
Hypodermis (exodermis). Just under the epidermis forming the outermost layer of the
cortex is a layer one or two cells in width called the hypodermis. Since its cell walls are
heavily suberized and impermeable to water its apparent function is to keep the water
and nutrients (which are absorbed in the root zone further down the root) from leaking
out through the cortex. The hypodermis is especially well developed in plants of arid
regions and in those with shallow root systems. It also deters the entrance of soil
microorganisms.
Endodermis. The innermost layer of the cortex is the endodermis, which is readily
identifiable by the presence of Casparian strips, bands of suberin present on
transverse and radial walls of its cells—the walls perpendicular to the surface of the
root. The endodermis regulates the passage of water and dissolved substances by
forcing them to move through living plasma membranes and plasmodesmata and not
simply diffuse through the porous cell walls. The absorption and translocation of
materials is thus selective; not everything in the surrounding soil gets through and into
the plant body. An endodermis almost always is present in roots and generally never in
stems.
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Document Summary
The organization of tissues in the primary root is simpler than in the primary stem because no leaves are produced on the roots and, consequently, there is no need to connect the vascular system laterally to offshoots. The primary body, produced by the three primary meristems, consists of a central cylinder of vascular tissue, the stele, surrounded by large storage parenchyma cells the cortex on the outside of which lies a protective layer of cells the epidermis. The root hairs of the young epidermal cells vastly increase the surface area through which movement of materials can occur. The thread-like hairs are simply enlargements of the protoplast that extend outward into the soil. They have little wall material and are extremely fragile and easily broken. The root epidermis of some plants is covered by a thin, waxy cuticle, which apparently isn"t thick enough to impede movement of substances through the epidermis.
