Meristematic regions are parts of the plant that are concerned with cell division and are found at the tips of the root, shoot and branches. These regions of growth are called apical meristems. Buds found between the leaf and stem are meristems.
Plants grow throughout their lifetime because they retain some meristematic cells that continue to divide as long as the plants live. This form of growth is called indeterminate. In contrast, animals cease growth after attaining a certain size. Such growth is called determinate.
Growth of the Root
Meristematic roots show three distinct zones of growth. At the tip of the root is the root cap which acts as which is a protective structure that is made up of nondividing parenchyma cells which are being replaced constantly by cells from the zone of division because they get rubbed off as the root pushes its way through the soil. These cells produce a gelatinous substance that lubricates the root tip and thereby facilitate the pushing of the root tip through the soil as it elongates.
a) zone of division
Located just behind the root cap is the apical meristematic region, a small region that consists of small actively dividing cells. These dividing cells give rise to the two other zones, namely the zone of elongation and the zone of differentiation.
b) zone of elongation
The zone of cell elongation is found just behind the meristem. It consists of immature undifferentiated cells that are undergoing elongation. It extends only few millimetres along the root. In a maize plant, the fastest elongation is only about 4mm from the tip and cells more than 10 mm from the tip have completed their elongation. The elongation makes it possible for the roots to push faster through the soil than it would have been if it were only pushed by the production of new cells from the zone of division.
c) zone of maturation
The zone of elongation gives to the zone of maturation or differentiation in which the various internal structures of the root assume their characteristic roles. The zone of mature consists of mature cells that have differentiated into various tissues. This zone is easy to identify because it contains root hairs.
Primary tissues of roots
A cross - section of the young root immediately near the tip shows three meristematic zones. These are the protoderm, the ground tissue and the provascular cylinder.
The protoderm will give rise to the epidermis, the ground tissue forms the cortex and the provascular cylinder will form the stele. The stele consists of the precursors for the primary xylem, primary phloem, the pericycle and vascular cambium. The protoderm, ground tissue and provascular cylinders give rise to the primary tissues of the plant.
Cross-section of young plant root
Primary Growth of Roots
The protoderm gives rise to the epidermis of the root, a single layer of cells that protects the young roots. Root hairs are extension of the epidermal cells.
The cortex makes up the bulk of the young plant. It is the region between the epidermis and the vascular system. It is mainly made up of dividing parenchyma cells with numerous intercellular spaces. The cortex store large quantities of starch in its cells. It also channels absorbed water and mineral salts to the vascular system. In young roots , the cortex is prominent but in older roots it is much reduced or even absent where both the cortex and the epidermis are replaced by a corky periderm.
The innermost layer of the cortex, one cell thick, is the endodermis.
The endodermis, which separates the cortex from the stele, consists of a layer of cells whose walls are bound by a waterproof band, the Casparian strip, a band of waxy material called suberin. The Casparian strip runs through the radial (side) and end walls of the endodermal cells. This strip does not allow water and solutes to pass between adjacent cell walls.
Therefore, the only way to the vascular cylinder is through the endodermal cells themselves. In this way, the strip regulates the substances that enter the vascular system. The endodermis selectively regulates the substances that enter the vascular system.
The endodermis forms the outer boundary of a central core of the root called the stele that consists of the vascular cylinder. Inside the endodermis is the pericycle, is a thin layer, one cell thick, of parenchymatous cells. The cells of the pericycle are meristematic and give rise to lateral or secondary roots.
The central region of the dicot stele surrounded by the endodermis and pericycle consists of the vascular vessels, specifically the phloem and the xylem. These constitute the transport system of the plant. Substances manufactured by the plant in the leaves are transported within the phloem while substances absorbed from the soil are carried within the xylem.
Cross-sections of monocot and dicot young stems
Large monocot plants have a region of parenchyma tissue , the pith, located at the centre of the stele and because of this the xylem does not form the star – shaped figure characteristic of dicots.
Primary Growth of the Stem
Angiosperms or flowering plants consist of herbaceous and woody plants. Herbaceous plants are soft non-woody plants. They can be dicots or monocots. They usually complete their reproductive cycles in one year and die.
Monocots differ from the dicots in the arrangement of the vascular bundles.
The vascular bundles of monocots are scatted throughout the stem while those of dicots show a definite arrangement in which the vascular bundles are between the cortex and the pith. In monocots, the pith is continuous with the cortex but it is inside the ring formed by the vascular vessels in dicots.
The primary tissues of the stem arise from the division of the cells of the terminal bud at the tip of the stem. Like the root, division of the terminal buds results in the formation of protoderm, procambium and ground tissue meristems.
Secondary Growth of the Stem
In mature dicots, the division of the vascular cambium results into secondary xylem to the inside towards the pith and secondary phloem to the outside towards the cortex. This increases the diameter of the stem. Thus, as more layers are added, the phloem and the cortex are pushed further outwards until the epidermis can no longer withstand the pressure and splits. Consequently, it is shed as bark.
Cross-section of woody stems showing primary and secondary tissues
Another meristematic region, the cork cambium, located in the cortex, produces cork, a shiny waxy substance that forms part of the periderm or the new surface coat of the stem.
The woody part of the stem is predominantly made up of layers of secondary xylem. Most of the secondary phloem is lost as bark in mature trees.
In temperature regions where seasonal changes are marked, it is possible to tell the age of a tree by the rings of layers due to secondary xylem growth.
Insert diagram showing annual growth rings of secondary xylem
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