Tissue is a cellular organizationallevel intermediate between cells anda complete organism. Hence, a tissueis an ensemble of cells, not necessarilyidentical, but from the same origin,that together carry out a specificfunction.
The study of tissue isknown as histology or, inconnection with disease,histopathology.
Plant Tissues Permanent Tissues Meristimatic Tissues (Cells with speciallised structure and function that have lost their(These cells continuously divide throughout the life of the plant) ability to divide further) Simple Permanent Tissue Apical Meristem Complex Permanent Tissue Parenchyma Xylem Lateral Meristem Collenchyma Phloem Intercalary Meristem Sclerenchyma
Meristematic tissue: Cells of this tissue continue to dividethroughout the life of the plant. Some of these cells lose theirability to divide and become part of other tissues. Name of the Location Function tissue Apical Meristem Present at the growing tip of stem Increase in length of stem and root and root Lateral Meristem Found on the lateral sides of Increase in girth of stem (also called roots and stem and root cambium) Intercalary Present at the base of leaves or Growth of leaves and Meristem internodes branches
A longitudinal section through a growing shoot tip showing apical meristematic tissue. Note that the cells are small, have dense cytoplasm, and are very tightly packed.
High power view of a longitudinal section of the Coleus apicalmeristem. The apical meristem is adome-shaped mass of dividing cells at the tip of the shoot. The apical meristem will produce the three primary meristems: protoderm, procambium, and ground meristem. These three meristems in turn will produce new cells that will differentiate into the epidermis, primary vascular tissues, and ground tissues (pith and cortex).
A longitudinal section through a root tip. The meristematic tissue islocated just above the root cap. This too is apical meristem; divisionof these cells followed by cell elongation results in the root growing in length.
It is a cross section of a dicotstem.Focus on the two largevascular bundles in the centerof the slide.The xylem tissue is stainedred.Just above the xylem is alayer of meristematic tissue,the vascular cambium.The phloem tissue is foundoutside of the vascularcambium.
This is a high-power view of a cross-section showing a lateral meristem, the vascular cambium, in thesame plant shown in previous slide. Again, the xylem tissue is stained red, and the large cells on the top of the slide are phloem. The green brick-like cells between the xylem and phloem is the area in which the vascular cambium is located. The new cells produced by the cambium are initially like those of the cambium itself, but, as they grow and mature, their characteristics slowly change as they differentiate into other tissues. The vascular cambium is a single layer of cells within this brick like region; it is responsible for the growth in diameter of a stem. The tissues produced by the vascular cambium are secondary tissues.
Permanent tissue: Cells of this tissue have lost their ability todivide and they have a specialized structure to performspecific functions.Based on the type of cells present in the tissue, thePermanent tissue is divided into two categories:Simple Permanent TissueandComplex Permanent Tissue.While the simple permanent tissue consist of only one typeof cells (eg. Parenchyma),the complex permanent tissue consists of more than onetype of cells (eg. Xylem and phloem)
S i m p l e P e r m a n e n t T i s s u e sParenchymaStructure:It is the fundamental tissue composed of thin walled, livingcells whose cell wall is composed of cellulose. Small intercellular spacesare present between the cells.Location and function: It occurs in all soft parts of plants and ismeant for storage of food and to provide turgidity to softer parts of plants.Parenchyma tissue in stem and roots store nutrients and water.Types of parenchyma:i) Chlorenchyma :Certain parenchymatous tissue contain chloroplastand synthesize food by the process of photosynthesis.ii) Aerenchyma: In aquatic plants parenchymatous cells have air cavitiesbetween them to store air, such a tissue is called Aerenchyma. It providesbuoyancy to the aquatic plants so that they can float in water.
CollenchymaStructure: This tissue is composed of somewhat elongated cells withcell walls that are irregularly thickened at corners due to deposition ofcellulose or pectin. They may be oval, circular or polygonal. Very littleintercellular spaces are present.Location: It occurs below the epidermis of stem and petiole (stalk ofthe leaf) and around veins.Function: This tissue provides mechanical support and flexibility andin some cases it may possess chloroplasts to perform Photosynthesis.The stem and leaves are able to bend easily and then come back totheir original position due to the presence of collenchyma.
SclerenchymaStructure: It is a tissue of dead and thick walled cells, having nointercellular spaces. The thickenings are of cellulose or lignin or both.Several unlignified areas called pits often develop on walls.Location: This tissue is usually found in the hard and stiff parts of theplant like seed coat, husk of coconut, in the stem around vascularbundles, veins of leaves and hard covering of fruits and nuts.Function: It is the chief mechanical tissue in plants and is able to bearpush, pull, strain and shearing forces. It provides strength to plant partsand also protects the delicate parts of the plants.They are of two types: fibres and sclereids.
Epidermis and Bark The protective tissuesThe epidermis usually consists of a single-layered group of cells that coversplants leaves, flowers, roots and stems. It forms a boundary between the plantand the external world.Bark is formed from the meristem that appears later in the life cycle of a plant.Woody stems and some other stem structures produce a secondary coveringcalled the secondary meristem or periderm or cork cambium that replaces theepidermis as the protective covering.The periderm replaces the epidermis, and acts as a protective covering like theepidermis.Cells produced on the outside by periderm form the cork. Cells of have suberinin their walls to protect the stem from drying and pathogen attack. Older corkcells are dead, as is the case with woody stems. As the stem grows, the corkcambium produces new layers of cork which are impermeable to gases andwater.
A high-power view of one glandular hair. Secretory hairs may provide a chemical defense against insects.
Another type of surface tissue, the outer bark or periderm (stained red in this slide). Periderm is found on the surface of woody plants; it includes the cork cells on thesurface of older woody stems. The periderm replaces the epidermis in plants that have secondary growth. The cork cells are dead; it is their waterproofed cell walls that function as the protective outer covering of plants. Meristematic cells within the periderm (cork cambium, the other lateral meristem) produce the cork cells.
XylemIt is a complex permanenttissue, which is specializedfor the conduction of waterand mineral substances inthe plant body. Xylem is aheterogenous tissue madeup of four different types ofcellular elements.They are:•Xylem tracheids•Xylem tracheae or vessels•Xylem fibers and•Xylem parenchyma
Phloem:Phloem is a complexpermanent tissue, which isspecialized for theconduction of food andother organic substances.Phloem is also aheterogenous tissue, madeup of four different types ofcellular elements, namely,•Sieve tubes•Companion cells•Phloem parenchyma and•Phloem fibres