The permanent tissues may be simple or complex. A simple tissue is made up of one type of cells forming a uniform or homogeneous system of cells.
The common simple tissues are parenchyma, collenchyma and sclerenchyma. A complex tissue is composed of more than one type of cells working together as a unit. The complex tissues consist of parenchymatous and sclerenchymatous cells; collenchymatous cells are not present in such tissues. The common instances are—the xylem and the phloem.
Type # 1. Simple Tissues:
1. Parenchyma:
The parenchyma tissue is made-up of living cells which are variable in their morphology and physiology, but generally having thin walls and a polyhedral shape, and concerned with vegetative activities of the plant. The individual cells are called parenchyma cells.
The parenchyma includes isodiametric, thin-walled and equally expanded cells. The parenchyma cells are oval, rounded or polygonal in shape having well evolved spaces among them. The cells are not greatly elongated in any direction. The cells of this tissue are living and contain sufficient amount of cytoplasm in them. Usually each cell contains one or more nuclei.
Parenchyma makes up large parts of various organs in several plants. Pith, mesophyll of leaves, the pulp of fruits, endosperm of seeds, cortex of stems and roots, and other organs of plants consists mainly of parenchyma. The parenchyma cells also exist in xylem and phloem.
In the aquatic plants, the parenchyma cells in the cortex contain well developed air spaces (intercellular spaces) and such tissue is known as aerenchyma. Parenchyma may be specialized as water storage tissue in many succulent and xerophytic plants.
In several plants, chlorophyll-free, thin-walled and water-turgid cells are found which represent water storage tissue. When the parenchyma cells are exposed to light they develop chloroplasts in them, and this tissue is known as chlorenchyma. The chlorenchyma contains well developed aerating system. Intercellular spaces are abundant in the photosynthetic parenchyma (chlorenchyma) of stems too.
The turgid parenchyma cells help in giving rigidity to the plant body. Partial conduction of water is also maintained through parenchymatous cells. The parenchyma acts as special storage tissues to store food material in the form of starch grains, proteins, fats and oils.
The parenchyma cells that possess chloroplasts in them make chlorenchyma which are responsible for photosynthesis in green plants. In water plants the aerenchyma keep up the buoyancy of the plants. These air spaces also facilitate exchange of gases. In many succulent and xerophytic plants this tissues store water and are known as water storage tissue. Vegetative propagation by cuttings takes place because of meristematic potentialities of the parenchyma cells which divide and evolve into buds and adventitious roots.
2. Collenchyma:
Collenchyma is a living tissue made -up of somewhat elongated cells with thick primary non-lignified walls. Important characteristics of this tissue are its early evolution and its adaptability to change in the rapidly growing organ, especially those of increases in length. When the collenchyma becomes functional, no other strongly supporting tissues have appeared. It gives support to the growing organs which do not evolve much woody tissue. Morphologically, collenchyma is a simple tissue, for it consists of one type of cells.
Collenchyma is a typical supporting tissue of growing organs and of those mature herbaceous organs which are only slightly modified by secondary growth or lack this growth completely. It is the first supporting tissue in stems, leaves and floral parts.
It is the main supporting tissue in many dicotyledonous leaves and stems of monocotyledons. Collenchyma chiefly exists in the peripheral regions of stems and leaves. It is commonly found just beneath the epidermis. In stems and petioles with ridges, collenchyma is particularly well developed in the ridges. In leaves it can be differentiated on one or both sides of the veins and along the margins of the leaf blade.
The collenchyma includes elongated cells, various in shape, with unevenly thickened walls, rectangular, oblique or tapering ends, and persistent protoplasts. The cells overlap and interlock, forming fibre like strands. The cell walls include cellulose and pectin and have high water content.
They are extensible, plastic and adapted to rapid growth. In the beginning the strands are of small diameter but they are added to, as growth continues, from surrounding meristematic tissue. The border cells of the strands can be transitional in structure, passing into the parenchyma type. The walls of collenchyma are chiefly composed of cellulose and pectic compounds and contain much water. In several plants collenchyma is a compact tissue lacking intercellular spaces.
The mature collenchyma cells are living and contain protoplasts. Chloroplasts also exist in variable numbers. They are found abundantly in collenchyma which approaches parenchyma in form. Collenchyma consisting of narrow cells possesses only a few small chloroplasts or none. Tannins can be present in collenchyma cells.
The chief primary function of the tissue is to give support to the plant body. Its supporting value is increased by its peripheral position in the parts of stems, petioles and leaf mid-ribs. When the chloroplasts are present in the tissue, they carry on photosynthesis.
3. Sclerenchyma:
The sclerenchyma (Greek sclerous = hard; enchyma = an infusion) includes thick-walled cells, often lignified, whose major function is mechanical. This is a supporting tissue that withstands various strains which result from stretching and bending of plant organs without any damage to the thin-walled softer cells.
The individual cells of sclerenchyma are called sclerenchyma cells. Collectively sclerenchyma cells make sclerenchyma tissue. Sclerenchyma cells do not contain living protoplasts at maturity. The walls of these cells are uniformly and strongly thickened. Most commonly, the sclerenchyma cells are grouped into fibres and sclereids.
Fibres:
The fibres are elongate sclarenchyma cells, generally with pointed ends. The walls of fibres are generally lignified. Sometimes their walls are so much thickened that the lumen or cell cavity is reduced very much or altogether obliterated. The pits of fibres are always small, round or slit like and often oblique.
The pits on the walls can be numerous or few in number. The middle lamella is conspicuous in the fibres. In most types of fibres, however, on maturation of cells the protoplast disappears and the permanent cell becomes dead and empty. Very rarely the fibres retain protoplasts in them.
The fibres are abundantly found in several plants. They may occur in patches, in continuous bands and sometimes singly among other cells. They provide strength and rigidity to the various organs of the plants to able them to withstand various strains caused by outer agencies. The average length of fibres is 1-3 mm in angiosperms.
The fibres are divided into two large groups-xylem fibres and extraxylary fibres. The xylem fibres evolve from the same meristematic tissues as the other xylem cells and constitute an integral part of xylem. In other word, some of the continuous cylinders in monocotyledonous stems arise in the ground tissue under the epidermis at variable distances. They are called cortical fibres.
The fibres forming sheaths around the vascular bundles in the monocotyledonous stems arise partly from the same procambium as the vascular cells, partly from the ground tissue. The fibres present in the peripheral region of the vascular cylinder, often close to the phloem, are called pericyclic fibres. The extraxylary fibres are sometimes combines into a group known as bast fibres.
Sclereids:
The sclereids are broadly distributed in the plant body. They are usually not much longer than they are broad, occurring singly or in groups. Generally these cells are isodiametric but some are elongated too. They are commonly observed in the cortex and pith of gymnosperms and dicotyledons arranged singly or in groups. In many species of plants, the sclereids occur in the leaves.
The leaf sclereids may be few to abundant. In certain leaves the mesophyll is completely permeated by sclereids. Sclereids are also common in fruits and seeds. In fruits they are disposed in the pulp singly or in groups (viz., Pyrus). The hardness and strength of the seed coat is due to the presence of abundant sclareids.
The secondary walls of the sclereids are typically lignified and vary in thickness. In several sclerides the lumina are almost filled with massive wall deposits, and the secondary wall shows prominent pits. Commonly the pits are simple and rarely bordered pits may also exist.
Type # 2. Complex Tissues:
Here the vascular tissues have been known as complex tissues. The most important complex tissues are—xylem and phloem.
Elements of Xylem:
1. Xylem:
Xylem is a conducting tissue, which conducts water and mineral nutrients upwards from the root to the leaves. The xylem is made- up of different kinds of elements.
They are:
(a) Tracheids,
(b) Fibres and fibre-trancheids,
(c) Vessels or tracheae,
(d) Wood fibres and
(e) Wood parenchyma.
The xylem is also meant for mechanical support to the plant body.
(a) Tracheids:
The trancheid is a fundamental cell kind in xylem. It is an elongate tube-like cell having tapering, rounded or oval ends and hard and lignified walls. The walls are not much thickened. It is without protoplast and non-living on maturity. In transverse section the tracheid is typically angular, by more or less rounded forms occur.
The tracheids of secondary xylem have fewer sides and are more sharply angular than the tracheids of primary xylem. The end of a tracheid of secondary xylem is somewhat chisel-like. They are dead empty cells. Their walls are provided with abundant, bordered pits arranged in rows or in other patterns.
The cell cavity or lumen of a tracheids is large and without any contents. The trancheids contain various kinds of thickenings in them and they may be distinguished as annular, spiral, scalariform, reticulate or pitted trachieds.
Tracheids alone make the xylem of ferns and gymnosperms, while in the xylem of angiosperms they exist associated with the vessels and other xylary elements. The tracheids are specially adapted to function of conduction. The thick and rigid walls of tracheids also aid in support and where there are no fibres or other supporting cells, the tracheids play a prominent part in the support of an organ.
(b) Fibres and Fibre-Tracheids:
In the phylogenetic evolution of the fibre, the thickness of the wall increases while the diameter of the lumen decreases. In most kinds the length of the cell also decreases and the number and size of the pits found on the walls also decrease. Sometimes the lumen of the cell becomes too much narrow or altogether obliterated and simultaneously pits become quite small in size. At such stage it is assumed that either there is very little conduction of water or no conduction through such type of cells, typical fibres are formed. Between this cells (i.e., fibres) and normal tracheids there are many transitional forms which are neither typical fibres nor typical tracheids.
These transitional kinds are designated as fibre-tracheids. The pits of fibre tracheids are smaller than those of vessels and typical tracheids. However, a line of demarcation cannot be drawn in between tracheid and fibre- tracheids and between fibre-tracheids and fibres. When the fibres contain very thick walls and reduced simple pits they are known as libriform wood fibres due to their similarity to phloem fibres (liber—phloem fibres).
The libriform wood fibres mainly occur in woody dicotyledons (e.g., in Leguminosae). The walls of fibre tracheids and fibres of many genera of different families possess gelatinous layers. The cells possessing such layers are called gelatinous tracheids, fibre-tracheids and fibres. In certain fibre-tracheids the protoplast persists after the secondary wall is mature and may divide to produce two or more protoplasts.
These protoplasts are separated by thin transverse partition walls and remain enclosed within the original wall. These fibre-tracheids are called septate fibre-tracheid. In fact, they are not individual cells but rows of cells. Here, the transverse partitions are true walls, and each chamber has a protoplast with nucleus.
(c) Vessels:
In the phylogenetic evolution of the tracheids the diameter of the cell has increased and the wall has become perforated by large openings. Because of these adaptations and specializations water can move from cell to cell without any resistance. In the more primitive kinds of vessel, the general form of the tracheid is retained and increase in diameter is not much.
In the most advanced types, increase in diameter is much and the cell becomes drum-shaped (viz., Quercus alba). The tracheids is sufficiently longer than the cambium cell from which it is derived. The primitive vessel is slightly longer than the cambium cell. The most advanced kind of vessel remains the length of cambium cell or is somewhat shorter, with a diameter greater than its length (drum shaped vessel).
The ends of the cells change in shape in the series from least to highest specialization. The angle produced by the tapering end wall becomes greater and greater until the end wall is at right angle to the side walls (as in drum-shaped vessel in Quercus alba). Some intermediate forms contain tail-like tips beyond the ends wall. Usually the diameter of vessel is much greater than that of tracheids and because of the presence of perforations in the partition walls they form long tubes through which water is conducted from root to leaf.
The pits are often more numerous and smaller in size than are those of tracheids and cover the wall closely. When observed in abundance they are either scattered or arranged in definite patterns on the walls of the vessels.
The openings in vessel-element walls are known as perforations. These openings are restricted to the end walls except in some slender, tapering types. The area in which the perforations occur is called as perforation plate. Commonly this is an end wall.
The strips of cell wall between scalariform perforations are the perforation bars. The perforation plate when bears single opening is described as having simple perforation. If there are two or more openings, they are called multiple perforations.
The secondary walls of vessel elements develop in a wide variety of patterns. Usually, in the first-formed part of the primary xylem a more limited area of the primary wall is covered by secondary wall layers than in the later-formed primary xylem and in the secondary xylem.
The secondary thickenings are deposited in the vessel as rings, continuous sprials or helices, with the individual coils of a helix here and there interconnected with each other, giving the wall a ladder-like appearance. These secondary thickenings are called—annular, sprial or helical and scalariform, respectively. In a still later ontogenetic type of vessel-elements, the reticulate vessel-element, the secondary wall appears like a reticulum. When the meshes of the reticulum are transversely elongated, the thickening is known as scalariform-reticulate. The pitted elements are characteristic of the latest primary xylem and of the secondary xylem.
Vessels are characteristic of the angiosperms. However some angiospermic families lack the vessels—the Winteraceae. Trochodendraceae and Tetracentraceae. In several monocotyledons (e.g., Yucca. Dracaena) they are absent from the stems and leaves.
Ontogeny of the Vessel:
The vessels are produced from procambium cells or derivatives of cambium by the fusion of the cells end to end during the last stages of development. During such fusion the end walls are lost and the lumina of the series of the cells are freely open into one another, forming a long tube. From the meristematic stage the vessel elements increase greatly in diameter.
(d) Wood Parenchyma:
The parenchyma cells, frequently exists in the xylem of most plants. In secondary xylem, this cell occurs vertically more or less elongated and placed end to end, called wood or xylem parenchyma. The radial transverse series of the cells form the wood rays and are called wood or xylem ray parenchyma.
The xylem parenchyma cells are noted for storage of food in the form of starch or fat Tannins, crystals and various other substances also exist in xylem parenchyma cells. These cells assist directly or indirectly in the conduction of water upward through the vessel and tracheids.
2. Phloem:
The xylem and phloem have evolved along more or less on similar lines. In xylem a series of tracheids, structurally and functionally united, has become a vessel whereas in phloem a series of cells in same way united, forms a sieve tube.
The fundamental cell type of xylem is tracheid, whereas in phloem the basic cell type is the sieve element. There are two forms of sieve element— the more primitive form is the sieve cell of gymnosperms and lower forms where series of united cells do not occur, the unit of a series, the sieve tube element.
Phloem like xylem is a complex tissue, and includes the following elements:
(a) Sieve elements,
(b) Companion cells,
(c) Phloem fibres, and
(d) Phloem parenchyma.
In the pteridophytes and gymnosperms only sieve cells and phloem parenchyma are present. In certain gymnosperms, sieve cells, phloem parenchyma and phloem fibres are present. In angiosperms, sieve tubes companion cells, phloem parenchyma, phloem fibres, sclerids and secretary cells are present.
(a) Sieve Elements:
The conducting elements of the phloem are collectively called sieve elements. They may be segregated into the less specialized sieve cells and the more specialized sieve tubes or sieve tube elements. The morphologic specialization of sieve elements is expressed in the evolution of sieve areas on their walls and in the peculiar modifications of their protoplasts.
The sieve areas are depressed wall areas with clusters of perforations by which the protoplasts of the adjacent sieve elements are interconnected by connecting strands. In a sieve area each connecting strand remains encased in a cylinder of substance known as callose.
The wall parts bearing the highly specialized sieve areas are known as sieve plates. If a sieve plate consists of a single sieve area, it is a simple sieve plate. Several sieve areas, arranged in scalariform, reticulate, or any other manner constitute a compound sieve plate. However, just as vessel may have perforation plates in their side walls, sieve tube elements can have sieve plate in their lateral walls.
The most important characteristic feature of the sieve-element protoplast is that it absents a nucleus when the cell completes its development and becomes functional. The loss of the nucleus exists during the differentiation of the element. In the meristematic state the sieve element resembles other procambial or cambial cells in having a more or less vacuolated protoplast with a conspicuous nucleus. Later the nucleus disorganizes and disappears.
(b) Companion Cells:
The companion cell is a specialized kind of parenchyma cell which is closely associated in origin, position and function with sieve tube elements. When visible in transverse section the companion cell is usually a small, triangular rounded or rectangular cell beside a sieve-tube element.
These cells are living having abundant granular cytoplasm and a prominent elongated nucleus which is retained throughout the life of the cell. Generally the nuclei of the companion cells serve for the nuclei of sieve tubes as they lack them. The companion cells do not contain starch.
They live only so long as the sieve- tube element with which they are associated and they are crushed with those cells. The companion cells are produced by longitudinal division of the mother cells of the sieve-tube element before specialization of this cell begins. One daughter cell becomes a companion cell and other a sieve-tube element.
The companion cell initially can divide transversely several times producing a row of companion cells so that one to several companion cells may accompany each sieve-tube element. A companion cell or a row of companion cells produced by the transverse division of the single companion-cell initial may extend the full length of the sieve-tube element.
The number of companion cells accompanying a sieve-tube element is fairly constant for a particular species. The solitary and long companion cells exist in primary phloem and herbaceous plants whereas numerous companion cells exist in the secondary phloem of woody plants.
The companion cells exist only in angiosperms where they accompany most sieve-tube elements. In the phloem of several monocotyledons, they are abundant, together with sieve tubes making up the entire tissue. The sieve cells of the gymnosperms and vascular cryptogams have not companion cells.
(c) Phloem Fibres:
In several lowering plants, fibres form a prominent part of both the primary and secondary phloem. The phloem fibres are rarely found or absent in phloem of living pteridophytes. They are also not found in certain gymnosperms and angiosperms. Only simple pits are found on the walls of phloem fibres. The walls may be lignified or non-lignified.
The Cannabis (hemp) fibres are lignified, whereas fibres of Linum (flax) are of cellulose and without lignin. Due to the strength of strands of phloem fibres, they have been used for a long time in the manufacture of cords, ropes, mats and cloth. The fibres used in this way have been known since early times as bast or bass—such way the phloem fibres are also known as bast fibres.
(d) Phloem Parenchyma:
The phloem contains parenchyma cells that are concerned with many activities characteristic of living parenchyma cells, this as storage of starch, fat and other organic substances. The tannins and resins are also observed in these cells.
The parenchyma cells of primary phloem are elongated and are oriented, like the sieve elements. There are two systems of parenchyma observed in the secondary phloem. The phloem parenchyma is not found in several or most of monocotyledons.
3. Secretory Tissue:
The tissues that are concerned with the secretion of gums, resins, volatile oils, nectar latex and other substances are known as secretory tissues.
These are further subdivided into two groups:
(a) Laticiferous tissue and
(b) Glandular tissue.
(a) Laticiferous Tissue:
Usually latex is present in the families of several flowering plants. This substance may be white, yellow or pinkish in colour. This is a viscous fluid and established to be colloidal in nature. Several substances like sugars, proteins, gums, alkaloids, enzymes, rubber, etc., remain suspended in a matrix of watery fluid. Starch grains can be abundantly present.
The latex of some plants is of great importance, especially as a source of rubber (e.g., Hevea, Ficus, etc.), chicle (Achras), and papain (Carica). The laticiferous ducts, in which latex is found, can be of two types —latex cells or non- articulate latex ducts and latex vessels or articulate latex ducts.
The functions and the contents of the two are same but they differ in their nature and morphology. They possess numerous nuclei in the thin layer of cytoplasm along the cell wall. The function of these tissues is not yet clearly understood. The can acts as food storage organs or as reservoirs of waste products.
Non-Articulate Latex Ducts or Latex Cells:
These ducts are independent units which extend as branched structure for long distances in the plant body. They originate as minute structures, elongate quickly and ramify in all directions of the plant body by repeated breaching but they do not fuse together, therefore, no netted structures are formed as they are formed in articulate ducts. The walls of the ducts are soft and very often thick. These ducts are commonly found in Calotropis, Euphorbia, Nerium, Vinca, etc.
Articulate Latex Ducts or Latex Vessels:
These ducts or vessels are the result of anastamosing of many cells together. They originate in the meristems from rows of cells by the absorption of the separating walls early in the ontogeny of the cells. They grow more or less as parallel ducts which by means of branching and frequent anastamoses produce a complex network. A duct of this type resembles with the xylem vessel only in the respect that it is made up of a series of cells united to produce a tube otherwise the latex tube is living and coencotyic. This latex vessels are commonly found in many angiospermic families—Papaveraceae, Compositae, Euphorbiaceae, Moraceae, etc.
(b) Glandular Tissue:
This tissue includes special structures, the glands. These glands contain some secretory or excretory products. The glands may include isolated cells or small groups of cells with or without a central cavity. They are of various kinds, and more common types are those which secrete digestive enzymes, called digestive glands and those which secrete nectar, known as nectaries. They can be internal or external.
The common internal glands which are usually lying embedded in the interior tissues of plant body are oil glands, mucilage secreting glands, glands secreting gums, resin and tannin, etc., digestive glands secreting enzymes, water secreting glands also called hydathodes. The common external glands which exist on the epidermis may be glandular epidermal hairs, nectaries, etc.
Digestive Glands:
In certain insectivorous plants there are special glands which secrete protein-digesting enzymes. These enzymes act upon insects so that the products of digestion may be absorbed by the plant in Drosera, the secretory tissue is at the tips of the leaf tentacles. Here in addition to the digestive enzymes there are secreted viscid substances which hold the insects.
These are internal glands.
Oil Glands:
These are internal glands which frequently possess essential oils in them. These oils are volatile and odoriferous. These glands originate due to split of certain cells, but they are produced in abundance by the breaking down of cells containing the volatile oil. On the disintegration of the cells the oil stores up in the large cavities of glands. These cavities are laysigenous in nature. These oil glands are commonly found in Citrus, Eucalyptus and other plants.
Hydathodes (Water Secreting Glands):
Many plants contain special structures which exudate water under situation of low transpiration and abundant soil moisture. These modified special structures are known as hydathodes or water stomata.
The water stoma resembles an ordinary stoma, and morphologically it is considered to be enlarged stoma which has lost the power of movement and serves for water secretion. Generally, the hydathodes occur at the tips of the leaves of Pistia and other avoids water hyacinth, grasses, garden nasturtium and several other plants of humid climate. The hydathodes are internal glands.
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