Roots anchor plants and absorb water and minerals in solution. A germinating seed radicle becomes the first root. Four zones, or regions, of young roots are recognized: (1) A protective root cap that also aids in the perception of gravity. (2) A region of cell division. Its apical meristem subdivides into a protoderm, which produces the epidermis; a ground meristem, which produces the cortex; and a procambium, which produces primary xylem, phloem, pith. (3) A region of elongation in which the cells produced by the apical meristem become considerably longer and slightly wider. (4) A region of maturation in which the cells mature into the distinctive cell types of primary tissues. Cells of the apical meristem are generally cuboidal in shape. The perception of gravity by root is amyloplasts. Some of the epidermal cells in the region of maturation develop root hairs; the root hairs greatly increase the absorptive surface of the root.
A longitudinal section through a dicot root tip. A. Regions of the root. B. Locations of the primary meristems of the root.
The tissues that mature in this region are similar to those of stem tips, but pith is absent in most dicot roots and originates from the procambium in monocot roots. At its inner boundary the cortex has an endodermis with suberized Casparian strips. Some endodermal cells, called passage cells, may remain thin-walled Next to the endodermis toward the center of the root are parenchyma cells constituting the pericycle. Branch roots arise of the pericycle. The vascular cambium in young roots of many dicots and conifers arise of parts of the pericycle and parenchyma between the arms of xylem and patches of phloem. The endodermis consists of a single-layered cylinder of compactly arranged cells whose primary walls are impregnated with suberin. The suberin bands, called Casparian strips, are on the radial and transverse walls. The plasma membranes of the endodermal cells are fused to the Casparian strips, which are perpendicular to the root s surface; they prevent water from passing through the otherwise permeable (porous) cell walls.
The pericycle, which is usually one cell wide, may in some plants be a little wider. The cells of the pericycle may continue to divide even after they have matured. Lateral (branch) roots and part of the vascular cambium of dicots arise within the pericycle. In dicot roots, the primary xylem usually first forms a solid core with two to several arms in the center of the root; a pith may be present in monocot roots. Vessel element is relatively thin-walled. Primary phloem first is produced in discrete patches between the primary xylem arms, but the tissues eventually appear as concentric cylinders. In woody plants, a cork cambium usually arises in the pericycle and produces cork tissues similar to those of stems. Roots may graft together naturally. There are no leaves in roots. In many dicot plants, a cork cambium producing cork and phelloderm cells develops near the surface of the root. Pith (parenchyma) tissue, which originates from the ground meristem, is generally present in stems but is absent in most dicot roots. Grass roots and those of most other monocots, however, do have pith tissue.
A longitudinal section through a monocot root. A. Regions of the root. B. Locations of the primary tissues of the root.
A cross section of a dicot (buttercup Ranunculus) root. A. Complete view. 40. B. Enlargement of the root center (vascular cylinder). 400.
A portion of the endodermis of a buttercup (Ranunculus) root. 1,000.
Part of the center of a buttercup root, showing endodermal cells with Casparian strips. 600.
A cross section through a dicot (willow Salix) root showing the origin of a lateral (branch) root.
Each stem has an apical meristem at its tip that produces tissues resulting in increase in length. Leaf primordia develop into mature leaves when growth begins. Three primary meristems develop from an apical meristem: the protoderm gives rise to the epidermis; the procambium produces primary xylem and primary phloem; and the ground meristem produces pith and cortex (parenchyma cells ). The primary xylem and primary phloem are in discrete vascular bundles. As leaves and buds develop from primordia, traces of xylem and phloem branch off from the main cylinder, leaving leaf gaps or bud gaps. A vascular cambium, producing secondary tissues, may arise between primary xylem and phloem. Secondary xylem cells include tracheids, vessel elements, and fibers. Secondary phloem cells include sieve tube members and companion cells
In many plants, a cork cambium producing cork and phelloderm cells develops near the surface of the stem. Cork cells, which are part of the outer bark (periderm), have suberin in their walls. Suberin is impervious to moisture, and the outer bark, therefore, aids in protection. Lenticels in the bark permit gas exchange. As woody stems age, lenticels develop beneath the stomata. As cork is produced, the parenchyma cells of the lenticels remain so that exchange of gases (e.g., oxygen, carbon dioxide) can continue through spaces between the cells. Primary vascular tissues and the pith, if present, constitute the stele. Protosteles have a solid core of xylem, usually surrounded by phloem; siphonosteles are tubular, with pith in the center; eusteles have the vascular tissues in discrete bundles. The cortex would not be part of a stele. Herbaceous dicots have vascular bundles arranged in a ring in the stem. The pith in woody plants eventually will be crushed and replaced by new tissues produced from within as growth occurs.
A. A cross section of an alfalfa (Medicago) stem. x40. The tissue arrangement is typical of herbaceous dicot stems. B. An enlargement of a small portion of the outer part of the stem. x400
Woody dicots have most of their secondary tissues arranged in concentric layers. The most conspicuous tissue is wood (secondary xylem). In broadleaf trees, spring wood usually has relatively large vessel members, while summer wood has smaller vessels and/or a predominance of tracheids. An annual ring is 1 year s growth of xylem. A tree s age and other aspects of its history can be determined from annual rings. Rays, which function in lateral conduction, radiate out from the center of the trunk. Older wood toward the center (heartwood) ceases to function when its cells become plugged with tyloses. Younger, functioning wood (sapwood) is closer to the surface. A tree s functions are not particularly affected by the rotting of its heartwood. In woody plants, older tissues composed of thin-walled cells become crushed and functionless, and some are sloughed off.
A cross section of a portion of a young linden (Tilia) stem. ca. X300.
This tree was 100 years old when it was cut down. Note the proportion of sapwood, which consists of functional cells, to heartwood, in which the cells are no longer capable of conduction.
The vascular rays consist of parenchyma cells that may remain alive for 10 or more years. Laticifers are latex-secreting cells or ducts found in various flowering plants. The latex of some plants has considerable commercial value. Monocot stems have scattered vascular bundles and no cambia. The parenchyma tissue is not divided into pith and cortex. Each vascular bundle is surrounded by a sheath of sclerenchyma cells. Numerous bundles and a band of sclerenchyma cells and thicker-walled parenchyma cells just beneath the surface of monocot stems aid in with standing stresses. The ground tissue or fundamental tissue of monocots is composed of parenchyma. The dry part of wood consists primarily of cellulose and lignin. Resins, gums, oils, dyes, tannins, and starch are also present. Properties of wood that play a role in its use include density, specific gravity, and durability.
A cross section of a monocot (corn Zea mays) stem. X20. A single vascular bundle of corn (Zea mays) enlarged. X400.
All leaves originate as primordia. The lower and often the upper surfaces of leaves have pores (stomata) that permit air circulation and facilitate photosynthesis. Leaves also respire, accumulate wastes, and eliminate excess moisture via transpiration. Stomata are very numerous, ranging from about 1,000 to more than 1.2 million per square centimeter (6,300 to 8 million per square inch) of surface. The epidermis is a surface layer of cells that is coated with a cuticle. Waxes, glands, hairs, and an occasional cellular crystal may also be present. The lower epidermis usually contains numerous stomata, each formed by a pair of guard cells that regulate both evaporation of water vapor from the leaf and gas exchange between the interior and the atmosphere. Guard cell have chloroplasts. The mesophyll (chlorenchyma) between the upper epidermis and the lower epidermis may be divided into an upper palisade layer, which consists of rows of parenchyma cells containing numerous chloroplasts, and a lower spongy layer in which the parenchyma cells are loosely arranged.
A dicot stoma. A. Surface view. B. View in transverse section.
Calcium carbonate crystal. Transverse section of upper portion of rubber plant (Ficus elastica) leaf blade
A stereoscopic view of a portion of a typical leaf.
This region is called the palisade mesophyll and may contain more than 80% of the leaf s chloroplasts (more photosynthesis). The lower region, consisting of loosely arranged parenchyma cells with abundant air spaces between them, is called the spongy mesophyll. Its cells also have numerous chloroplasts. Veins (vascular bundles) of various sizes are scattered throughout the mesophyll. They consist of xylem and phloem tissues surrounded by a jacket of thicker-walled parenchyma cells (fibers) called the bundle sheath. The veins give the leaf its skeleton. Leaves change color as green chloroplast pigments break down, revealing pigments of other colors, and different pigments accumulate in cell vacuoles. Water-soluble anthocyanin and betacyanin pigments may also accumulate in the vacuoles of the leaf cells in the fall. Anthocyanins, the more common of the two groups, are red if the cell sap is slightly acidic, blue if it is slightly alkaline, and of intermediate shades if it is neutral. Betacyanins are usually red.
Portions of cross sections of maple (Acer) leaves. The normally green chloroplasts are stained red. A leaf exposed to full sun.
Portions of cross sections of maple (Acer) leaves. The normally green chloroplasts are stained red. A leaf exposed to shade. Note the reduction in mesophyll cells and chloroplasts in the shade leaf.
The leaf pigments mostly responsible for gold to orange leaf colors in the fall are carotenes. Environmental factors and hormonal changes that occur in autumn in the abscission zone at the petiolar base of each deciduous leaf cause leaves to drop. The phloem tissue is the first to become blocked when an abscission layer forms at the base of a leaf petiole in the fall. Closest to the stem, the cells of the protective layer, which may be several cells deep, become coated and impregnated with fatty suberin. Leaves are a source of many useful products, including food, beverages, dyes, fuel, spices, flavorings, oils, medicinal and narcotic drugs, insecticides, and waxes. In some plants, there are special openings called hydathodes at the tips of leaf veins. Root pressure forces liquid water out of hydathodes, usually at night when transpiration is not occurring. The loss of water through hydathodes is called guttation.
The abscission zone of a leaf.
Monocot leaves, besides having parallel veins, usually do not have the mesophyll differentiated into palisade and spongy layers. Some monocot leaves (e.g., those of grasses) have large, thin-walled bulliform cells on either side of the main central vein (midrib) toward the upper surface. Under dry conditions, the bulliform cells partly collapse, causing the leaf blade to fold or roll; the folding or rolling reduces transpiration. Most spines are modifications of the whole leaf, in which much of the normal leaf tissue is replaced with sclerenchyma. In leaves, the epidermal cell walls perpendicular to the surface often assume bizarre shapes that, under the microscope, give them the appearance of pieces of a jigsaw puzzle.
Part of a cross section of a grass leaf. When the bulliform cells are turgid, the leaf is expanded. When insufficient water is available, the bulliform cells collapse, causing the leaf blade to roll inward, thereby reducing water loss through the stomata. X100.