Chapter 2 — Plant Tissues

In a unicellular organism such as amoeba the single cell does everything: it moves, feeds, respires, excretes and reproduces. This works because the organism is tiny. In a large multicellular organism such as a mango tree it would be impossible for every cell to do every job, so cells become specialised. Some cells make food, some carry water, some give strength, some protect, some divide to produce new cells. This sharing out of work among different kinds of cell is called division of labour. It makes the organism more efficient, exactly as a factory works better when different workers specialise in different tasks, but it also makes the cells dependent on one another: a root cell cannot make its own food and a leaf cell cannot absorb its own water.

Cells that do the same job are usually found together, and cells that are together usually have the same origin and structure. A tissue is therefore defined as a group of cells that are similar in structure and origin and work together to perform a particular function. The study of tissues is called histology. Tissues combine to form organs (a leaf, a root), organs form organ systems and organ systems form the organism.

Plant tissues and animal tissues differ because plants and animals live differently. Plants do not move about; they stand in one place and grow throughout their life, so they need a great deal of supporting tissue, much of which is dead (wood is mostly dead cells) and consumes no energy. Their growth is confined to certain regions, the growing points, where dividing cells are concentrated. Animals move, so they need tissues that use energy constantly, nearly all their tissues are living, and growth is more uniform through the body. Plants also have far fewer kinds of tissue than animals, because their structure is simpler and their needs are fewer: make food, carry water and food, stand up, protect the surface and grow.

On the basis of their ability to divide, plant tissues are classified into two main groups. Meristematic tissue consists of cells that are actively dividing and is found in the growing regions of the plant. Permanent tissue consists of cells that have lost the power of division and have taken up a definite shape, size and function. Permanent tissue arises from meristematic tissue by a process called differentiation, in which a newly formed cell changes its structure to fit a particular job. The rest of this chapter follows this classification.

The word meristem comes from a Greek word meaning to divide. Meristematic tissue is the tissue of dividing cells found in the growing regions of a plant. It is the source of all other tissues; every cell in a plant was once a meristematic cell.

Meristematic cells have a characteristic appearance that you can recognise in a section of a root tip or shoot tip. They are small, roughly cubical or isodiametric (the same size in all directions), and packed closely together with no intercellular spaces. Each cell has a thin primary cell wall of cellulose, because a thick wall would prevent the cell from dividing and growing. The cytoplasm is dense and fills the cell, and the nucleus is large and prominent, often occupying much of the cell, because the cell is busy copying its DNA for the next division. Vacuoles are absent or very small, since a cell that is about to divide has no need to store water and sap. Plastids, if present, are in an immature form. The cells have a high rate of metabolism and use a great deal of energy.

The essential property of the meristem is that its cells divide continuously by mitosis. Each division produces two cells; one usually remains meristematic and keeps dividing, while the other is pushed away from the growing point, enlarges, and gradually changes its structure to become a permanent cell of one kind or another. This change is called differentiation. Thus a meristem is like a factory that never stops, producing new cells at one end and passing them on to become tissue at the other.

Because the meristems are the only places where cell division takes place, growth in a plant is localised: a plant grows only at its tips and, in woody plants, in a thin ring inside the stem. This is quite unlike an animal, which grows more or less all over. It also explains why a plant can keep growing all its life: the meristems remain active as long as the plant lives, so a tree adds new leaves, roots and wood year after year.

Meristematic tissue is classified according to where it lies in the plant: apical meristem at the tips, intercalary meristem at the bases of leaves and internodes, and lateral meristem along the sides of stems and roots. These are described in the next section.

Meristems are named according to their position in the plant body, and each position corresponds to a different kind of growth.

Apical meristem is found at the tips of roots and shoots, that is, at the apex. In the root it lies just behind the protective root cap; in the shoot it lies at the tip of the stem and in the tips of every branch and every bud. Apical meristems are responsible for the increase in length of the plant, which is called primary growth. Every new leaf, every new node and internode and every new root is produced by an apical meristem. Because of the apical meristem a stem grows from its tip, not from its base: if you tie a thread around a young stem 10 cm from the ground, after a year it will still be 10 cm from the ground, though the stem will be much longer.

Intercalary meristem is found between regions of permanent tissue, hence the name (intercalated means inserted between). It lies at the base of leaves and at the base of internodes, and is best developed in grasses, including wheat, rice, maize, sugarcane and bamboo. It is really a part of the apical meristem that has been left behind as the stem grew and remains active for some time. Intercalary meristem allows an internode or a leaf to lengthen from its base. This is why a lawn or a field of grass keeps growing after it has been cut or grazed: the meristem is at the base of the blade, out of reach of the blade of the mower or the teeth of the cow. It also allows a fallen stalk of wheat to bend upright again by growing faster on the lower side of the node.

Lateral meristem lies along the sides of the stem and root, parallel to the long axis, in the form of a cylinder. It includes the vascular cambium, a thin layer between the xylem and phloem which adds new xylem inside and new phloem outside, and the cork cambium in the bark. Lateral meristem is responsible for the increase in girth or thickness of the plant, called secondary growth. The trunk of a tree becomes thicker year by year because its vascular cambium adds a new ring of wood every year; these are the annual rings that reveal a tree's age. Lateral meristem is found in dicot stems and roots and in gymnosperms, but most monocots (grasses, palms, bananas) have no cambium and so do not increase in girth in the same way.

Cells produced by the meristem do not remain small and dividing for long. As they are pushed away from the growing point they enlarge, develop vacuoles, thicken their walls in different ways, and take on a shape suited to a particular function. Once this differentiation is complete the cells have lost the ability to divide and become permanent tissue.

The features of permanent tissue contrast with those of meristem point by point. The cells are larger and of definite shape and size. The cell walls are thin or thick depending on the tissue, and may be of cellulose alone or strengthened with pectin, lignin or suberin. The cytoplasm is less dense, usually forming only a thin lining, and the vacuole is large. The nucleus is small in proportion to the cell. There are usually intercellular spaces between cells. The cells may be living or dead: parenchyma and collenchyma are living, while sclerenchyma fibres, tracheids, vessels and cork cells are dead at maturity, having lost their protoplasm. Metabolic activity is low compared with meristem.

Permanent tissue is classified on the basis of the kinds of cells it contains. Simple permanent tissue is made of only one type of cell, all alike; there are three kinds, parenchyma, collenchyma and sclerenchyma, distinguished mainly by the thickness and material of their walls. Complex permanent tissue is made of more than one type of cell that together perform one function; there are two kinds, xylem and phloem, which together form the vascular tissue that conducts water and food through the plant. A third group, sometimes treated separately, is protective tissue: the epidermis that covers young parts and the cork that covers old ones. The sections that follow describe each of these in turn.

Parenchyma is the simplest and most abundant tissue of plants; it is the tissue of soft parts such as the pith and cortex of stems and roots, the flesh of fruits, the mesophyll of leaves and the storage tissue of seeds. The name comes from Greek words meaning poured beside, because it fills the spaces between other tissues.

Structure. Parenchyma cells are living, thin-walled and usually isodiametric, that is roughly spherical, oval or polygonal with the same diameter in every direction. The wall is a thin primary wall of cellulose. The cells are loosely packed with conspicuous intercellular spaces between them, which allow gases to move through the tissue. Each cell has a thin lining of cytoplasm, a nucleus, and a large central vacuole that stores water and dissolved substances.

Functions. The basic parenchyma of the pith and cortex stores food, such as starch in the potato tuber and the rice grain, and water. It packs and supports soft parts; when its cells are turgid with water they press on one another and hold a herbaceous stem or a leaf firm, which is why a plant wilts when its parenchyma loses water. It also carries on repair: parenchyma cells can regain the power of division, form callus over a wound, and give rise to secondary meristems.

Parenchyma is modified in several ways for special jobs. Chlorenchyma is parenchyma that contains chloroplasts; it forms the mesophyll of leaves and the green outer cortex of young stems and is the site of photosynthesis. Aerenchyma is parenchyma with very large air spaces between the cells, found in aquatic plants such as lotus, water hyacinth and Hydrilla; the air cavities give buoyancy so the plant floats and also allow oxygen to diffuse down to submerged roots. Storage parenchyma in tubers, seeds and fruits is packed with starch grains, oil or sugar. Parenchyma of the xylem and phloem helps in lateral transport and storage.

When you eat a potato, a banana or the flesh of a mango, you are eating parenchyma. The juiciness of a watermelon is the water in the vacuoles of enormous parenchyma cells. The tissue is so common that it is a good default guess: a thin-walled living cell with a big vacuole in a soft part of a plant is almost certainly parenchyma.

Collenchyma is the tissue that gives flexible mechanical support to young, growing parts of a plant. The name comes from the Greek colla, glue, referring to the glistening thickened walls. It is found just beneath the epidermis in the stems of dicots, in leaf stalks (petioles), in the midribs and veins of leaves, and around the vascular bundles of some stems. It is absent from roots and from most monocots.

Structure. Collenchyma cells are living and are elongated, often several times longer than broad, with oblique or tapering ends, and appear polygonal in cross-section. Their special feature is the uneven thickening of the cell wall: the wall is thickened with cellulose and pectin, but the thickening is deposited only at the corners where three or more cells meet, leaving the rest of the wall thin. This corner thickening is easily seen in a section as bright, refractive triangles at the angles between cells. Because pectin absorbs water the walls are pliable. The cells contain a thin cytoplasm, a nucleus and a vacuole, and sometimes a few chloroplasts. Intercellular spaces are few or absent.

Functions. Collenchyma provides tensile strength and elasticity. A young stem must be strong enough not to collapse, but it must also be able to bend in the wind without breaking and, most importantly, it must still be able to grow longer. A rigid, lignified tissue would prevent growth; collenchyma with its plastic, unlignified walls supports the stem while allowing it to stretch. This is why the tips of stems and the stalks of leaves can bend and sway and then spring back. When it contains chloroplasts collenchyma also carries out some photosynthesis.

The familiar strings in a stalk of celery, and the tough ridges along the stalk of a pumpkin leaf, are strands of collenchyma. If you bend a young sunflower or balsam stem it curves but does not snap, and it straightens when released; that is collenchyma at work. The same stem, when old and woody, snaps, because by then sclerenchyma and wood have replaced collenchyma as the supporting tissue.

Compare collenchyma with the two other simple tissues. Parenchyma is thin-walled all round and mainly for storage; collenchyma is thick only at the corners and gives flexible support; sclerenchyma, described next, is uniformly thick-walled and lignified and gives rigid support. All three are simple tissues because each is made of one kind of cell.

Sclerenchyma (from the Greek skleros, hard) is the tissue that makes plant parts hard, stiff and strong. It is found around vascular bundles, in the hard covering of seeds and nuts, in the husk of the coconut, in the stone of a mango and in the gritty specks of a guava or pear. Unlike parenchyma and collenchyma, sclerenchyma cells are dead at maturity.

Structure. The cells have a very thick, uniformly thickened secondary wall laid down inside the primary wall and heavily impregnated with lignin, a hard, water-resistant chemical that makes cell walls rigid. The wall is often so thick that the cavity (lumen) of the cell is reduced to a narrow slit or vanishes altogether. Because the wall shuts the cell off from its surroundings the protoplasm dies, so a mature sclerenchyma cell is an empty, lignified box. Small pits in the wall mark where thin spots once connected neighbouring cells. There are no intercellular spaces.

Sclerenchyma is of two types by shape. Fibres are long, narrow, spindle-shaped cells with pointed ends, often several millimetres long, and occur in bundles. They give tensile strength to stems and are the source of the commercial fibres jute (from the phloem of Corchorus), flax (linen), hemp and coconut coir. Sclereids (stone cells) are short, roughly isodiametric or irregularly shaped cells with extremely thick walls. They occur singly or in groups in the flesh of pear and guava, where they make the gritty texture, and in large masses in the shells of nuts and the stony endocarp of drupes such as mango, coconut and plum.

Functions. Sclerenchyma gives rigid mechanical support, allowing plants to bear the weight of their own branches, fruits and leaves and to resist bending and crushing. It protects seeds and embryos inside hard shells. Fibres associated with the xylem help the vessels resist the inward pull of the water column. Because the tissue is dead, it costs the plant nothing to maintain and it does not need food or oxygen; a tree trunk is largely dead supporting tissue.

Test yourself: parenchyma is thin-walled, living, for storage; collenchyma is corner-thickened, living, for flexible support; sclerenchyma is uniformly thick, lignified, dead, for rigid support. The three tissues thus form a series of increasing wall thickness and increasing rigidity.

A complex tissue is made of more than one kind of cell, all working together for a common purpose. The two complex tissues of plants, xylem and phloem, together form the vascular tissue, which runs as continuous strands (vascular bundles) from the root tips through the stem to the veins of every leaf. Because of vascular tissue the higher plants are called vascular plants; mosses and algae have none and so remain small.

Xylem (from the Greek xylon, wood) is the tissue that conducts water and dissolved minerals upward from the roots to the leaves and also gives mechanical support. Xylem is what wood is made of. It consists of four kinds of cell:

• Tracheids are long, tube-like, dead cells with tapering ends and thick lignified walls bearing pits. Water passes from one tracheid to the next through the pits. Tracheids are the only water-conducting cells in ferns and conifers.
• Vessels (tracheae) are long tubes formed from rows of dead cells placed end to end whose end walls have dissolved, so that they form a continuous pipe that may be many centimetres long. Their walls are lignified with rings, spirals or networks of thickening. Vessels are the main water conductors in flowering plants and are more efficient than tracheids.
• Xylem parenchyma consists of living, thin-walled cells that store food and help in the sideways movement of water.
• Xylem fibres are sclerenchyma fibres that give strength.

Of these, tracheids, vessels and fibres are dead at maturity; only xylem parenchyma is living. Because most of its cells are dead, hollow and lignified, xylem is both a good set of pipes and a strong skeleton.

Water moves upward through the xylem largely because water evaporates from the leaves (transpiration), which pulls the continuous column of water up the vessels, aided by the cohesion of water molecules and by root pressure from below. The flow in xylem is only upward (unidirectional). You can demonstrate it by standing a balsam stalk or a white flower in red ink: the ink rises through the xylem, and a cross-section shows red dots at the positions of the vessels.

Xylem formed by the apical meristem is primary xylem; that formed by the vascular cambium during secondary growth is secondary xylem, or wood. The annual rings of a tree are layers of secondary xylem, the wide-vesselled spring wood and the narrow-vesselled autumn wood of each year.

Phloem (from the Greek phloios, bark) is the complex tissue that conducts food, mainly sucrose made in the leaves by photosynthesis, to every part of the plant where it is needed or stored: to growing tips, roots, flowers, fruits and storage organs. In a stem it lies outside the xylem in each vascular bundle, and in an old trunk it forms the inner living layer of the bark. Phloem also consists of four kinds of cell:

• Sieve tubes are the conducting cells. They are long, tubular, living cells arranged end to end; the end walls are perforated like a sieve (the sieve plates) so that the cytoplasm of one cell is continuous with the next through the pores. Mature sieve tube cells are unusual in that they have lost their nucleus and most organelles, but they remain alive, their cytoplasm forming a thin lining through which the sugar solution flows.
• Companion cells are narrow, living cells with dense cytoplasm and a prominent nucleus lying beside each sieve tube element and connected to it by many plasmodesmata. Since the sieve tube has no nucleus, the companion cell controls its activities and helps load sugar into it; the two cells work as a unit.
• Phloem parenchyma consists of living, thin-walled cells that store food and help in lateral conduction.
• Phloem fibres (bast fibres) are dead sclerenchyma fibres that give strength; jute, flax and hemp fibres are phloem fibres.

Thus, in contrast with xylem, all the elements of phloem are living except the phloem fibres. Food moves through phloem as a solution of sucrose in water, pushed along by pressure differences between the leaf, where sugar is loaded, and the sink, where it is unloaded. The movement is called translocation. Unlike xylem, phloem conducts in both directions: downward from leaves to roots, and upward from leaves to growing tips and fruits, or from storage organs to new shoots in spring.

A classic experiment shows the role of phloem. If a complete ring of bark (which contains the phloem) is removed from a tree trunk, the part of the trunk above the ring swells because sugars coming down from the leaves accumulate there, while the roots below eventually starve and the tree dies. The xylem inside is untouched, so water still reaches the leaves.

Every young part of a plant, whether root, stem, leaf, flower or fruit, is covered by a single layer of cells called the epidermis. It is the outermost tissue and its job is to protect the tissues inside from water loss, mechanical injury and the entry of microbes, while still allowing the exchanges with the outside that the plant needs.

Structure. Epidermal cells are living, usually flat or tabular and fitted together so closely that there are no intercellular spaces; this tight fit is what makes the layer a barrier. Most epidermal cells have no chloroplasts, so the layer is transparent and lets light through to the photosynthetic tissue beneath. On the parts exposed to air the outer wall of each cell is coated with a waxy, waterproof layer called the cuticle, made of a substance called cutin. The cuticle cuts down evaporation of water from the plant surface, and it is thickest on plants of dry places such as cactus and on shiny leaves such as those of banyan and mango. On some leaves the cuticle is covered with a bloom of wax that makes water roll off in beads. The epidermis of a leaf is therefore an almost waterproof skin.

In the root, the epidermis is different. It has no cuticle, because its job is to absorb water, and many of its cells grow out into long thin tubes called root hairs. Root hairs are extensions of single epidermal cells; they push between soil particles and enormously increase the surface area through which water and minerals are absorbed. They live only a few days and are replaced as the root grows.

The epidermis may also bear outgrowths called trichomes or hairs: the soft hairs of a tomato stem, the stinging hairs of the nettle, the glandular hairs of tulsi and mint that release scented oils, and the hairs that reflect light and trap still air over the leaf surface in desert plants. These hairs protect against insects, reduce water loss and sometimes secrete substances. Epidermal cells of some grasses contain silica, which makes the leaf edge sharp.

In a leaf the epidermis is interrupted at many places by tiny pores called stomata, described in the next section, through which gases move. In woody plants the epidermis of the stem is eventually replaced by cork. But on leaves, on herbaceous stems and on young twigs the epidermis remains the protective layer for the whole life of the organ.

A plant that was sealed all over by cuticle could neither take in the carbon dioxide it needs for photosynthesis nor release oxygen or water vapour. The epidermis of leaves and young green stems is therefore pierced by thousands of microscopic pores called stomata (singular stoma, a Greek word for mouth). A square centimetre of the lower surface of a leaf may carry ten to thirty thousand stomata. In most dicot leaves they are more numerous on the lower (abaxial) surface; in floating leaves such as the lotus they are only on the upper surface; in grasses they are equal on both.

Structure. Each stoma is bounded by a pair of specialised epidermal cells called guard cells. In dicots the guard cells are kidney-shaped (bean-shaped) and in grasses they are dumb-bell shaped. Unlike other epidermal cells, guard cells contain chloroplasts. Their most important structural feature is that the wall on the inner side, facing the pore, is thick, while the wall on the outer side, away from the pore, is thin and elastic. The guard cells are surrounded by ordinary epidermal cells, sometimes of a special shape called subsidiary cells. Below the pore is a large air space in the leaf tissue.

Working. The stoma opens and closes because the guard cells change shape. When water enters the guard cells they become turgid and swell. Because the thin outer wall stretches more than the thick inner wall, each cell bends outward like a curved sausage, and the two cells pull apart, opening the pore between them. When the guard cells lose water they become flaccid, straighten, and come together, closing the pore. Water enters the guard cells when they accumulate potassium ions and sugars, which happens in light; so stomata generally open in the day and close at night, though in desert plants the timing is reversed to conserve water.

Functions. Stomata allow the exchange of gases: carbon dioxide enters for photosynthesis and oxygen leaves; at night the reverse happens for respiration. They are the main route for transpiration, the loss of water vapour from the plant, which cools the leaf and drives the upward flow of water in the xylem. By closing in hot, dry or windy conditions the guard cells prevent excessive water loss; by opening they permit photosynthesis. The stoma is thus a valve that balances the plant's need for carbon dioxide against its need to conserve water.

You can see stomata easily by peeling the lower epidermis of a Rhoeo, Tradescantia or balsam leaf, mounting it in water and looking under the microscope: bean-shaped green guard cells in pairs stand out against the colourless epidermal cells.

The single-layered epidermis is adequate for a leaf or a young shoot, but as a dicot stem or root grows older and thicker through the activity of the vascular cambium, the epidermis is stretched, cracks and dies. The plant then needs a new protective covering. This is provided by a secondary lateral meristem in the outer cortex called the cork cambium or phellogen. The cork cambium divides and produces layers of cells towards the outside, and these cells become cork (phellem), the tissue that Robert Hooke looked at in 1665.

Structure. Cork cells are dead, rectangular or box-shaped, arranged in regular radial rows with no intercellular spaces. Their walls are impregnated with suberin, a fatty, waxy substance that makes the cells impermeable to water and gases. Because suberin seals the cells off from the supply of water and food, the protoplasm dies and the cells become filled with air or with tannins and resins, which give bark its brown colour and bitter taste. Several layers of cork form each year, so an old trunk may carry a thick, cracked coat of it. The cork together with the dead tissues outside the cambium makes up the outer bark.

Functions. Cork is an excellent protective tissue. It prevents loss of water from the stem, a job the dead epidermis can no longer do. It protects against mechanical injury, against extremes of temperature and against the entry of fungi, bacteria and insects. Being full of air it is a poor conductor of heat, which is why trees survive frost and mild forest fires. Because cork cells are light, compressible, elastic, waterproof and rot-resistant, cork is used commercially for bottle stoppers, insulation, floats, sports goods and shoe soles; the cork of commerce is stripped from the cork oak, Quercus suber, whose bark regrows after each harvest.

Since cork is impermeable to gases, the living tissues inside would suffocate if it were continuous. At intervals the cork cambium produces loose, rounded, thin-walled cells with large intercellular spaces instead of tight cork; these rupture the surface as small raised spots called lenticels, through which oxygen and carbon dioxide can pass. Lenticels are easily seen as pale dots or lines on the bark of young twigs of mango, guava and neem, and on the skin of an apple or a potato.

Cork thus completes the story of protective tissue: epidermis with cuticle and stomata on young parts, cork with lenticels on old parts; both are barriers with controlled openings.

The best way to fix the tissues in mind is to see them in place. In the laboratory you cut a thin transverse section of a young dicot stem (sunflower, balsam, bean) or a monocot stem (maize) with a blade, stain it with safranin, and mount it. Going from outside to inside a young dicot stem shows the following layers.

• Epidermis: a single layer of closely fitting cells with a cuticle, sometimes bearing hairs.
• Cortex: beneath the epidermis, a few layers of collenchyma (the hypodermis) giving flexible support, then several layers of parenchyma, often green, with intercellular spaces, and finally an innermost layer called the endodermis which may contain starch.
• Vascular bundles: arranged in a ring. Each bundle has phloem on the outside, xylem on the inside, and a thin strip of cambium between them (the bundle is open, meaning it can grow). The bundle is often capped on the outside by a patch of sclerenchyma fibres. The xylem shows large red-stained vessels with the small first-formed vessels (protoxylem) towards the centre.
• Pith: the large central region of thin-walled parenchyma that stores food and water; the strips of parenchyma between the bundles are medullary rays.

A monocot stem such as maize differs: the vascular bundles are many and scattered throughout the ground tissue instead of in a ring, each is surrounded by a sheath of sclerenchyma, there is no cambium (bundles are closed), so no secondary growth, and there is no distinct cortex or pith. A monocot stem such as sugarcane or bamboo therefore does not get thicker with age the way a dicot tree does.

A dicot root section shows an epidermis with root hairs and no cuticle, a wide parenchymatous cortex, a distinct endodermis, and in the centre a solid core in which xylem and phloem lie on alternate radii, the xylem arms forming a star with usually four points, with no pith or only a small one. The vascular cambium arises later between the arms. In a monocot root the xylem groups are many and a large central pith is present.

Knowing these arrangements lets you answer the standard examination question of identifying a tissue from its position: a corner-thickened tissue just under the epidermis of a stem is collenchyma; thick-walled dead cells capping a bundle are sclerenchyma; large hollow lignified cells inside the bundle are xylem vessels; thin-walled cells with sieve plates outside them are phloem; the loose cells of the centre are pith parenchyma.

The tissues of this chapter are not only microscope subjects; they are the materials of everyday life, and connecting them to familiar objects is the best revision.

Food. Nearly all the plant food we eat is parenchyma: the starch-storing parenchyma of rice, wheat, potato and tapioca; the sugar-storing parenchyma of sugarcane, beetroot and fruits; the chlorenchyma of leafy vegetables such as spinach and gongura. The fibre in our diet, which aids digestion, is largely cellulose from cell walls and sclerenchyma from bran and skins.

Fibres and fabrics. Jute, flax, hemp and ramie are phloem (bast) fibres, bundles of sclerenchyma from the stem. Coir is sclerenchyma from the fruit wall of the coconut. Cotton is different: it is a mass of epidermal hairs from the seed coat of the cotton plant, each hair a single elongated epidermal cell filled with cellulose.

Wood and paper. Timber is secondary xylem laid down by the vascular cambium; its hardness comes from lignified vessel walls and xylem fibres. Paper is made by separating the fibres and tracheids of wood, bamboo or bagasse and matting them into sheets. The annual rings in a cut log count the years of the tree's life and record wet and dry seasons.

Cork and bark. Bottle stoppers, floats and insulation boards are cork, the dead suberised tissue of the cork oak. Cinnamon is the inner bark of a tree, containing phloem; quinine and many medicines are also extracted from bark.

Farming. Grasses recover from grazing and mowing because of their intercalary meristems. Gardeners take stem cuttings and graft fruit trees because parenchyma and cambium can divide and heal wounds. Girdling, the removal of a ring of bark, is sometimes used deliberately on grape vines to make the fruit sweeter, since sugar cannot pass the cut phloem and accumulates in the bunches.

Summary to remember. Meristematic tissue divides and produces growth (apical for length, lateral for girth, intercalary for internodes). Permanent tissue is differentiated and does not divide. Simple permanent tissues have one cell type: parenchyma (thin-walled, living, storage), collenchyma (corner-thickened, living, flexible support) and sclerenchyma (uniformly lignified, dead, rigid support). Complex tissues have several cell types: xylem (tracheids, vessels, parenchyma, fibres; water upward) and phloem (sieve tubes, companion cells, parenchyma, fibres; food both ways). Protective tissues are the epidermis with cuticle, stomata and root hairs on young parts and cork with lenticels on older parts. If you can place each of these in a stem section and name one everyday example of each, you know the chapter.