Several large organisms, animals or plants are made up of hundreds of cells. In some big plants, like trees there are several millions of cells; also in animals like man, there are millions of cells. Such organisms are said to be multicellular. In contrast, there are organisms that are made up of one cell only like bacteria, chlamydomonas, Amoeba' such organisms are said to be unicellular.
The tissue is a group of similar cells that perform same function. Generally, the cells which make up a tissue are contiguous. A few plant tissues are given below:
i) Meristematic Tissue:
A meristematic tissue is composed of meristematic cells. In tissue the cells are strongly fitted together. There are no air spaces between adjacent cells. Meristematic cells are able to cell division and so meristematic tissue is growing tissue. It is found at growing points at apices of stems and roots.
ii) Parenchymatous Tissue:
In the tissue cells are not strongly fitted. Air spaces take place among adjacent cells. Parenchymatous tissue is filling tissue. It forms ground tissue and may also act as storage tissues, string water or food.
iii) Collenchyma Tissue:
This is familiar to parenchyma tissue but cells are thickened at corners and their distribution is more limited. Presence of thickenings at corners of the cells gives some mechanical strength to organs where tissue takes place. Collenchyma tissue is generally found at periphery organs. Tissues are composed of cells which are all of same type. Other tissues are composed of different kinds of cells. Every cell type has different function.
As the complex tissue the xylem is composed of different cell types among which are the following; parenchyma fibre, sclerieds and vessels. Proportions in which different cell types are present vary in different xylem tissues. Despite number of different cells, each with its feature function, the xylem tissue as a whole, functions in conduction of water and mineral salts up plant.
Like xylem, phloem is a complex tissue and cell types are present are as follows: parenchyma, fibre, sieve tubes and companion cells. Parenchyma and fibre cells have the characteristic features. Sieve tubes are formed from the row of lengthened cells joined end to end. Cross walls between adjacent cells are perforated and are known as sieve plates. As sieve tube is maturing small portion of it is cut off by a longitudinal wall. This small portion is densely filled with cytoplasm and has original nucleus of parent cell. The mature sieve tube cell thus lacks a nucleus. Smaller cell called companion cell remains closely linked with sieve tube cell and this arrangement generally helps in recognizing phloem in plant section.
In multicellular organisms, group of similar cells with intercellular substances carry out particular function, such aggregation of cells is known as tissue. Organs are functional group of several tissues. Few examples of animal tissues are given below:
Epithelium is the sheet or tube of closely fitted cells, i.e. with minimal intersticial material between cells. It covers exposed surfaces of body and lines cavities and tubes. One surface of epithelium is thus free while other rests, generally, on the connective tissue. Epithelium may be categorized in different groups using number of different criteria. One of these classifications is based on height of cell relative to its breath. On this basis we have: columnar, cubical and squamous epithelia (in order of decreasing height) Epithelia may also be categorized on the basis of whether sheet is one cell thick (simple epithelium) or several cells thick (stratified epithelium).
ii) Connective Tissue:
This is the tissue which supports and binds together different organs and tissues of animal body. It is wide in distribution and strong in texture. Basically, it comprises of ground substance or matrix in which different structures are embedded. Matrix of areola tissue comprises of a gelatinous glycoprotein matrix and four kinds of cells and two kinds of fibres are present. Cell types of fibroblasts that synthesize fibres mast cells (that are phagocytic). Fibres present are collgen fibres (white fibres) and elastic fibres (yellow fibres). It is fibres which provide connective tissue its strength and toughness.
iii) Blood - a Liquid Tissue:
Blood is liquid and comprises of fluid plasma in which several different kinds of cells are suspended, suspended cells are known as formed elements. Plasma is a complex mixture of solution of inorganic salts and blood proteins. Salts present comprise sodium bicarbonate, sodium clhloride, potasssium sulphate and potassium phosphate. Presence of these salts makes blood somewhat alkaline. Proteins in blood are of three kinds: albumins, globumins and fibrinogen. Other substances present in blood plasma comprise glucose, fats, amino acids, hormones and urea. Cells (corpuscles) found in blood are red blood corpuscles (RB erythrocytes), white blood cells (WBC or leukocytes) and Platelets (thrombocytes).
Organs of plants and animals:
An organ is the portion of plant or animal that forms structural and functional unit. It is composed of number of tissues. For instance, leaf is a plant organ. It forms part of plant and carries out function of photosynthesis. Likewise, kidney is organ of excretion in some animals.
i) The Leaf:
It is composed of a number of different tissues. On the upper surface there is upper epidermis that is a tissue composed of epidermal cells. Each of the cells is tabular in shape; it is somewhat broader that it is high. Cells are in most part directly fitted together, therefore forming 100 continuous covering over surface of organ. There is layer of curtain above epidermis. This is the cuticle and it is impervious to water and gases. It is protective tissue. Within spongy mesophyll layer are ends of vascular tissue. This is composed of phloem and xylem tissues. Xylem brings water and mineral salts to leaves while phloem carries manufactured food from leave to other parts of plant. Both upper and lower epidermis is perforated by special pores known as stomata (sing stoma). Through these gases are exchanged between interior of leaf and surrounding atmosphere.
ii) The Root:
Root is plant organ that usually joins plant tightly to ground and absorbs water and mineral salts from soil. It is bounded on outside by layer that is composed of tabular directly fitted cells. This is known as piliferous layer and it is protective in function. Young root is perforated at intervals by stomata for gaseous exchange. At intervals few epidermal cells lengthened at right angles to axis of root. These extensions of epidermal cells are known as root hairs. Root airs are significant in absorption of water and mineral salts from soil.
ii) The Stem:
Stem the plant axis which has buds and shoots with leaves and, at basal end, roots. Stem is stalk of a plant or main trunk of tree. Stem carries water, minerals, and food to other parts of plant; it may also store food, and green stems themselves make food. In most plants stem is major vertical shoot, in some it is inconspicuous, and in others it is altered and resembles other plant parts (like, underground stems may look like roots).
iii) The Skin:
The skin is organ whose main function comprises protection, sensitivity to touch and excretion of waste substances like salts.
iv) The Kidney:
The kidney is mostly liable for excretion of urea from body. Waste substances tht are extracted from blood are poured in pelvis from where they are directed by ureter to bladder for temporary storage. Each kidney has approx one to two million nephrons. These are freely embedded in connective tissue and are richly supplied with blood vessels.
v) The Liver:
Liver performs several functions among which are:
i) Circulatory System:
This is the system by which blood carrying food and other dissolved substances are taken round the body. During circulation of blood substances diffuse from blood in organs in which they are needed. For example as liver, glucose and dissolve foods diffuse out of blood in liver, and at various tissues oxygen diffuse out of the blood in tissue cells. At the same time substances also enter blood from different organs
ii) The Digestive System:
As name suggests this system handles breakdown of foods in component parts. Starch is broken down in glucose units, protein in amino acid units and fats in fatty acids. Break down is by means of enzymes that are produced by different organs and glands and poured in digestive canal where actual breakdown occurs. Therefore digestive system comprises of tract, alimentary canal that runs from mouth and opens out at anus.
iii) The Nervous System:
The nervous system is concerned with perception of and reaction to stimuli. It comprises of central nervous system that is composed of brain and spinal cord. Associated with central nervous system is a system of peripheral nerves which end in different organs and tissues.
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