Tissues in Action

The difference lies in the type of supporting tissue present.

Coconut husk fibres are made of sclerenchyma. Sclerenchyma cells have very thick walls due to the deposition of lignin, and the cells are dead at maturity with no living contents. Lignin makes the walls hard, rigid and strong. This is why the husk fibres are hard and brittle — they are built for maximum mechanical strength, not for bending.

Coriander leaf stalks are made mainly of collenchyma. Collenchyma cells are living, with walls unevenly thickened at the corners by pectin (not lignin). Pectin gives flexibility, much like rubber, so the stalk can bend easily without breaking and still support the leaf.

So the same job — support — is done by two different tissues: the lignified, dead sclerenchyma makes the coconut husk hard and brittle, while the pectin-rich, living collenchyma makes the coriander stalk soft and flexible.

The cuticle is a waxy, water-repellent layer of cutin secreted over the epidermis. Its main effect is to reduce the loss of water from the plant surface and to make that surface waterproof.

Why it helps a desert plant. In a desert, water is extremely scarce and the air is hot and dry, so a plant loses water very rapidly by transpiration. A thick cuticle greatly cuts down this evaporation, helping the plant conserve the little water it has. It also protects the plant against intense heat and mechanical injury. So a thick cuticle is a valuable adaptation for survival in a dry habitat.

Why it harms an underwater plant. A plant living underwater is surrounded by water, so it is in no danger of drying out — there is no water to conserve. Instead it must exchange gases (carbon dioxide and oxygen) and absorb dissolved minerals directly through its surface from the surrounding water. A thick, waterproof cuticle would block this exchange of gases and nutrients that the plant needs to survive. That is why submerged aquatic plants have a very thin cuticle, or none at all.

Water is raised through the plant by a partnership between the dead conducting cells of the xylem and the living cells of the leaves.

Role of the dead xylem cells. The xylem is made of tracheids and vessels. Because these cells are dead and empty — with no cytoplasm blocking the way — they join end to end to form continuous, hollow pipes that carry water without obstruction. Their walls are thickened with lignin, which makes them strong enough to withstand the pull of the water column without collapsing.

Role of the living leaf cells. The pulling force comes from the leaves. Water evaporates from the surface of the living mesophyll cells and escapes as vapour through the stomata — this is transpiration. As water leaves the top, it creates a suction called the transpiration pull.

How they work together. Water molecules cling to one another (cohesion) and to the walls of the xylem (adhesion), forming one continuous, unbroken column of water stretching from the roots to the leaves. When water is pulled out at the top by transpiration, the whole column is dragged upward, so more water is pulled up from below — against gravity. In short, the dead xylem provides the strong, hollow pathway, while the living leaf cells provide the pulling force that keeps the water moving.

Stomata are tiny pores in the epidermis that allow gaseous exchange and transpiration. If they were completely absent, several vital processes would be affected:

Gaseous exchange would stop. Carbon dioxide could not enter the leaf for photosynthesis, and oxygen could not diffuse in and out for respiration. With no carbon dioxide reaching the mesophyll, photosynthesis would slow down or stop, so the plant would make little or no food and would eventually starve.

Transpiration would stop. Water vapour could not escape from the leaves, so there would be no transpiration pull. Without this pull, water and minerals would not be drawn up efficiently through the xylem from the roots, and the whole transport system would be weakened.

The plant could overheat. Transpiration cools the plant. With no stomata to release water vapour, the leaves could become too warm on a hot day.

Removal of excess water and wastes that normally leaves through the stomata would also be affected.

Overall, a land plant with no stomata would be unable to photosynthesise, transport materials or regulate its temperature properly, and it would not survive normally.

The dance poses use almost every movable joint in the body. Each joint can be recognised by the kind of movement it allows:

Ball and socket joint (shoulder and hip). This joint allows movement in all directions — forward, backward, sideways and rotation. It is used whenever a dancer raises and circles the arms, or lifts and swings a leg out to the side.

Hinge joint (knee and elbow). A hinge joint allows bending and straightening in one plane only, like a door hinge. It is used when the dancer bends the knees into a sitting pose (as in the aramandi of Bharatanatyam) or bends the elbows.

Pivot joint (between the skull and the first neck vertebra). This joint allows rotation, so the head can turn from side to side. It gives the graceful side-to-side neck and head movements typical of Indian classical dance.

Gliding joints (wrist and ankle). These allow small sliding movements, producing the delicate hand gestures (mudras) and the pointing and flexing of the feet.

So a single dance pose usually combines several joints at once — ball-and-socket at the shoulder and hip, hinge at the knee and elbow, pivot at the neck, and gliding at the wrist — each contributing its own type of movement.

Correct answer: (iii) — They have thin walls, dense cytoplasm and a large prominent nucleus.

Meristematic cells are small and thin-walled, with dense cytoplasm packed with organelles and a large, prominent nucleus. They have little or no vacuole and are tightly packed with no intercellular spaces. These features let them divide rapidly and continuously, which is exactly what a dividing tissue must do.

Why the others are wrong. (i) Thick walls are a feature of sclerenchyma, whose cells are dead and cannot divide. (ii) Large vacuoles belong to mature, permanent cells; a dividing cell has very small or no vacuoles. (iv) Once a cell has become functionally differentiated, it has lost the ability to divide.

Correct answer: (ii) — Phloem.

Phloem is the conducting tissue that carries food — the sugars made in the leaves during photosynthesis — to all other parts of the plant, including the roots. This downward movement of food is called translocation. If food cannot reach the roots, it is the phloem that is malfunctioning.

Why the others are wrong. (i) Xylem transports water and minerals upward from the roots, not food. (iii) The epidermis is a protective covering, not a transport tissue. (iv) Sclerenchyma provides mechanical support and does not conduct food.

Correct answer: (iii) — To allow quick exchange of materials across them.

The epithelium that lines organs such as the lungs (alveoli), the blood vessels and the intestine is only one cell thick so that substances can diffuse across it rapidly. A thin layer means a very short distance for gases, nutrients and other materials to travel, so exchange is fast and efficient.

Why the others are wrong. (i) Exchange, not storage, is the job of these linings. (ii) Maximum strength comes from many layers of cells (as in the skin), but that gives protection, not exchange. (iv) Friction is reduced by secretions such as mucus, not by the tissue being thin.

In the normal jump the joints move freely; in the straight-leg jump they are held locked. This changes both how high you jump and how you land.

Normal jump. Before pushing off, you bend at the hip, the knee and the ankle (you crouch down). Bending these joints first stretches the leg muscles, which can then contract powerfully, so you spring up higher. On landing, the joints bend again to absorb the shock, so the landing is soft.

Straight-leg jump. The hip, knee and ankle are kept stiff and almost straight throughout. The muscles cannot pre-stretch, so far less force is produced and the jump is very low. On landing there is no bending to cushion the impact, so the shock passes straight up the leg and the landing feels hard and jarring.

This shows how the hinge joints at the knee and ankle and the ball-and-socket joint at the hip work together to produce a powerful movement and to soften a landing.

Correct answer: (ii) — Hinge.

The knee and the ankle are hinge joints. Like a door hinge, they allow bending and straightening in one plane only (back and forth), which is exactly the movement made when you bend your knees and ankles.

Why the others are wrong. (i) A ball-and-socket joint (shoulder or hip) allows movement in all directions. (iii) A pivot joint (between the skull and the first neck vertebra) allows only rotation, such as turning the head.

Plotting the data from Table 3.7 gives two rising curves — one for the diameter of the stem and one for the number of annual rings — both drawn against the age of the tree:

(i) Analysis of the graph. As the age of the tree increases, the diameter of the stem increases steadily — the tree grows thicker (increases in girth) year after year. The growth is more or less continuous, so an older tree can be recognised by its greater diameter.

(ii) Relation between diameter and annual rings. One new annual ring is added to the stem each year, so the number of annual rings equals the age of the tree in years. As each ring is added, the diameter also increases. Therefore the diameter is directly related to the number of rings: the more annual rings a tree has, the greater its diameter. Counting the rings tells us both the age of the tree and, indirectly, how much its girth has grown.

(iii) Tissue responsible for girth, and its location. The increase in girth is brought about by the lateral meristem (the cambium). It is located along the circumference of the stem and root, as a ring of dividing cells. Its cells divide and add new cells towards the inside and the outside, so the stem becomes thicker — and this year-by-year activity produces the annual rings.

(i) Functions hampered by debarking. The outer part of the bark contains the phloem. Removing the bark damages the phloem, so the downward transport of food (sugars) from the leaves to the roots and other parts is hampered. The tree also loses its protective outer covering, which normally prevents water loss and keeps out microbes and injury.

(ii) Tissue affected by further damage. Just beneath the bark lies the lateral meristem (cambium). Further damage to the trunk would injure this cambium, stopping the tree’s growth in girth and its ability to form new phloem and xylem and to heal the wound.

(iii) Function hampered if inner tissues are damaged. Beneath the cambium lies the xylem (the wood). If these inner tissues were severely damaged, the upward transport of water and minerals from the roots to the leaves would be hampered, and the tree would also lose much of its mechanical support. The leaves would wilt and the tree could die.

(iv) Assumptions. The answers above assume that only the outer bark (phloem) has been removed while the cambium and xylem beneath are still intact, and that the bark has been stripped right around the trunk (ringing). If instead only a small patch of bark were removed, the tree could survive and heal the wound, because food and water could still move through the undamaged parts. But if the damage went all the way round and reached the cambium and xylem, transport would be cut off completely and the tree would very likely die.

The flexibility is due to collenchyma. Collenchyma is a living supporting tissue whose cells have walls unevenly thickened at the corners by pectin. It provides mechanical support together with flexibility, which is why young stems and leaf stalks can bend in the wind without breaking.

If collenchyma were replaced by sclerenchyma: sclerenchyma cells have thick, lignified, rigid walls and are mostly dead. They give great strength but no flexibility. The young stem would become hard, stiff and brittle. In strong monsoon winds it would no longer be able to bend and sway — it would resist the wind and then snap or break instead. So replacing collenchyma with sclerenchyma would leave the sapling unable to survive strong winds. Sclerenchyma is suited to mature, woody parts of a plant, not to a flexible young sapling.

Sugarcane grows back from the buds present at its nodes. A node carries a bud along with meristematic tissue (the intercalary meristem), which can divide and grow into a new shoot; the region between two nodes (the internode) has no such buds.

(i) The type ‘B’ cuttings sprouted because they included a node with its bud, from which a new shoot could grow. The type ‘A’ cuttings did not sprout because they were pieces of internode only, without a node or bud, and so had no meristematic cells to start new growth.

(ii) The difference is that type ‘B’ contained a node (with a bud and meristematic tissue), while type ‘A’ was an internode without a node.

(iii) To find out whether this difference had an effect, one would observe and record how many cuttings of each type sprouted, and measure the number, length and health of the new shoots that grew from each type over the same period.

(iv) For a fair comparison, every other condition should be kept the same for both types: the same variety and age of sugarcane, cuttings of the same length and thickness, the same soil, water, temperature and sunlight, planted in the same way and observed for the same length of time. Only the presence or absence of a node should differ.

Both boys are partly right, and Rajiv’s refinement is correct.

Rohan’s statement fits simple tissues. A simple permanent tissue — such as parenchyma, collenchyma or sclerenchyma — is made of only one type of cell. All the cells are similar in structure and work together for one function, so here a tissue really is ‘a group of similar cells performing a similar function’.

But complex tissues are different. Xylem and phloem are complex tissues, each made up of more than one type of cell. Xylem, for example, contains tracheids, vessels, xylem parenchyma and xylem fibres — cells that differ in structure — yet they act together as a unit for one common function, the conduction of water. Phloem is similar, with sieve tubes, companion cells, phloem parenchyma and phloem fibres.

A better definition is therefore: a tissue is a group of cells, similar or different in structure, that have a common origin and work together to perform a specific function. So Rajiv is right — ‘similar cells’ describes simple tissues well, but in complex tissues different types of cells cooperate to carry out a single function.

The strength comes from sclerenchyma. Sclerenchyma cells are long and narrow, with very thick walls due to the deposition of lignin, and they are dead at maturity with no living contents. Lignin makes the walls hard, rigid and strong, so sclerenchyma fibres are tough and fibrous and can bear a great deal of mechanical stress — ideal for coir, ropes and mats.

Why living parenchyma could not do the same job. Parenchyma cells are living, with thin cellulose walls and large vacuoles, and they are loosely packed with intercellular spaces between them. They are soft and provide very little mechanical strength; their main tasks are storage and photosynthesis. Such thin-walled, living cells would tear easily and could not withstand the constant pulling and wear that a mat must endure. Only the thick, lignified, dead walls of sclerenchyma give the required toughness.

Vibha’s statement is incorrect. Apical meristem is indeed found at the tips of roots and shoots, but it is not the only meristem. Meristematic tissue is also found in two other places:

Lateral meristem (cambium) — along the sides, or circumference, of stems and roots. It increases the girth (thickness) of the plant.

Intercalary meristem — at the base of the internodes or leaves, for example at the nodes of grasses and sugarcane. It helps the plant regrow after being cut or grazed.

So meristematic cells occur in three locations, not just at the apices.

A question Neha can ask Vibha to make her rethink: “If meristem were present only at the tips, then how does a tree trunk keep growing thicker over the years, and how does grass grow back after it is mowed or grazed?” These everyday observations cannot be explained by apical meristem alone, so they would lead Vibha to realise that lateral and intercalary meristems must also exist.

(i) The plant cell will have the larger vacuole. A mature plant cell usually has a single large central vacuole that can take up most of the cell’s volume (often about 80–90%). It stores cell sap, water, minerals and wastes, and the pressure it creates (turgor) keeps the cell firm. An animal cell has only small, temporary vacuoles, or none at all. So for two cells of the same overall size, the plant cell’s vacuole is much larger.

(ii) Assumptions.

• That the plant cell is a mature plant cell — a young or meristematic plant cell has many small vacuoles rather than one big central one, so the answer assumes maturity.

• That the animal cell is a typical animal cell with only small vacuoles.

• That ‘size’ means the overall volume of the cell, and that both cells are in their normal, healthy (turgid) state.

If, for example, the plant cell were a young meristematic cell, it would not yet have a large central vacuole, and the answer could change.

The statement is an over-simplification. It can be examined critically by asking questions such as these:

“Does any single tissue carry out more than one function?” — Take parenchyma as an example. It stores food, but it also carries out photosynthesis when it contains chloroplasts (chlorenchyma), and in aquatic plants it stores air to help the plant float (aerenchyma). So one tissue has several functions.

“Can the same function be performed by more than one tissue?” — Take mechanical support, which is provided by collenchyma, by sclerenchyma and also by turgid, water-filled parenchyma. So one function is not tied to a single tissue.

“In a complex tissue, do different cells perform different jobs?” — Take xylem, which both conducts water and provides mechanical support, and phloem, which conducts food while its fibres give support.

“Does a tissue’s role change with the plant’s age or conditions?” — Take meristematic tissue, which divides and then differentiates into several different permanent tissues.

Examining these examples — parenchyma, xylem and phloem, and the supporting tissues — shows that a tissue can perform more than one function, and that a single function can be performed by more than one tissue. So the textbook statement is not fully correct.