Chapter 3 — ଟିସୁ ତନ୍ନ (Tissues)

In an Amoeba or a bacterium a single cell carries out every process of life: it takes in food, respires, excretes, moves, senses and reproduces. Such an arrangement works only while the organism stays small. In a multicellular organism, a mango tree with crores of cells or a human being with lakhs of crores, it would be wasteful and impossible for every cell to do everything. Instead, cells become specialised: some take up the work of making food, some of conducting water, some of contracting, some of carrying messages. This is called division of labour, and it makes the organism far more efficient, because a cell built for one job does it better than a cell that must do all jobs.

Specialised cells of the same kind are grouped together in the body. A tissue is a group of cells that have a common origin, are usually similar in structure, and work together to perform a particular function. The term was introduced by the French anatomist Bichat, and the branch of biology dealing with tissues is histology. Muscle tissue in the arm is a mass of contractile cells; the tissue at the tip of a root is a mass of dividing cells; blood is a tissue of cells floating in a fluid. Tissues combine to form organs (a leaf, the heart), organs form organ systems (the shoot system, the circulatory system), and organ systems make up the organism.

Plants and animals differ in their ways of life, and their tissues differ accordingly. Plants are fixed in one place and do not move; they therefore need a great deal of supporting tissue, much of it dead, with thick walls, so that a tree can stand for centuries with very little expenditure of energy. Plants grow throughout their life, but growth is confined to certain regions, the tips of roots and shoots and the cambium, where dividing tissues are located; most of the plant body is made of permanent, non dividing tissue. Animals move about in search of food and shelter and so need more energy and more living tissue; most of their tissues are living, and growth is more uniform through the body until adult size is reached. Animals also need tissues that plants do not: muscle for movement and nerves for rapid coordination.

Plant tissues are therefore classified first on the basis of whether their cells divide, into meristematic and permanent tissues, and animal tissues on the basis of function, into epithelial, connective, muscular and nervous tissues. The chapter follows this order. In studying any tissue the student should ask three questions: what do its cells look like, where is it found, and what does it do, and should notice how the answers to the first and the third are connected.

Growth in plants is limited to certain regions where cells divide continually. The tissue in these regions is called meristematic tissue or meristem (Greek meristos, divisible). The cells of a meristem are small, roughly cubical or polygonal and closely packed with no intercellular spaces. Each has a thin cellulose wall, dense cytoplasm, a large prominent nucleus, and vacuoles are absent or very small, because the cell is not storing anything but is fully occupied with dividing. Plastids are present only as proplastids. These cells are the source of every other tissue in the plant: as they divide, the cells left behind enlarge, become vacuolated and take on a specialised form, a process called differentiation, and become permanent tissue.

Meristems are classified according to their position in the plant body.

• Apical meristem is present at the growing tips (apices) of roots and shoots, protected by a root cap in the root and by young leaves in the shoot bud. Its division adds cells behind it and increases the length of the plant; this is primary growth. If the tip of a shoot is removed, growth in length stops and side branches develop, which is the principle behind pruning of hedges and tea bushes.
• Lateral meristem lies along the sides of stems and roots, parallel to their long axis, in the form of cylinders. It includes the vascular cambium between the xylem and phloem, which produces new xylem to the inside and new phloem to the outside, and the cork cambium near the surface. Its division increases the girth or thickness of the stem and root; this is secondary growth, and it produces the wood and bark of trees. The annual rings seen in a cut log are layers of xylem laid down by the vascular cambium, one per year.
• Intercalary meristem is a portion of apical meristem left behind at the base of leaves or at the nodes and internodes, and is characteristic of grasses, bamboo, wheat and rice. It allows the internodes to elongate and is why grass grows again after being grazed or mown and why a fallen paddy stem can lift itself.

Meristems are also classified by origin as promeristem (the earliest embryonic cells at the very tip), primary meristem (derived from it, forming the primary plant body) and secondary meristem (arising later from permanent tissue, such as cork cambium).

The functions of meristematic tissue may be summed up: it brings about growth in length and thickness, it forms all the permanent tissues, it heals wounds by producing new cells, and it makes possible vegetative propagation from cuttings, since cambium can produce new roots and shoots. The perpetual youth of plants, which unlike animals can keep growing and producing new organs throughout their lives, is due to their meristems.

Cells produced by the meristem soon stop dividing, grow to their final size and shape, and take up a definite function. Such tissues are called permanent tissues, and the process by which a meristematic cell acquires its permanent form is differentiation. Permanent cells may be living or dead, thin walled or thick walled, and have intercellular spaces; they have large vacuoles and a comparatively small nucleus. Permanent tissues are classified as simple, made of one kind of cell only (parenchyma, collenchyma, sclerenchyma), protective (epidermis, cork), and complex, made of more than one kind of cell working together (xylem, phloem).

Parenchyma (Greek para, beside, and enchyma, infusion) is the most common and simplest permanent tissue and forms the bulk of the soft parts of a plant. Its cells are living, thin walled (the wall of cellulose), and isodiametric, that is, roughly equal in all directions, oval, rounded or polygonal. They have a large central vacuole, a peripheral cytoplasm and a nucleus. The cells are loosely packed with conspicuous intercellular spaces between them, which allow gases to move through the tissue. Parenchyma is found in the cortex and pith of stems and roots, in the mesophyll of leaves, in the pulp of fruits, in the endosperm of seeds and in the packing around the vascular bundles.

Parenchyma cells retain the power to divide when needed, and they are modified in several ways:

• Chlorenchyma is parenchyma containing chloroplasts, found in the mesophyll of leaves and in green stems; it carries out photosynthesis.
• Aerenchyma is parenchyma with very large air spaces, found in the stems and leaves of aquatic plants such as lotus, water hyacinth and Hydrilla; the air gives buoyancy and allows the plant to float, and stores oxygen for submerged parts.
• Storage parenchyma stores starch (potato tuber, rice endosperm), sugar (sugar cane stem), oil (groundnut) or water (the fleshy leaves of aloe and the stems of cactus).

The functions of parenchyma follow from its structure. It stores food and water. Being thin walled and turgid, it provides support to soft parts such as young stems and leaves, which wilt when the parenchyma loses water. Its intercellular spaces allow exchange of gases. Chlorenchyma manufactures food. Parenchyma also heals wounds and gives rise to new meristems such as cork cambium, and in tissue culture a single parenchyma cell can be grown into a whole plant, a property called totipotency. When we eat a potato, a watermelon or the flesh of a mango, we are eating parenchyma.

Collenchyma (Greek kolla, glue) is a living supporting tissue found just below the epidermis in the stems and leaf stalks of dicotyledonous plants, in the ridges of stems such as those of the pumpkin and cucumber, and along the veins of leaves; it is generally absent from roots and from monocots. Its cells are elongated, polygonal in cross section, living with a nucleus and cytoplasm, and often contain chloroplasts. Their distinctive feature is the uneven thickening of the cell wall with cellulose and pectin, deposited mainly at the corners where three or more cells meet; the thickening does not contain lignin. Intercellular spaces are few or absent. Because the walls are thick yet not lignified, collenchyma is both strong and flexible: it gives mechanical support to the growing parts of the plant while allowing them to bend without breaking and to keep growing in length. It is collenchyma that lets a young stem sway in the wind, and it is the tissue that makes a stalk of celery stringy. When it contains chloroplasts it also photosynthesises.

Sclerenchyma (Greek skleros, hard) is the principal strengthening tissue of the mature plant. Its cells are dead at maturity, having lost their protoplasm, and their walls are uniformly and heavily thickened with lignin, a hard, waterproof substance, so that the cell cavity (lumen) is reduced to a narrow channel or is completely closed. There are no intercellular spaces. Sclerenchyma occurs in two forms. Fibres are long, narrow, pointed cells, often several millimetres in length, occurring in bundles around the vascular bundles, in the pericycle and in the bark, and in the hard covering of seeds and nuts. Jute, flax (linen), hemp and coir are commercial fibres, and it is sclerenchyma fibres that make a husk of coconut so tough. Sclereids (stone cells) are short, irregular, very thick walled cells found in the gritty flesh of guava and pear, in the hard shells of nuts and the stones of mango, plum and date, and scattered in the leaves of tea.

The function of sclerenchyma is to give hardness, rigidity and mechanical strength to the plant, protecting it from bending and from the stress of wind and weight, and to protect seeds and fruits. Being dead, it demands no energy to maintain, which suits the plant's economy. Its lignified walls also make it resistant to decay, which is why wood and jute last.

The outermost layer of the plant body forms a protective covering against water loss, injury and infection. In young plants and in the leaves, flowers and fruits, this is the epidermis; in older stems and roots that have undergone secondary growth, it is replaced by cork.

The epidermis is usually a single layer of living cells, flat and often elongated, fitted together like tiles with no intercellular spaces, so that it forms a continuous sheet. The cells have no chloroplasts, except the guard cells, and have a thin cellulose wall on the inner side but a thicker outer wall. On the parts exposed to air, the epidermis secretes a waxy, waterproof layer called the cuticle, made of cutin, over its outer surface; the cuticle is thin on shaded leaves and very thick in desert plants such as cactus and in the shiny leaves of the banyan, where it cuts down the loss of water. In dry habitats the epidermis may be several layers thick. The epidermis of the leaf and young stem is pierced by tiny pores called stomata (singular stoma), each surrounded by two bean shaped guard cells that contain chloroplasts. When the guard cells take in water and swell, the pore opens; when they lose water, it closes. Through the stomata the plant exchanges oxygen and carbon dioxide with the air and loses water vapour in transpiration. In the root, the epidermal cells behind the tip send out long, thin, tubular extensions, the root hairs, which enormously increase the surface for absorbing water and minerals from the soil. On leaves and stems the epidermis may bear hairs (trichomes) that reduce water loss and discourage insects.

As a stem or root grows in thickness, the epidermis is stretched and ruptured. A secondary meristem, the cork cambium (phellogen), then arises in the outer cortex and cuts off layers of cells to the outside which form cork (phellem). Cork cells are dead, rectangular, and arranged in compact rows without intercellular spaces. Their walls are impregnated with suberin, a fatty substance that makes them impervious to water and gases. Cork is therefore an excellent protective tissue: it prevents water loss from the trunk, keeps out fungi and bacteria, cushions the living tissue beneath against mechanical injury, and insulates it against heat and cold, which is why a forest fire that scorches the bark may leave a tree alive. Because the cork is gas tight, small regions of loosely arranged cells called lenticels occur in it as raised spots on the bark, through which the living tissues inside can breathe. Commercial cork, used for bottle stoppers, floats, insulation and shuttlecocks, is the thick bark of the cork oak Quercus suber of Spain and Portugal. Cork is light, does not absorb water and does not catch fire easily.

A complex tissue consists of more than one type of cell, all working together as a unit. The two complex tissues of plants, xylem and phloem, are both concerned with conduction, and together they form the vascular bundles that run as continuous strands from the root tips through the stem to every leaf, like the pipes of a building. Their presence is what distinguishes the vascular plants (pteridophytes, gymnosperms and angiosperms) from mosses and algae, and it is what allows a plant to grow tall.

Xylem (Greek xylon, wood) is the water conducting tissue. It is made of four kinds of cells, three dead and one living.

• Tracheids are long, narrow, tube like cells with tapering ends and thick lignified walls, dead at maturity. Water passes from one to the next through thin areas called pits. They are the only water conducting cells of ferns and gymnosperms.
• Vessels (tracheae) are the main conducting cells of flowering plants. A vessel is a long tube formed of a row of cells placed end to end whose end walls have dissolved, so that water flows through a continuous pipe that may run for a metre or more. The walls are lignified with rings, spirals or networks of thickening, which keep the vessel from collapsing under the suction of transpiration. Vessels are dead and hollow.
• Xylem fibres are sclerenchyma fibres, dead, with thick walls and narrow lumen, which give mechanical strength to the xylem.
• Xylem parenchyma is the only living component; its thin walled cells store food (starch) and help in the sideways movement of water.

The functions of xylem are two. First, it conducts water and dissolved minerals absorbed by the roots upward to the stem and leaves; the movement is in one direction only, from root to leaf, driven mainly by the pull of transpiration. Second, because most of its cells have thick lignified walls, it gives mechanical support to the plant. The wood of a tree trunk is almost entirely xylem, laid down year after year by the vascular cambium; the older, central heartwood no longer conducts but is filled with tannins and resins and supports the tree, while the outer sapwood carries the water. Timber, therefore, is xylem, and its strength, grain and durability depend on the arrangement of vessels and fibres. In a young dicot stem the xylem lies on the inner side of each vascular bundle, facing the pith, and in the root it forms a star shaped core at the centre.

Phloem (Greek phloios, bark) is the food conducting tissue of the plant. It lies on the outer side of the vascular bundle in the stem, next to the cortex, and in old trees it forms the inner living layer of the bark, which is why stripping a ring of bark from a tree kills it: the roots are starved of the food that the phloem carried down from the leaves. Like xylem it is a complex tissue, but unlike xylem most of its cells are living. It has four components.

• Sieve tubes are the conducting cells. They are long tubes made of elongated cells, the sieve tube elements, placed end to end; the end walls are perforated by groups of pores like a sieve, forming sieve plates, through which the strands of cytoplasm and the dissolved food pass from cell to cell. A mature sieve tube element is living but has no nucleus, having lost it during development, and its cytoplasm is reduced to a thin lining; the walls are thin and of cellulose, not lignified.
• Companion cells are narrow, elongated, living cells with dense cytoplasm and a prominent nucleus, lying alongside each sieve tube element and connected to it by plasmodesmata. Since the sieve tube has no nucleus, the companion cell controls its activities and loads sugars into it; the two are formed from a single mother cell and die together. Companion cells are found in angiosperms only.
• Phloem parenchyma consists of living thin walled cells that store food and help in its sideways conduction; it is absent in most monocots.
• Phloem fibres (bast fibres) are the only dead component, sclerenchyma fibres that give strength; the fibres of jute, flax and hemp are phloem fibres.

The function of phloem is the translocation of food. The sugars made by photosynthesis in the leaves, mainly as sucrose, together with amino acids and hormones, are carried by the sieve tubes to all parts of the plant that need or store them: to the growing tips, the roots, the developing fruits and seeds, and the storage organs such as tubers. Unlike the flow in xylem, transport in phloem is bidirectional: in spring sugar stored in the roots moves upward to the opening buds, and in summer sugar from the leaves moves downward to the roots. The movement requires energy from the living cells and stops if the phloem is killed. Aphids that feed on plants insert their fine mouth parts precisely into a sieve tube and drink the sugary sap, and the honeydew they excrete is phloem sap.

Animal tissues are grouped by function into four types: epithelial tissue, which covers and lines; connective tissue, which binds, supports and transports; muscular tissue, which contracts to produce movement; and nervous tissue, which receives stimuli and conducts messages. Every organ of the body contains several of these tissues; the stomach, for example, is lined by epithelium, wrapped in connective tissue, moved by muscle and controlled by nerves.

Epithelial tissue (epithelium) forms the covering of the outer surface of the body and the lining of every cavity, tube and organ within it: the skin, the mouth and gut, the air passages and lungs, the blood vessels, the kidney tubules and the glands. It is the first tissue to appear in the embryo. Its cells are closely packed, with little intercellular substance and no intercellular spaces, and are joined firmly by junctions so that the sheet is continuous and forms a barrier. The sheet rests on a thin non cellular basement membrane that separates it from the underlying tissue, and it has no blood vessels of its own, receiving nutrients by diffusion from below. Epithelial cells divide rapidly to replace those worn away; the lining of the intestine is renewed every few days and the outer skin every few weeks.

Epithelia are classified by the shape of their cells and the number of layers.

• Squamous epithelium is made of thin, flat, scale like cells with irregular outlines, fitted together like the tiles of a floor (hence also called pavement epithelium). As a single layer it lines the alveoli of the lungs, the blood vessels (where it is called endothelium), the heart and the mouth; being extremely thin it allows rapid diffusion of gases and fluids. Where protection against wear is needed, as in the skin, the oesophagus and the lining of the mouth, the cells are piled in many layers as stratified squamous epithelium; the outer layers of the skin are dead and filled with the protein keratin.
• Cuboidal epithelium has cube shaped cells with a central round nucleus, found in the tubules of the kidney, the ducts of salivary glands and the thyroid follicles; it is concerned with secretion and absorption.
• Columnar epithelium has tall, pillar like cells with the nucleus near the base, lining the stomach and intestine, where it absorbs digested food and secretes mucus and enzymes; the intestinal cells bear microscopic finger like microvilli that increase the absorbing surface.
• Ciliated epithelium is cuboidal or columnar epithelium whose free surface bears hair like cilia that beat in waves to move material along; it lines the windpipe and bronchi, where it sweeps dust laden mucus up to the throat, and the oviducts, where it moves the egg towards the uterus.
• Glandular epithelium is columnar or cuboidal epithelium folded inward to form glands that secrete sweat, saliva, digestive juices, mucus, hormones or milk.

The functions of epithelium are protection against injury, drying and infection; absorption in the gut and kidney; secretion by glands; exchange of gases in the lungs; excretion of waste; and sensation, since the sense organs of taste, smell and hearing are modified epithelia.

Connective tissue is the most abundant and widely distributed tissue of the animal body. Its name describes its main job: it connects, binds and supports the other tissues and organs and fills the spaces between them. Unlike epithelium, in which cells are packed tight, connective tissue has cells scattered loosely in a large amount of non living intercellular substance called the matrix, which the cells themselves secrete. The nature of the matrix decides the character of the tissue: it may be a jelly (areolar tissue), a liquid (blood), a firm rubbery solid (cartilage) or a hard mineral (bone). Most connective tissues also contain protein fibres in the matrix: white collagen fibres, which are strong and unstretchable, and yellow elastin fibres, which are elastic. All connective tissues develop from the mesoderm of the embryo.

Areolar tissue (loose connective tissue) is the most widespread. It consists of a semi fluid jelly like matrix containing a loose network of collagen and elastin fibres and several kinds of cells: fibroblasts that make the fibres, macrophages that engulf bacteria and debris, mast cells that release histamine in inflammation, and fat cells. It is found under the skin, joining it to the muscles beneath; around blood vessels and nerves; between muscles; and around and inside every organ as packing material. It fills spaces, binds tissues together, supports organs, helps repair after injury and defends against infection. When you pinch and lift the skin on the back of your hand, the layer that stretches is areolar tissue.

Adipose tissue (fat) is areolar tissue in which the cells are chiefly fat cells (adipocytes), each swollen by a single large droplet of fat that pushes the nucleus and cytoplasm to one side. It is found below the skin, around the kidneys and heart, in the abdomen, in the marrow of long bones and in the breast. It stores energy as fat; it insulates the body against cold, which is why whales and seals have thick blubber; it cushions organs such as the kidney and eyeball against shocks; and it fills out the contours of the body. Over eating enlarges the fat cells and leads to obesity.

Tendons and ligaments are dense connective tissues in which the fibres are packed tightly in bundles with few cells. A tendon attaches a muscle to a bone. It is a white, cord like structure made of parallel bundles of collagen fibres, very strong and inelastic, so that the full pull of the muscle is transmitted to the bone; the Achilles tendon at the heel and the tendons on the back of the hand are easily felt. A ligament joins bone to bone at a joint. It contains collagen fibres together with many yellow elastic fibres, so that it is strong yet elastic, holding the bones of the joint in place while allowing them to move and preventing dislocation. A sprain is a stretched or torn ligament. Both tendons and ligaments have a poor blood supply and therefore heal slowly.

Cartilage is a firm, smooth, flexible and slightly elastic supporting tissue. Its cells, called chondrocytes, lie singly or in groups of two to four inside small spaces called lacunae within a solid, rubbery matrix of chondrin, a protein carbohydrate complex secreted by the cells, strengthened by collagen fibres. Cartilage has no blood vessels or nerves; nutrients diffuse through the matrix from a surrounding membrane, the perichondrium, so it heals slowly. In the embryo the whole skeleton is first laid down in cartilage and is then gradually replaced by bone; in sharks and rays the skeleton remains cartilage for life. In the adult human cartilage is found at the tips of the nose, in the external ear (pinna), in the rings of the windpipe and larynx, at the ends of long bones where it forms the smooth, slippery surface of joints and reduces friction, and as the discs between the vertebrae, which act as shock absorbers. Its functions are support, giving shape to flexible parts, allowing smooth movement at joints and cushioning against shock.

Bone is the hardest connective tissue and forms the skeleton of vertebrates. Its matrix is made of about one third collagen, which gives toughness, and two thirds mineral salts, mainly calcium phosphate and calcium carbonate, which give hardness; a bone soaked in acid becomes rubbery and one heated in fire becomes brittle, showing that both components are needed. The bone cells, osteocytes, lie in lacunae arranged in concentric rings around central canals called Haversian canals, which carry blood vessels and nerves; fine channels (canaliculi) connect the lacunae so that nutrients reach every cell. Bone is therefore living and well supplied with blood, and it heals when broken. The outer part of a bone is compact and dense; the inner part is spongy with a network of plates; and the central cavity of long bones holds the bone marrow, where blood cells are made. Bone gives the body its framework and shape, supports its weight, protects delicate organs (the skull protects the brain, the ribs protect the heart and lungs), provides attachment for muscles which act on it as levers, stores calcium and phosphorus, and houses the marrow.

Blood is a connective tissue with a liquid matrix called plasma, in which cells float and which carries no fibres except when it clots. Plasma is a pale yellow fluid, about 90 per cent water, containing proteins (albumin, globulins, fibrinogen), glucose, amino acids, salts, hormones and wastes. The cells are of three kinds. Red blood cells (erythrocytes), about 50 lakh per cubic millimetre, are biconcave discs without a nucleus in mammals, packed with the red pigment haemoglobin which carries oxygen from the lungs to the tissues; they are made in the bone marrow and live about 120 days. White blood cells (leucocytes), 5,000 to 10,000 per cubic millimetre, are larger, colourless, nucleated cells of several types that defend the body by engulfing bacteria and producing antibodies. Platelets (thrombocytes) are small cell fragments that start the clotting of blood at a wound. Blood transports oxygen, carbon dioxide, food, wastes and hormones; distributes heat and keeps the body temperature even; and defends against disease. An adult human has about 5 litres of blood, and lymph, the fluid that leaks from the capillaries and bathes the cells, is also counted as a fluid connective tissue.

Muscular tissue is made of elongated cells called muscle fibres that have the special property of contractility: they can shorten forcefully when stimulated and relax again. Every movement of the body, walking, blinking, breathing, the beating of the heart and the churning of the stomach, is produced by muscle. The cytoplasm of a muscle fibre, called sarcoplasm, contains long protein threads (myofibrils) made of actin and myosin, which slide over each other to shorten the fibre using energy from ATP. Muscle makes up about 40 per cent of the weight of the human body. There are three types of muscle, distinguished by structure, location and control.

Striated muscle (skeletal or voluntary muscle) is attached to the bones by tendons and moves the limbs, trunk, jaw and face. Its fibres are very long (up to 30 cm), cylindrical, unbranched, and multinucleated, with many nuclei lying at the edge of the fibre just under the membrane (sarcolemma). Under the microscope the fibres show alternate dark and light bands across their width, the striations, produced by the regular arrangement of actin and myosin. These muscles are under the control of the will and are therefore called voluntary; they contract rapidly and powerfully but tire quickly, as anyone who has held a bucket at arm's length knows. The muscles of the biceps, calf and back are striated.

Smooth muscle (non striated, unstriped or involuntary muscle) forms the walls of the internal hollow organs: the stomach, intestines, bladder, uterus, blood vessels, bronchi and the iris of the eye. Its fibres are short, spindle shaped (pointed at both ends), unbranched, with a single central nucleus and no striations. They are not under the control of the will, being worked by the autonomic nervous system, and they contract slowly and rhythmically but can stay contracted for a long time without tiring. The wave like movements of the gut (peristalsis) that push food along, the narrowing of arteries, and the contractions of the uterus in childbirth are all smooth muscle at work.

Cardiac muscle is found only in the wall of the heart. It combines features of the other two. Its fibres are striated like skeletal muscle but are involuntary like smooth muscle. They are short, cylindrical, branched, joined end to end by special junctions called intercalated discs that appear as dark cross lines and pass the contraction quickly from one fibre to the next so that the whole heart beats as a unit; each fibre has one or two central nuclei. Cardiac muscle contracts rhythmically about 72 times a minute throughout life and never tires; it generates its own beat and is only regulated, not started, by nerves.

Nervous tissue is the tissue of control and coordination. It receives stimuli from inside and outside the body, converts them into electrical impulses, conducts the impulses rapidly from one part of the body to another, and brings about a response. It forms the brain, the spinal cord and the nerves, and it is the most highly specialised tissue of the body. Its cells have the properties of excitability, the ability to respond to a stimulus, and conductivity, the ability to carry the impulse along their length. Nervous tissue consists of nerve cells or neurons, which do the actual work of conduction, and supporting cells called neuroglia, which nourish, protect and insulate them. Neurons are formed in the embryo and, unlike most cells, do not divide after birth, which is why damage to the brain or spinal cord is permanent.

A neuron is the structural and functional unit of the nervous system. It has three parts.

• The cell body (cyton or soma) contains the nucleus, cytoplasm and organelles including characteristic granules of rough ER called Nissl granules; it maintains the cell and gathers incoming signals. Cell bodies lie mostly in the brain and spinal cord (grey matter) and in swellings called ganglia.
• Dendrites are short, branching, tapering processes that arise from the cell body and carry impulses towards it. Their many branches give a large surface for receiving signals from sense organs or from other neurons.
• The axon is a single, long, thin fibre that carries impulses away from the cell body to another neuron, a muscle or a gland. It may be over a metre long, running from the spinal cord to the toe. Many axons are wrapped in a fatty, white myelin sheath formed by Schwann cells, interrupted at intervals by gaps called nodes of Ranvier; the sheath insulates the axon and speeds up conduction, so that impulses in myelinated fibres travel at up to 120 metres per second. The axon ends in fine branches with swollen tips, the synaptic knobs.

Neurons do not touch one another. The junction between the axon ending of one neuron and the dendrite of the next is a microscopic gap called a synapse. When an impulse reaches the synaptic knob, a chemical (neurotransmitter) is released, crosses the gap and starts a fresh impulse in the next neuron; the synapse ensures that impulses travel in one direction only. A nerve is a bundle of many axons bound together by connective tissue, like wires in a cable. Sensory neurons carry impulses from the sense organs to the brain and spinal cord; motor neurons carry commands from them to muscles and glands; and association neurons within the brain and cord connect the two. Together they make possible sensation, thought, memory, reflexes and every coordinated action of the body.

Having studied the tissues of both kingdoms, it is worth drawing together how and why they differ, since this is a favourite examination question and it links structure to way of life. Plants are autotrophic and stationary; animals are heterotrophic and mobile. From this single difference most of the contrasts between their tissues follow.

• Growth: plants grow throughout life, but only at the meristems at their tips and in the cambium; the rest of the body is permanent tissue. Animals grow more uniformly and stop growing when they reach adult size; cell division for repair continues in most tissues, but there is nothing like a meristem.
• Living and dead tissue: much of the plant body, xylem, sclerenchyma and cork, is made of dead cells that give support and protection at no running cost, suiting an organism that does not move and must economise on energy. Almost all animal tissues are living, because a moving animal needs active cells everywhere and consumes more energy.
• Support: plants are supported by thick cellulose and lignin walls and by turgor; animals by an internal skeleton of bone and cartilage and by muscle tone.
• Movement and coordination: plants have no muscular or nervous tissue; their movements are slow growth or turgor changes, and coordination is chemical, through hormones. Animals have muscle for rapid movement and nervous tissue for rapid coordination.
• Transport: plants conduct water in dead xylem and food in living phloem, without a pump; animals circulate blood, a fluid connective tissue, driven by the heart.
• Organisation: plant tissues are simpler and fewer in type; animal tissues are more varied and organised into more complex organs and organ systems.

Finally, tissues are one step in a ladder of organisation. In sponges the cells are only loosely cooperative; in Hydra and jellyfish true tissues appear but hardly any organs; in flatworms tissues combine into organs; and from roundworms upward, organs form organ systems. The same ladder can be seen in the individual: a leaf is an organ made of epidermis, mesophyll parenchyma, collenchyma, xylem and phloem; the shoot is an organ system of stems, leaves and buds. In the human body the four tissue types combine in every organ. The stomach wall, from inside out, is columnar epithelium with glands, areolar connective tissue with blood vessels, three layers of smooth muscle, and a network of nerves, all covered by a thin squamous epithelium. Knowing the tissues, therefore, is knowing the material from which every organ is built, and it is the necessary foundation for the study of physiology, of nutrition, transport, respiration and control, that follows in the next classes.