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The root apical meristem, or root apex, is a small area at the tip of the root in which all cells are capable of repeated division and from which all major root tissues are derived. The root tip is protected as it moves through the soil by an outer zone of living parenchyma cells called the root cap. As the root cap cells are destroyed and removed, new parenchyma cells are added from a special inner layer of meristematic cells called calyptrogen. Root hairs also begin to develop as simple extensions of protodermal cells near the tip of the root. They greatly increase the surface area of ​​the root and facilitate the absorption of water and minerals from the soil.

What Is The Function Of Phloem In Plants

Along the longitudinal axis of the root, beginning with the root cap and moving away from the root tip, there are five distinct zones in which certain specific growth patterns dominate: cell division, cell elongation, primary tissue maturation, tissue mature primary and secondary. tissue growth (the latter is found in woody roots – i.e., those of perennial eudicots). There is a gradual transition between these regions.

Plant Cells And Tissues

The region of cell division includes the apical meristem and the primary meristems—protoderm, ground meristem, and procambium—that arise from the apical meristem. As is generally true of non-meristematic regions elsewhere in the plant body, root length in the second region increases as a result of cell elongation rather than cell division. The maturation region that follows is where cells differentiate (ie, the change in structure and physiology into cells of a specific type) and where phloem and the first primary xylem, as well as mature root hairs, are clearly seen. The region of mature primary tissues is where the anatomy of the primary root body is most apparent and where all elements of the vascular cylinder, cortex, and epidermis are visible. Finally, in the secondary growth region, the secondary xylem and phloem, as well as the periderm add to the circumference of the plant.

Stems) and all converge into a single central vascular cylinder in the root, forming a continuous system of vascular tissue from root tips to leaves. At the center of the vascular cylinder of most roots is a hard, striated (or ridged) core of primary xylem (Figure 9). The main phloem lies between these flutes or ridges. Parenchyma cells are distributed throughout the vascular cylinder.

The vascular cylinder of the root is surrounded by a layer of pericyclic parenchymatous cells. As the root ages, many of these cells become fibers, especially in monocotyledons and many herbaceous eudicots. As defined above (

Tissue systems: Terrestrial tissue), a characteristic feature of parenchyma cells is their ability to differentiate into cells of a different type under appropriate conditions. The parenchymal cells of the pericycle can therefore be considered meristematic in that they give rise to new lateral meristems and lateral roots. In woody roots the vascular cambium (the lateral meristem that gives rise to secondary phloem and secondary xylem) originates in the pericycle as well as the procambium; procambium is the primary meristematic tissue between primary phloem and xylem. The first cork cambium is a lateral meristem arising in the pericycle; successive cork cambium arises in the secondary phloem. Because lateral roots begin in the pericycle and grow through the cortex and epidermis, they are said to have an internal or endogenous origin, in contrast to the external or exogenous origin of leaves and stem apical meristems.

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Ground tissue called the cortex surrounds the vascular cylinder and pericycle. The root cortex is generally composed of parenchymal cells with large intercellular air spaces. The endoderm (the innermost layer of the cortex adjacent to the pericycle) consists of closely packed cells that have within their walls Casparian strips, water-impermeable deposits of suberin that regulate water and mineral uptake by roots. The cortex is surrounded by the dermal system consisting of a single layer of epidermal cells.

The few variations that occur in root anatomy are found mainly in monocotyledons. Monocotyledon roots lack secondary growth. Also, monocotyledons generally have a parenchymal pit in the center of the vascular cylinder and fibers or sclereids, or both, in the cortex; and well-developed pericyclic fibers. Orchids have a multi-layered epidermis called the velamen, which consists of compact non-living cells with lignified bands of secondary walls. These cells provide support, prevent water loss and help the plant absorb water. When dry, the orchid root appears white, and when wet, the root appears green because the velamen cells absorb water, become translucent, and reveal the green cortical cells.

The shoot apical meristem and primary meristems extend to the tip of the stem and give rise to primary stem tissues. The shoot apical meristem produces leaves and axillary buds exogenously; as a result, the stem and leaf epidermis is continuous. (In contrast, as mentioned above, lateral roots are produced endogenously, and the dermal system of the lateral roots is discontinuous with that of the parent root.)

The stem has growth periods similar to those of the root, but the longitudinal regions are not as apparent as in the root until the nodes differentiate and the internodes increase in length. Internodal elongation involves many cell divisions and is followed by cell elongation. At this point, the increase in thickness involves a radial cell division and cell expansion.

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Primary tissue systems appear after internodal elongation. The procambium differentiates as an essentially continuous hollow cylinder or discrete procambial strands, which differentiate into primary xylem and phloem. The ground tissue that lies outside the procambial cylinder is the cortex, and that inside is the shaft. Ground tissue called interfascicular parenchyma lies between the procambial strands and remains continuous with the cortex and pith. As the vascular tissue grows, xylem and phloem develop, vascular bundles mature, the single-layered epidermis differentiates as epidermal cells, trichomes, and some stomata, and the parenchymal pit may develop as collenchyma or contain sclereids or fibers or both; unequal spreading and expansion of the core produces the flattened stems (pads) of spiny cacti (

; Cactaceae). The parenchymatous cortex may also develop some collenchyma, sclereids, or fibers; Uneven growth and expansion of the cortex produces cladodes of epiphytic cacti (eg, night-blooming cereus,

; Cactaceae). In most aquatic angiosperms, the parenchymatous cortex contains large intercellular spaces. As a rule, angiosperm stems do not have a defined endoderm or pericycle.

The most common arrangement of primary xylem and phloem is called a collateral bundle; the outer part of the procambium (near the cortex) becomes the phloem, and the inner part (near the pit) becomes the xylem. In a dicotyledonous plant, the phloem is both outside and inside the xylem, as in Solanaceae (potato family) and Cucurbitaceae (cucumber family). In monocots, phloem can surround xylem, or xylem can surround phloem.

Cross Section Through A Dicotyledonous Leaf

The vascular bundles of the stem are continuous not only with the primary vascular system of the root but also with the vascular bundles of the leaves. At each node, one or more longitudinal stem bundles enter the leaf base as leaf traces, connecting the vascular system of the stem to that of the leaf. The point at which the stem bundle diverges from the vascular cylinder toward the leaf is a leaf gap, called a lacuna. The number of lacunae varies among angiosperm groups and remains a characteristic for the classification of different species.

Several leaves in a line along the stem have common stem bundles. In some species, all stem bundles and their leaf traces are interconnected, while in others each stem bundle and associated leaf trace remain laterally independent of the others. An arrangement of two trace leaves and a single gap is found among some primitive angiosperm families and in all gymnosperms and is the arrangement from which other nodal patterns are derived.

In woody eudicots, the vascular cambium is formed in parts that grow towards each other and fuse. Each vascular bundle develops a meristematic zone of growth from an undifferentiated (parenchymal) layer of cells between the primary xylem and primary phloem, called the fascicular cambium. This meristematic zone spreads laterally from each tuft and eventually becomes continuous, forming a complete vascular cambium.

The arborescent (tree-like) stems of monocots have a different growth pattern and anatomy than eudicots. Scattered throughout the ground tissue are vascular bundles without a fascicular cambium and without a defined pit or cortex. Secondary growth, when it occurs, is different because a thickening secondary meristem forms beneath the epidermis. This thickening secondary meristem produces secondary parenchyma (connective tissue) internally, and then secondary vascular bundles develop within this connective tissue. Thus, there are no rings of secondary xylem or secondary phloem as in woody eudicots.

Composite Of Cross Section Of A Lilac Leaf

Many arboreal monocots have only massive primary growth without secondary growth. This primary growth flows

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