In the 1660s, an English scientist named Robert Hooke sliced a piece of cork as thinly as he could and placed it under a microscope he had built himself. He expected to see a solid, uniform material. What he actually saw surprised him: the cork was not solid at all. It was made of hundreds of tiny, empty, box-like compartments packed neatly side by side, each shut off from its neighbours by a wall. They reminded him of the small rooms in a monastery, so he called them cells. He published his drawings in his book Micrographia, and the word has stayed with biology ever since.
What Hooke was actually looking at were the leftover walls of cells that had already died. That is why cork is so light, why it floats, and why it has been used for centuries to seal bottles. But this raises a bigger question. If those cells were dead, something living must have made them — and must have kept making them, layer after layer, as the tree aged. That living layer lies hidden just inside the bark, and it is only one of several tissues quietly at work inside every plant you see.
Cutting a plant open and studying these internal tissues is called anatomy, and this chapter is your microscope. As you read, keep the outline in Fig. 8.0 in mind.
This chapter will answer each of these questions as you work through it.
8.2 Meristematic Tissue
A meristematic tissue is a group of young cells that are still capable of dividing. They are living cells, and wherever they are present, that part of the plant is able to grow. Because these cells have not yet matured into any specialised form, they are described as immature.
The cells are polyhedral or isodiametric in shape, which simply means they are roughly equal in length and breadth rather than long and drawn out. They are packed so closely that intercellular spaces are absent between them. This tight packing is important: a dividing cell needs support from its neighbours on all sides, and gaps would weaken the growing region.
The cell wall is thin and elastic, and is mainly composed of cellulose. A thin, stretchable wall lets the cell enlarge immediately after each division instead of being locked into a fixed size by a rigid wall. Inside, the protoplasm is dense, with a distinct nucleus at the centre. Vacuoles are either absent or, if present, are very small — so the cell is filled with active cytoplasm rather than with stored water and sap.
These cells show a very high rate of metabolism. Division is expensive work: new DNA, new proteins and new wall material must all be made, and that demands a steady, rapid supply of energy. The general appearance of such cells is shown in Fig. 8.1.
Meristematic tissue is a group of young, living, immature cells with thin cellulosic walls, dense protoplasm and the ability to divide, present in the regions of the plant where growth occurs.
meristematic cell (Thin elastic cellulosic wall → cell can enlarge soon after dividing → rapid growth is possible. Dense cytoplasm with little or no vacuole → maximum machinery for protein and DNA synthesis in a small volume → high rate of division. No intercellular spaces → cells brace one another → the growing tip stays firm.)
Classification of Meristems
Meristems are not all alike. Three different criteria are used to classify them — their origin, their position in the plant body, and the function they eventually perform. The same meristem can therefore have more than one name, depending on which criterion you are using. All three schemes are brought together in FC 8.2.
Meristems Classified by Origin
The primordial meristem, also called the promeristem, is the embryonic meristem. It usually occupies a very minute area right at the tip of the root and the shoot. It is the original source from which the other meristems arise.
Primary meristem originates from the primordial meristem and is present in the plant body from the very beginning, at the root and shoot apices. Its cells keep dividing, and the different permanent tissues of the young plant are produced from these primary meristems.
Secondary meristematic tissues are different in their history. They develop from living permanent tissues during the later stages of plant growth, and are therefore called secondary meristems. In other words, cells that had already matured and stopped dividing regain the power of division — a reversal called dedifferentiation. This tissue occurs in the mature regions of the root and shoot of many plants. A secondary meristem is always lateral in position, that is, along the sides of the central axis. Fascicular cambium, interfascicular cambium and cork cambium are examples.
Primary meristem vs Secondary meristem (A primary meristem comes from the promeristem and has been dividing since the beginning; a secondary meristem arises later from living permanent cells that regain the ability to divide.)
Meristems Classified by Position
Apical meristem is produced from the promeristem and forms the growing point at the apices of the root, the shoot and their lateral branches. It brings about an increase in the length of the plant body, and its cells are called apical initials. In the shoot the apical meristem is terminal in position — it sits right at the tip. In the root it is subterminal, that is, it is located behind the root cap. The reason is protective: a root has to push through hard, abrasive soil, so the delicate dividing cells are kept just behind a cap of cells that takes the damage.
Intercalary meristematic tissue is present in the top or the base area of a node. Its activity is mainly seen in monocots, and it is short lived — it functions for a while and then matures into permanent tissue.
Lateral meristem is present along the sides of the central axis of organs. It takes part in increasing the girth of the stem or root, for example intrafascicular cambium (the fascicular cambium named above). It is found in the vascular bundles of gymnosperms and dicot angiosperms. The three positions are compared in Fig. 8.2.
Apical vs Intercalary vs Lateral meristem | Feature | Apical meristem | Intercalary meristem | Lateral meristem | |—|—|—|—| | Position | Apices of root, shoot and lateral branches | Top or base area of a node | Along the sides of the central axis | | Origin | From promeristem | Left behind by apical meristem | Usually secondary, from permanent tissue | | Growth caused | Increase in length | Increase in length | Increase in girth | | Duration | Long lasting | Short lived | Long lasting | | Chiefly seen in | All plants | Mainly monocots | Gymnosperms and dicot angiosperms |
Intercalary meristem is the reason a lawn survives being mowed and a grazed grassland recovers so quickly. In grasses the growing region lies at the base of the node, well below the blade tips, so cutting off the top does not remove the dividing cells and the leaf simply pushes up again. If the growing region were at the tip, as in most dicots, a single mowing would stop growth for good.
Meristems Classified by Function
A young growing region of the plant contains three functionally distinct groups of meristem, each destined to build a particular set of tissues.
Protoderm is the outermost of the three, and forms the protective covering — the epidermis — around the various organs. The meristem called procambium is involved in developing the primary vascular tissue, that is, the conducting tissue of the young plant. The remaining structures, such as the cortex, endodermis, pericycle, medullary rays and pith, are formed from the region of the ground meristem. These are the three groups of meristem based on function.
Notice the logic of their arrangement: the outermost meristem makes the outer covering, the innermost regions make the packing tissue, and the meristem in between makes the conducting strands. The position of a meristem therefore predicts what it will become.
the three functional meristems (Outside in — Protoderm makes the Protective layer, Procambium makes the Plumbing, ground meristem makes everything on the ground floor in between.)
The apical meristem is used commercially in plant tissue culture. Viruses spread through a plant along its vascular tissue, but they usually fail to reach the fast-dividing cells right at the shoot tip. Growers therefore excise a tiny piece of shoot apical meristem and raise whole new plants from it — a technique used to produce virus-free banana, potato and sugarcane planting material on a large scale.
In examinations, this section is most often tested through match-the-following and assertion–reason questions that pair a meristem with its position, its origin or the tissue it produces. It is worth being able to move between the three classification schemes for the same meristem rather than memorising one list.
- List the characteristics of meristematic tissue.
- Classify meristematic tissue on the basis of origin.
8.3 Permanent Tissue
Permanent tissue is a group of cells which have lost the capacity of division and have acquired a permanent size, shape and function. Once a cell leaves the meristem, it stops dividing and settles into a fixed job for the rest of its life.
This settling down is the result of morphological, physiological and functional changes that occur during the maturation of the cell. Its shape changes, its wall may thicken, its vacuole may enlarge, and in some cases the protoplasm is lost altogether. Together these changes are called differentiation.
Depending upon the types of cells present, permanent tissues are of two kinds — simple permanent tissue and complex permanent tissue. The distinction is exactly the one introduced earlier: one type of cell, or several types working as one unit. The full classification is set out in FC 8.3.
Simple Permanent Tissues
These are made up of only one type of cell, and all the cells carry out similar functions. Such a tissue may be either living or dead. There are three types of simple permanent tissue — parenchyma, collenchyma and sclerenchyma.
Parenchyma
The cells of this tissue are thin walled and isodiametric, and may be round, oval to polygonal, or elongated in shape. The cell wall is composed of cellulose. The cells are living, with a prominent nucleus and cytoplasm and a large vacuole. Because it has undergone the least change during maturation, parenchyma is the least specialised permanent tissue.
Parenchyma has distinct intercellular spaces between its cells, though sometimes the cells may show a compact arrangement instead. The cytoplasm of adjacent cells is interconnected through plasmodesmata — fine strands of cytoplasm that pass through pores in the walls — and the tissue therefore behaves as one continuous living mass rather than as a set of sealed boxes.
These cells are distributed in all the parts of the plant body, namely the epidermis, cortex, pericycle, pith, mesophyll cells, endosperm, xylem and phloem. Wherever you look inside a plant, parenchyma is somewhere in the picture.
Its functions follow from its simple structure. Parenchyma cells store food and water, help in gaseous exchange, increase buoyancy, perform photosynthesis, and carry out several other functions in the plant body. A thin wall lets gases and solutes pass easily; a large vacuole gives room for storage; intercellular spaces form an internal air network. Notice how each function is traceable to a structural feature, as shown in Fig. 8.3.
Parenchyma also holds a reserve of possibility. Dedifferentiation in parenchyma cells develops the vascular cambium and the cork cambium at the time of secondary growth. In other words, this ordinary-looking tissue is the source of the secondary meristems that later thicken the plant.
parenchyma cell (Thin cellulosic wall → gases and dissolved substances cross easily → gaseous exchange and photosynthesis are possible. Large vacuole → bulk storage space → food and water are stored. Intercellular air spaces → a continuous air network and lower density → aeration and buoyancy, which is why aquatic plants float.)
The floating leaves and stems of aquatic plants such as the water hyacinth are packed with parenchyma containing unusually large air spaces. This air-filled tissue lowers the density of the plant enough to keep its leaves at the surface where light is available. The same air channels also carry oxygen down to roots buried in oxygen-poor mud.
Collenchyma
Collenchyma is a simple permanent tissue made up of living cells. Its cell wall is cellulosic, but shows an uneven deposition of cellulose and pectin, especially at the corners where several cells meet. The walls may also show the presence of pits.
The cells are similar to parenchyma, containing cytoplasm, a nucleus and vacuoles, but they are smaller in size and are without intercellular gaps. They therefore appear compactly packed. In transverse section the cells are either circular, oval or angular.
Collenchyma is a living mechanical tissue and serves several functions in plants. It gives mechanical strength to the young stem and to parts like the petiole of a leaf. It allows bending and pulling action in plant parts and also prevents the tearing of a leaf. Growth of organs and elongation are its other functions.
The combination of strength with flexibility is the key point here. Because the extra wall material is only pectin and cellulose and not a hard, rigid substance, the wall can still stretch. This is why a young stem can bend in the wind and spring back instead of snapping, and why it can continue to elongate while still being supported.
Collenchyma is usually absent in monocots and in the roots of dicot plants. A root growing inside soil is held on all sides and is pulled rather than bent, so a flexible anti-bending tissue is of little use there. Its typical appearance is shown in Fig. 8.4.
collenchyma (Extra cellulose and pectin deposited at the corners → strength is concentrated exactly where neighbouring cells meet and stress is highest → the organ resists bending. The wall stays unlignified and stretchable → the cell can still elongate → the young stem grows and supports itself at the same time.)
Sclerenchyma
Sclerenchyma is a simple permanent tissue made up of compactly arranged, thick walled, dead cells. The cells are living at the time of production, but at maturity they become dead. As the cells are devoid of cytoplasm, their thickened walls are due to a uniform deposition of lignin — a hard, waterproof substance that makes the wall rigid. The cells remain interconnected through several pits, which are thin unlignified spots in the wall.
Sclerenchyma is of two types, namely fibres and sclereids.
Fibres are thread-like, elongated and narrow structures with tapering and interlocking end walls. They are mostly found in bundles, their pits are narrow, unbranched and oblique, and they provide mechanical strength. Interlocking ends matter: when tapering ends overlap and grip one another, a bundle of fibres behaves like a rope rather than like a stack of separate rods.
Sclereids are usually broad, with blunt end walls. These occur singly or in loose groups, and their pits are deep, branched and straight. They are developed due to secondary thickening of parenchyma cells and provide stiffness only. The two forms are shown in Fig. 8.5.
This tissue functions as the main mechanical tissue of the plant. It permits bending, shearing and pulling. It gives rigidity to leaves and prevents them from falling, and it also gives rigidity to epicarps and seeds. Commercial fibres are also produced from sclerenchyma fibres, for example jute, flax and hemp.
Fibres vs Sclereids (Fibres are long, narrow and tapering with narrow oblique pits, occur in bundles and provide mechanical strength; sclereids are short and broad with blunt ends and deep branched pits, occur singly or in small groups, and provide stiffness only.)
Bite into a pear and you feel a faint grittiness. Those hard specks are sclereids, often called stone cells, scattered singly through the soft parenchyma of the fruit. The same cells make up the stony shell around a mango or coconut seed — a lignified armour that protects the embryo until conditions are right for germination.
Parenchyma vs Collenchyma vs Sclerenchyma | Feature | Parenchyma | Collenchyma | Sclerenchyma | |—|—|—|—| | Nature of cells | Living | Living | Dead at maturity | | Cell wall | Thin, cellulosic | Cellulosic with uneven cellulose and pectin at corners | Thick, uniformly lignified | | Intercellular spaces | Distinct (sometimes compact) | Absent | Absent | | Cell shape | Isodiametric, round, oval, polygonal or elongated | Circular, oval or angular in T.S. | Elongated fibres or broad sclereids | | Main function | Storage, photosynthesis, gaseous exchange, buoyancy | Mechanical strength with flexibility | Main mechanical tissue, rigidity | | Typical location | All parts of the plant body | Young stem, petiole; absent in monocots and dicot roots | Hard parts, seed coats, epicarps, fibres |
Complex Permanent Tissues
These tissues are heterogenous, comprising more than one type of cell, and all of them function together as a single unit. This tissue is involved in conducting sap and food from the source area to the sink area. Xylem and phloem are the complex tissues present in plants.
The word “source” means the part where a substance is available or made, and “sink” means the part where it is used or stored. A single tissue made of only one cell type could not do this job, because conduction needs open channels, and open channels need support and maintenance from other cells alongside them.
complex permanent tissue (Zoom in: xylem is built from four different cell types, each a specialised cell. Zoom out: xylem and phloem together form a vascular bundle, vascular bundles run through the root, stem and leaf, and together they form the plant’s vascular tissue system.)
Xylem
Xylem is a dead complex tissue. The components of xylem are tracheids, vessels, xylem parenchyma and xylem fibres. Besides conduction, the xylem also provides mechanical strength to the plant body.
Tracheids and vessels conduct water and minerals, and these two together are also known as hadrome. In pteridophytes and gymnosperms, tracheids are the conducting elements, while vessels are the conducting elements in angiosperms and also in Selaginella (a pteridophyte) and Gnetum (a gymnosperm). These two genera are important exceptions and are frequently asked about. The tissue and its position in a vascular bundle are shown in Fig. 8.6.
Tracheids
Tracheids are elongated, tubular and dead cells. Their ends are oblique and tapering. The cell walls are thick, having an uneven deposition of lignin, and this provides mechanical strength. Being dead and hollow is not a defect here — an empty tube with a stiff wall is exactly what is needed to carry a column of water without collapsing.
Tracheids contribute 95% of the wood in gymnosperms and 5% in angiosperms. This single pair of figures explains why gymnosperm wood is uniform and fine-grained while angiosperm wood is not.
Different types of thickening patterns are seen on their walls, such as annular (in the form of rings), spiral (in the form of a spring or helix), scalariform (ladder like), and pitted, which is the most advanced type (small circular areas) and may be simple or bordered. These patterns are compared in Fig. 8.7.
reading the thickening patterns in Fig. 8.7 (Move from left to right across the figure. The rings of annular thickening leave most of the wall unthickened, so the tube can still stretch as the organ elongates. Moving right, the thickening covers more and more of the wall surface, until in the pitted type nearly the whole wall is lignified and only small circular pits are left. So the trend is: less thickening means more flexibility and less support; more thickening means more support and less capacity to stretch. That is why the earliest formed elements carry the least thickening.)
Vessels
Vessels are longer than tracheids and are formed by the dissolution of the end walls of a row of cells, that is, of vessel elements. When these end walls break down, a series of separate cells becomes one long continuous pipe. Vessels are involved in the conduction of water and minerals.
Their lumen is wider than that of tracheids, and the thickening is due to an uneven deposition of lignin, similar to tracheids. A wider tube offers less resistance to flow, so vessels move water faster than tracheids can. In monocots the vessels are rounded, whereas they are angular in dicot angiosperms.
The first formed xylem vessels, called protoxylem, are small and have either annular or spiral thickenings, while the later formed larger vessels, called metaxylem, have reticulate or pitted thickenings. The reason lies in timing: protoxylem forms while the organ is still elongating and so must be able to stretch, whereas metaxylem forms after elongation is over and can afford heavy thickening.
The relative position of these two decides an important anatomical term. When protoxylem is arranged towards the pith and metaxylem towards the periphery, it is called endarch, as in the stem. When the position is the reverse, as in roots, it is called exarch.
Endarch vs Exarch (Endarch — protoxylem lies towards the centre or pith and metaxylem towards the outside; typical of stems. Exarch — protoxylem lies towards the outside and metaxylem towards the centre; typical of roots.)
Tracheids vs Vessels (Both are dead, lignified and conduct water. Tracheids are single cells with intact oblique tapering end walls and a narrower lumen; a vessel is a long tube made of many vessel elements whose end walls have dissolved, and its lumen is wider.)
Xylem Parenchyma and Xylem Fibres
Xylem parenchyma cells are small and are associated with tracheids and vessels. This is the only living tissue among this complex tissue. Its function is to store food, in the form of starch, and sometimes tannins. This parenchyma is also involved in the lateral or radial conduction of water or sap — that is, it moves material sideways, across the width of the organ, while the vessels and tracheids move it lengthwise.
Xylem fibres are sclerenchymatous cells and serve mainly for mechanical support. These are called wood fibres. They are also elongated, narrow and spindle shaped, the cells taper at both ends, and their walls are lignified.
the four components of xylem (Two conduct, one supports, one lives — tracheids and vessels conduct, fibres support, and xylem parenchyma is the only living component.)
Phloem
Phloem is a living tissue. It is also called bast. Phloem is responsible for the conduction of organic food material from a source, which is generally the leaf, to a sink, which is any other part of the plant. Phloem was named leptome by Haberlandt, as a term matching the one used for xylem.
On the basis of origin, phloem is of two kinds — protophloem, which is first formed, and metaphloem, which is later formed. Phloem is composed of sieve cells, sieve tubes, companion cells, phloem parenchyma and phloem fibres. Its structure is shown in Fig. 8.8.
Gottlieb Haberlandt was an Austrian botanist working in the early twentieth century, and his contribution to plant science went well beyond naming leptome. He was the first to propose that a single isolated plant cell, given the right nutrients, could be grown into a whole plant. He could not make the experiment succeed with the methods available to him, but the idea was correct, and modern plant tissue culture rests on it.
Sieve Tubes and Sieve Cells
Sieve tubes are the long tubular conducting channels of phloem. These are placed end to end, with bulging at the end walls. The sieve tube has a sieve plate, formed by a septum having small pores. The sieve plates connect the protoplast of adjacent sieve tube cells, so that the living contents of a whole column of cells form one continuous stream.
The sieve tube cell is a living cell with only a thin layer of cytoplasm, and it loses its nucleus at maturity. Losing the nucleus clears the tube of obstruction, but it also leaves the cell unable to control its own activities — a problem solved by its partner cell described below. The sieve tube cell is connected to the companion cell through phloem parenchyma by plasmodesmata.
Sieve cells are found in lower plants like pteridophytes and gymnosperms. The cells are narrow and elongated with tapering ends, and the sieve area is located laterally, on the side walls rather than at the ends.
sieve tube (End walls perforated into a sieve plate → the cytoplasm of one cell is continuous with the next → food solution flows along an unbroken channel. Nucleus lost and cytoplasm reduced to a thin lining → the lumen is left clear → resistance to flow is low.)
Companion Cells
Companion cells are narrow, elongated and living. These cells are laterally associated with sieve tube elements. Companion cells have dense cytoplasm and a prominent nucleus. Their arrangement alongside the sieve tubes can be seen in both views of Fig. 8.9.
The nucleus of the companion cell regulates the functions of the sieve tube cell through simple pits. This is the arrangement that rescues the enucleate sieve tube: one cell provides the clear channel, the neighbouring cell provides the control.
From the point of view of origin, the sieve tube cell and the companion cell are derived from the same cell. Their partnership is so close that the death of one results in the death of the other type.
a sucrose molecule travelling from a leaf to a root 1. Sucrose is made in the mesophyll of the leaf, which acts as the source. 2. It is loaded into a sieve tube, with the companion cell supplying the energy and control for this step. 3. It moves along the column of sieve tubes, passing from one cell to the next through the pores of the sieve plates. 4. Phloem parenchyma alongside carries some of it sideways, by lateral conduction, to nearby tissues. 5. It reaches the root, which acts as the sink, and is unloaded there to be used for respiration or stored as starch.
Phloem Parenchyma and Phloem Fibres
Phloem parenchyma cells are living and elongated, and are found associated with sieve tubes and companion cells. Their chief function is to store food, latex, resins, mucilage and similar substances. These cells carry out the lateral conduction of food material. Phloem parenchyma is absent in most of the monocots.
Phloem fibres are the only dead tissue among this unit. They are sclerenchymatous. They are generally absent in primary phloem, but present in secondary phloem. These cells have lignified walls and provide mechanical support. They are used in making ropes and rough clothes.
Xylem vs Phloem | Feature | Xylem | Phloem | |—|—|—| | Nature of the tissue | Dead complex tissue | Living tissue | | Other name | Hadrome (for its conducting elements) | Bast; named leptome by Haberlandt | | Material conducted | Water and minerals | Organic food material | | Direction of transport | From root towards the rest of the plant | From source (usually leaf) to sink | | Conducting elements | Tracheids and vessels | Sieve tubes and sieve cells | | Only living component | Xylem parenchyma | — (only the fibres are dead) | | Only dead component | — (only the parenchyma is living) | Phloem fibres | | Extra role | Provides mechanical strength | Storage of food, latex, resins, mucilage |
Questions from this section often take the form of “which is the only living element of xylem” or “which is the only dead element of phloem”, or ask you to place protoxylem and metaxylem correctly in a diagram of a root or a stem. Reasoning from structure to function, rather than recalling lists, is what such questions reward.
- Write a note on parenchyma.
- Describe sclerenchyma fibres.
- Sketch and label a T.S. of phloem tissue.
8.4 Tissue Systems
All plant tissues are derived from meristems, and their structure and functions depend on the position they occupy in the plant body. A tissue system is the next level of organisation above a tissue: it is a group of tissues that occupy a particular region and together perform one broad task for the whole plant.
On the basis of their structure and location, three types of tissue systems are present, namely the epidermal tissue system, the ground tissue system and the vascular tissue system. Whether you cut a root, a stem or a leaf, these same three systems appear, one inside the other, as shown in FC 8.5.
tissue system (Zoom in: each tissue system is built from the simple and complex tissues you have already met — parenchyma, collenchyma, sclerenchyma, xylem, phloem. Zoom out: the three tissue systems together make up an organ such as a root, stem or leaf.)
Epidermal Tissue System
The epidermal tissue system forms the outer covering of the plant body and is derived from the protoderm, which is also called the dermatogen. Two types of structures are seen in this system, namely the epidermis and the epidermal appendages.
Epidermis is the outermost protective cell layer, made up of compactly arranged cells without intercellular spaces. Leaving no gaps is essential for a protective layer — a gap would be an open door for water loss and for microorganisms. The cells show the presence of a central large vacuole, thin cytoplasm and a nucleus, so they are living but not heavily specialised.
The outer side of the epidermis is often covered with a waxy, thick layer called the cuticle, which prevents the loss of water. Since the cuticle is deposited outside the cell wall and is waterproof, it seals the plant surface in much the same way that a coat of wax seals a polished floor. The epidermis may also bear hairs, and its general appearance in a leaf is shown in Fig. 8.10.
epidermis (Cells compactly arranged with no intercellular spaces → no gaps for water or microbes → effective protective covering. Outer wall coated with a waxy cuticle → water cannot pass out through the surface → transpiration loss is restricted. Cells kept thin-walled and living → the layer stays flexible and can bear appendages → the surface can still absorb, secrete or exchange where needed.)
Epidermal Appendages
Root epidermis has root hairs. These are unicellular and elongated, and are involved in the absorption of sap from the soil. Being unicellular, a root hair is simply an outgrowth of a single epidermal cell — an inexpensive way of adding an enormous amount of absorbing surface without building new tissue.
In the stem, the epidermal hairs are called trichomes. These are generally multicellular, may be branched or unbranched, and may be stiff or soft or even secretory. Trichomes help in preventing water loss due to transpiration, because a covering of hairs traps a still layer of moist air against the surface and slows down the escape of water vapour.
Root hairs vs Trichomes (Root hairs are unicellular outgrowths of the root epidermis and their job is absorption of sap from the soil; trichomes are the generally multicellular hairs of the stem epidermis and their job is to reduce water loss.)
The dense, silvery trichomes on the leaves of many desert and hill plants are the reason such leaves feel woolly. Growers of aromatic plants exploit the secretory kind: the fragrance of a mint or tulsi leaf comes from oils stored in glandular trichomes on its surface, which is why the scent is released the moment the leaf is crushed.
Stomata and the Stomatal Apparatus
Small gateways in the epidermal cells are called stoma. Such stoma are controlled or guarded by specially modified cells called guard cells. These guard cells may be kidney shaped, as in dicots, or dumbbell shaped, as in monocots, and along with the pore they are collectively called stomata.
The stoma, the guard cells and the subsidiary cells together form a unit called the stomatal apparatus. Stomata are further covered by subsidiary cells, which are the epidermal cells immediately surrounding the guard cells and which support their movement. The whole unit is shown in surface view in Fig. 8.11.
Guard cells have a few chloroplasts and can carry out photosynthesis, which sets them apart from the other epidermal cells around them. Guard cells change their turgor pressure, causing the opening and closing of the stoma, and thus they play a vital role in the exchange of gases and water vapour.
The movement depends on an asymmetry in the wall. The wall of a guard cell facing the pore is thick, while the outer wall is thin, so when water enters and turgor pressure rises, the thin outer wall stretches more than the thick inner wall. The cell therefore bends outward and the pore between the pair opens. When water is lost, turgor falls, the cells straighten and the pore closes.
guard cell (Inner wall facing the pore thick and the outer wall thin → the two walls stretch unequally when turgor rises → the cell curves outward and the pore opens. Chloroplasts present → energy and solutes are made locally → turgor changes can be driven quickly.)
Because a stoma must open for carbon dioxide to enter but water vapour escapes through the same pore, every plant lives with a compromise between feeding and drying out. Farmers see the result directly at midday in summer, when crops wilt and stomata close even though sunlight is at its strongest — photosynthesis slows down because the plant has chosen to conserve water.
Ground Tissue System
All the plant tissues excluding the epidermal and vascular tissues make up the ground tissue. It is made up of simple permanent tissue, for example parenchyma, which is why the ground tissue is often the bulk of an organ by volume.
It is present in the cortex, pericycle, pith and medullary rays in the primary stem and root. Collenchyma and sclerenchyma in the hypodermis, and the chloroplast-containing mesophyll tissue in leaves, also constitute ground tissue.
So the ground tissue system is defined by exclusion rather than by a single cell type. Whatever lies between the outer covering and the conducting strands — packing, storing, supporting or photosynthesising — belongs to it.
Vascular Tissue System
The vascular tissue system consists of distinct patches of complex tissue, namely xylem and phloem. These patches are not scattered at random; the way they are arranged is one of the most reliable clues for identifying a plant organ under the microscope.
On the basis of their arrangement in the plant body, the bundles are described as radial when both the complex tissues are situated separately on separate radii, as separate bundles. This is a common feature of roots.
In the stem, the complex tissues are collectively present as neighbours of each other on the same radius, in the form where xylem is inside and phloem is outside. Such bundles are hence called conjoint, collateral, vascular bundles. The word conjoint means the two tissues are joined in one bundle, and collateral means they lie side by side on the same radius.
Radial vs Conjoint bundles (Radial — xylem and phloem lie on separate radii as separate bundles, typical of roots. Conjoint — xylem and phloem lie together in a single bundle on the same radius, typical of stems.)
Types of Vascular Bundles
These bundles may be further of the open type, in which secondary growth takes place, containing cambium in between them, and of the closed type if cambium is not present, in which case secondary growth is absent. This single feature — the presence or absence of a strip of cambium — decides whether the organ will ever be able to thicken.
When phloem is present in a vascular bundle on both the sides of the xylem, with intervening cambium tissue, it is called a bicollateral vascular bundle. It is a feature of the family Cucurbitaceae, the family of gourds, pumpkins and cucumbers.
When one vascular tissue is completely encircling the other, it is called a concentric vascular bundle. This may be leptocentric, in which phloem is encircled by xylem, or hadrocentric, in which xylem is encircled by phloem.
When one complex tissue is encircling on both the faces of the other, it is amphicribral, in which xylem is encircled by phloem on both faces, and amphivasal, in which phloem is encircled by xylem on both faces. The types are compared in Fig. 8.12.
reading the names of vascular bundles (Split each name into its two halves. Lepto- stands for phloem and hadro- for xylem, while -centric tells you which tissue is in the centre — so leptocentric has phloem at the centre, encircled by xylem, and hadrocentric has xylem at the centre, encircled by phloem. In the other pair, cribrum means a sieve, so amphicribral means phloem is the tissue lying on both faces, and vas means a vessel, so amphivasal means xylem is the tissue lying on both faces.)
Open vs Closed vascular bundle | Feature | Open vascular bundle | Closed vascular bundle | |—|—|—| | Cambium | Present between xylem and phloem | Absent | | Secondary growth | Takes place | Does not take place | | Increase in girth | Possible | Not possible | | Typically seen in | Dicot stems | Monocot stems |
In examinations, this section is tested mostly through identification: you are given a labelled section or a description and asked to name the bundle type, or to say which organ or group the section belongs to. Working from the arrangement — same radius or different radii, cambium present or absent, phloem on one face or both — is far more reliable than trying to recall the names as a list.
8.5 Secondary Growth in Plants
Primary Growth and Secondary Growth
The vertical growth of the roots and stems in length, with the help of the apical meristem, is called primary growth. This is the growth that makes a seedling taller and pushes its root deeper into the soil.
Growth in length alone, however, cannot support a large plant. A tall stem carrying heavy branches would bend and snap unless it also became thicker. Dicotyledonous plants and gymnosperms therefore exhibit an increase in the girth of the root and the stem, and this increase in thickness is what we call secondary growth.
Primary growth is the increase in length of the root and shoot brought about by the apical meristem; secondary growth is the increase in girth of the root and stem brought about by lateral meristems.
Primary growth vs Secondary growth | Feature | Primary growth | Secondary growth | |—|—|—| | Meristem responsible | Apical meristem | Lateral meristems (vascular cambium and cork cambium) | | Direction of growth | Increase in length | Increase in girth | | Tissues produced | Primary tissues | Secondary tissues | | Occurs in | All plants | Dicots and gymnosperms |
Activity of the Cambium Ring
Once formed, the cambium ring cuts off new cells towards both the sides, inner and outer. Each division adds one cell to the growing tissue and leaves one cell behind as cambium, so the ring itself is never used up.
The cells that are cut off towards the pith, that is, on the inner side, mature into secondary xylem, and the cells that are cut off towards the periphery mature into secondary phloem. Generally, the amount of secondary xylem is more than the secondary phloem. The result of many such seasons of activity is seen in Fig. 8.13.
what happens to one cambium cell during a favourable season 1. The cambium cell, lying in the ring between xylem and phloem, divides. 2. One of the two daughter cells remains as cambium, keeping the ring intact. 3. If the other daughter cell lies on the inner side, it matures into an element of secondary xylem. 4. If it lies on the outer side, it matures into an element of secondary phloem. 5. The cambium ring is pushed slightly outward by the xylem piling up inside it. 6. The process repeats through the season, and the stem grows measurably thicker.
Q2. A young sapling is thinner than your wrist, yet the same tree later has a trunk you cannot hug. If plant cells cannot simply stretch, how does the trunk get thicker?
A. The trunk does not thicken by stretching its existing cells. A continuous ring of vascular cambium forms inside the stem and keeps dividing season after season, adding secondary xylem on its inner side and secondary phloem on its outer side. Since far more secondary xylem is added than phloem, layer upon layer of it piles up inside the trunk, and the girth increases from within.
Importance of Secondary Tissues
In woody plants, the secondary tissues constitute the bulk of the body. What looks from outside like a solid trunk is, almost entirely, tissue added after the stem first formed.
These tissues provide support, conduction of water and minerals, and protection. Lateral meristems play a major role in the development of secondary tissues — which is why only those plants that possess lateral meristems can become woody at all.
Grafting and budding, used routinely to propagate mango, rose, citrus and apple, work only because of the cambium. The gardener has to align the cambium of the scion with the cambium of the stock, because only these dividing cells can produce the new tissue that fuses the two pieces into one continuous plant. If the cut is made carelessly and the two cambium rings do not meet, the graft simply fails to take.
Secondary Growth in Roots
Secondary growth is also observed in most of the dicot and gymnospermic roots, by producing secondary vascular tissue and periderm. Secondary growth is produced by the vascular cambium and the cork cambium respectively — the vascular cambium making the secondary vascular tissue, and the cork cambium making the periderm.
The starting point in a root is different from that in a stem, because the root has radial bundles with no cambium inside them. Here, the conjunctive parenchyma cells present on the inner edges of the primary phloem bundles become meristematic. These cells add secondary xylem and secondary phloem on the inner and outer side respectively. Apart from this difference in origin, these events are similar to secondary growth in stems, as set out in FC 8.8.
Board questions on this section often ask you to arrange the steps of cambial ring formation in the correct sequence, or to explain why a monocot stem cannot show this kind of growth. The reasoning to hold on to is simple: no lateral meristem means no secondary growth, whatever the size of the plant.
8.6 Wood
Wood is nothing other than the secondary xylem that piles up inside a trunk year after year. Because the cambium that produces it works at different rates at different times of the year, this wood is not uniform, and the differences within it can be read like a record.
Spring Wood and Autumn Wood
During favourable conditions, spring wood, also called early wood, is formed. It has broader xylem bands and a lighter colour; its tracheids have thin walls and a wide lumen, its fibres are less in number, and its density is low. Water is plentiful and the plant is growing fast, so the cambium produces wide, roomy pipes quickly.
Whereas during the unfavourable season, autumn wood, also called late wood, is formed. It has a narrow xylem band and a darker colour, the lumen is narrow, the walls are thick, abundant fibres are present, and the wood is of high density. With less water available, the emphasis shifts from rapid conduction to strength.
Together, one band of spring wood and the band of autumn wood that follows it make up the wood formed in a single year. This pair is seen in a cut trunk as one growth ring or annual ring, and counting the rings gives a good estimate of the age of the tree. The contrast is shown in Fig. 8.19.
Spring wood vs Autumn wood | Feature | Spring wood (early wood) | Autumn wood (late wood) | |—|—|—| | Season of formation | Favourable season | Unfavourable season | | Width of xylem band | Broader | Narrow | | Colour | Lighter | Darker | | Tracheid wall and lumen | Thin walls, wide lumen | Thick walls, narrow lumen | | Fibres | Less in number | Abundant | | Density | Low | High |
reading the rings in Fig. 8.19 (Each light-and-dark pair is one year, so the number of pairs gives the age of the tree. The width of a pair reports the growing conditions of that year — a wide ring means a season with plenty of water and a long favourable period, while a narrow ring means a poor or dry year. Reading outward from the centre therefore gives you the tree’s life story in order, from its earliest years at the middle to the most recent year just under the bark.)
Q3. Why do the cut logs of some trees show clear rings, while logs of other plants show none at all?
A. A ring appears only when a cambium alternates between a fast, favourable season and a slow, unfavourable one, producing a light band of spring wood followed by a dark band of autumn wood. Where the seasons contrast sharply, the rings are distinct. Where the climate stays uniform through the year, the cambium works at a steady rate and the boundaries are faint or absent. And a plant that has no vascular cambium at all forms no secondary xylem, so its cut stem shows no rings whatsoever.
In the early decades of the twentieth century, the American astronomer A. E. Douglass noticed that trees growing in the same region showed the same pattern of wide and narrow rings, because they had lived through the same good and bad years. By matching the ring patterns of living trees with those of older timber, he was able to build a continuous calendar reaching far back in time. This method, tree-ring dating, is still used to date wooden beams in ancient buildings and to reconstruct the rainfall of past centuries.
Heartwood and Sapwood
As a trunk ages, its innermost wood stops conducting. The tracheary elements of heartwood are plugged by in-growths of the adjacent parenchyma cells, and these balloon-like in-growths are called tyloses. Once tyloses block a vessel, water can no longer pass through it.
These blocked elements are then filled by oils, gums, resins and tannins, which are together called extractives. This inner, non-functional, durable part, which is resistant to pathogens, is called duramen or heartwood. Its darkness and its resistance to decay both come from the same extractives, which are toxic or unpalatable to fungi and insects.
The outer, light, functional part of the secondary xylem is called sap wood, or alburnum. Here the cells are living, there is no such deposition, the wood is lighter in colour and less durable, it is more susceptible to pathogens, and it is involved in the conduction of sap. A useful way to see the difference is shown in Fig. 8.20.
Heartwood vs Sapwood (Heartwood is the inner, darker, dead and non-conducting wood plugged with tyloses and filled with extractives, and it is durable and pathogen-resistant. Sapwood is the outer, lighter wood with living cells, no such deposition, and it still conducts sap, but it is less durable.)
The durability of heartwood is exactly what timber merchants sell. The dark heartwood of teak and sal resists termites and fungal decay for decades and is used for door frames, furniture and boats, while the pale sapwood of the same log is worth far less and is often trimmed away. Tyloses have a commercial use of their own: the wood of certain oaks is so thoroughly plugged by them that barrels made from it hold liquid without leaking, which is why such barrels are used for ageing wine and whisky.
8.7 Cork Cambium and Secondary Growth
The Need for a New Protective Layer
The increase in diameter of the stem by secondary growth is mainly due to the activity of the vascular cambium, and a second lateral meristem then appears in the outer cortical layer to deal with the consequences of that thickening.
The consequence is mechanical. The epidermis is a single layer of cells that cannot keep pace with a trunk expanding from within, so it gets ruptured. Once that happens, it becomes necessary to replace these cells by new cells, because a plant with a torn outer covering would lose water freely and would be open to infection.
Periderm and Bark
The secondary cortex is parenchymatous in nature, so the tissue formed on the inner side of the phellogen remains living and can store food. Phellogen, phellem and phelloderm together constitute the periderm.
The activity of the cork cambium develops a pressure on the other cells lying outside it, and these cells die. Cut off from water and food by the impervious cork layer inside them, the outer tissues cannot survive, and they gradually dry and flake away.
Bark is a non-technical term referring to all the cell types found external to the vascular cambium, including the secondary phloem. Being non-technical, the word covers a mixture of living and dead tissue rather than one defined tissue. Bark of the early season is soft and that of the late season is hard.
Periderm vs Bark (Periderm is a precise term for just three layers — phellogen with the phellem outside it and the phelloderm inside it. Bark is a loose, non-technical term for everything lying outside the vascular cambium, and therefore includes the periderm and the secondary phloem.)
the three layers of periderm (All three share the root phell-. Phellogen generates the other two; phellem is the layer thrown outside as cork; phelloderm lies inside as the secondary cortex — think of “derm” as the deeper skin.)
The cork used in bottle stoppers, insulation board and the core of a cricket ball is stripped from the thick bark of the cork oak, a tree of the Mediterranean region. The stripping is done carefully so that the phellogen underneath is left alive; because that meristem survives, it simply makes a fresh layer of cork, and the same tree can be harvested again and again over a lifetime of more than a century.
Lenticels
Sealing a stem completely would create a new problem: the living cells inside still respire and need to exchange gases. The plant solves this with small openings in the bark.
Lenticels are aerating pores present as raised scars on the surface of the bark. These are portions of the periderm where phellogen activity is more, so that loosely arranged cells are produced instead of tightly packed cork, leaving a passage between them. Lenticels are the means for gaseous and water vapour exchange, as seen in Fig. 8.22.
Stomata vs Lenticels (Stomata are pores in the epidermis, guarded by living guard cells, and they can open and close. Lenticels are pores in the bark, formed of loosely arranged periderm cells, and they remain permanently open.)
The small pale dots scattered over the skin of a potato tuber or an apple are lenticels. Storekeepers pay attention to them: if tubers are kept in waterlogged or badly ventilated conditions, water fills these pores, gas exchange stops, and the tissue beneath begins to rot from the inside.
Secondary Growth in Monocots
Monocot stems lack cambium, hence secondary growth does not take place in them. Their vascular bundles are closed, so there is no strip of dividing cells that could be joined into a ring.
However, accessory cambium development in plants like Dracaena, Agave, palms and the root of sweet potato shows the presence of secondary growth. Here a fresh band of meristematic cells arises in an unusual position, outside the normal vascular arrangement, and produces additional tissue. Because it does not follow the standard pattern of a vascular cambium ring, this is called anomalous secondary growth.
Q4. A coconut palm grows very tall but its trunk stays almost the same thickness for years. Why can it not thicken the way a mango tree does?
A. A mango tree is a dicot: its vascular bundles are open, so the strips of cambium inside them join into a continuous ring that adds secondary xylem and phloem every season. A palm is a monocot, and its bundles are closed, with no cambium between xylem and phloem. Lacking a normal cambial ring, it grows in height using its apical meristem but cannot add regular layers of secondary tissue. Some monocots, palms among them, develop an accessory cambium in an unusual position and show a limited, anomalous form of secondary growth, but this is nothing like the year-on-year thickening of a dicot trunk.
- Concentric vascular bundles are always closed. Describe.
- How is the arrangement of vascular bundles in dicot and monocot stem?
- How is the structure of the vascular bundles of the root?
- Why are the vascular bundles of a dicot stem described as conjoint, collateral and open?
8.8 Anatomy of Root, Stem and Leaf
Everything studied so far — the tissues, the tissue systems and the ways they thicken — now comes together. In this section the same three tissue systems are traced through actual organs, and the small differences in their arrangement are exactly what allow you to identify an unknown section under the microscope.
Anatomy of Dicot Root
The transverse section of a typical dicotyledonous root shows the following anatomical features, arranged from the outside inward as summarised in FC 8.11.
The outermost single layer of cells, which is without a cuticle, is the epiblema. The absence of a cuticle is significant here: a root has to take in water, and a waterproof covering would defeat that purpose. Some of its cells are prolonged into unicellular root hairs.
Next to it on the inner side is the cortex, which consists of several layers of typical parenchymatous cells. After the death of the epiblema, the outer layer of the cortex becomes cutinized and is called the exodermis, taking over the protective role that the epiblema had been performing. The cortical cells store food and water.
The innermost layer of the cortex is called the endodermis. Its cells are barrel-shaped, and their radial walls bear the Casparian strip, also called Casparian bands, composed of suberin. Because suberin is waterproof, these bands seal the wall route between neighbouring endodermal cells, so that water and dissolved minerals are forced to pass through the living cytoplasm instead of slipping past unchecked. Near the protoxylem there are unthickened passage cells, which lack this thickening and therefore allow material through.
endodermis (Suberin laid down as a Casparian band on the radial walls → water cannot travel along the wall between cells → every ion must cross a living membrane, so the root controls what enters. Unthickened passage cells left near the protoxylem → a regulated doorway → water and minerals reach the xylem at exactly the right place.)
The Casparian strip is the reason a plant is not simply at the mercy of its soil. Because everything entering the stele must cross a living membrane at the endodermis, the root can admit the nutrients it needs and exclude much of what it does not, including excess sodium in saline soils and several toxic metals. Crop varieties bred for salt-affected land depend heavily on how effective this checkpoint is.
A single layer of parenchymatous pericycle is present just below the endodermis, and it bounds the stele or vascular cylinder. The stele consists of 2 to 6 radial vascular bundles, and the xylem is exarch. Based on the number of groups of xylem and phloem, the stele may be diarch to hexarch — that is, having two groups up to six groups.
A parenchymatous connective tissue, also called conjunction tissue, is present between the xylem and the phloem. This is the same tissue that will later become meristematic when the root begins secondary growth.
The central part of the stele or vascular cylinder is called the pith. It is narrow and made up of parenchymatous cells, with or without intercellular spaces. At a later stage, a cambium ring develops between the xylem and the phloem, which causes secondary growth in thickness. The complete section is shown in Fig. 8.14.
Epiblema vs Epidermis (Both are the outermost single layer, but the epiblema of the root has no cuticle and bears unicellular root hairs for absorption, while the epidermis of the stem carries a cuticle and multicellular trichomes and is built to prevent water loss.)
Anatomy of Monocot Root
The monocot root resembles that of a dicot root in its basic plan — the same epiblema, cortex, endodermis, pericycle and radial bundles appear in the same order. However, it possesses more than six xylem bundles, which is the polyarch condition. The pith is large and well-developed, and secondary growth is absent. The section is shown in Fig. 8.15.
naming the stele by number of xylem groups (Count the xylem groups and attach the Greek number: two → diarch, three → triarch, four → tetrarch, five → pentarch, six → hexarch. Beyond six there is no separate name — it is simply polyarch, “many”, and that “many” points to a monocot root.)
Dicot root vs Monocot root | Feature | Dicot root | Monocot root | |—|—|—| | Number of xylem bundles | 2 to 6 (diarch to hexarch) | More than six (polyarch) | | Pith | Narrow | Large and well developed | | Cambium | Develops later between xylem and phloem | Absent | | Secondary growth | Present | Absent |
Anatomy of Dicot Stem (Sunflower)
A transverse section of a dicot stem shows the following structures, again read from the outside inward as set out in FC 8.12.
The epidermis is single, and is the outermost layer with multicellular outgrowths called trichomes. A layer of cuticle is usually present towards the outer surface of the epidermis.
The cortex is situated inner to the epidermis and is usually differentiated into three regions, namely the hypodermis, the general cortex and the endodermis. The hypodermis is situated just below the epidermis and is made of 3-5 layers of collenchymatous cells, in which intercellular spaces are absent. Placing a flexible mechanical tissue just under the surface is efficient design — like a hollow pipe, a stem resists bending best when its strengthening material lies near the outside.
The general cortex is made up of several layers of large parenchymatous cells with intercellular spaces. The endodermis is the innermost layer of the cortex, made up of barrel shaped cells. It is also called the starch sheath, because its cells are characteristically loaded with starch grains.
The stele is the central core of tissues, differentiated into pericycle, vascular bundles and pith. The pericycle is the outermost layer of the vascular system, situated between the endodermis and the vascular bundles. In sunflower it is multilayered and sclerenchymatous in the region of the vascular bundles, and it is called hard bast.
The vascular bundles are conjoint, collateral, open, and are arranged in a ring. Each one is composed of xylem, phloem and cambium. The xylem is endarch, and a strip of cambium is present between the xylem and the phloem — the strip that later becomes part of the cambial ring.
The pith is situated in the centre of the young stem and is made up of large-sized parenchymatous cells with conspicuous intercellular spaces. The complete section is shown in Fig. 8.16.
Anatomy of Monocot Stem
The monocot stem differs from the dicot stem in almost every one of these features. The epidermis is without trichomes and the hypodermis is sclerenchymatous — that is, the mechanical tissue below the surface is rigid and lignified rather than flexible.
The vascular bundles are numerous and are scattered in the ground tissue, rather than being arranged in a ring. Each vascular bundle is surrounded by a sclerenchymatous bundle sheath, which supplies the strengthening that a scattered bundle would otherwise lack.
The vascular bundles are conjoint, collateral and closed, that is, without cambium. The xylem is endarch and shows a lysigenous cavity — a space formed where some of the earliest protoxylem elements have broken down as the stem elongated. The pith is absent, because the scattered bundles and ground tissue occupy the centre, and secondary growth is also absent. The section is shown in Fig. 8.17.
Dicot stem vs Monocot stem | Feature | Dicot stem | Monocot stem | |—|—|—| | Trichomes on epidermis | Present | Absent | | Hypodermis | Collenchymatous | Sclerenchymatous | | Arrangement of vascular bundles | In a ring | Scattered in ground tissue | | Bundle sheath | Absent | Sclerenchymatous sheath around each bundle | | Nature of bundle | Conjoint, collateral, open | Conjoint, collateral, closed | | Lysigenous cavity | Absent | Present in the xylem | | Pith | Present, parenchymatous | Absent | | Secondary growth | Present | Absent |
identifying an unlabelled transverse section 1. Look at the outermost layer. Root hairs and no cuticle mean a root; a cuticle with trichomes means a stem. 2. If it is a root, find the xylem groups. Two to six means dicot; more than six with a large pith means monocot. 3. If it is a stem, look at the vascular bundles. Arranged in a ring means dicot; scattered through the ground tissue means monocot. 4. Check for cambium between xylem and phloem. Present means an open bundle capable of secondary growth; absent means a closed bundle. 5. Check the position of the protoxylem. Towards the periphery is exarch, so the organ is a root; towards the pith is endarch, so it is a stem. 6. Confirm with the hypodermis: collenchymatous points to a dicot stem, sclerenchymatous with bundle sheaths points to a monocot stem.
Anatomy of Leaf: Dorsiventral Leaf
The dorsiventral leaf is very common in dicotyledonous plants, where the mesophyll tissue is differentiated into palisade and spongy parenchyma. Such leaves are commonly horizontal in orientation, with distinct upper and lower surfaces. The upper surface, which faces the sun, is darker than the lower surface.
The reason for that difference in colour lies inside. The tissue just below the upper surface is packed with chloroplasts, while the tissue near the lower surface is loosely arranged and holds fewer of them, so the two faces of the same leaf look different.
T.S. of a Typical Dicot Leaf
The upper epidermis consists of a single layer of tightly packed, rectangular, barrel shaped, parenchymatous cells which are devoid of chloroplast. A distinct layer of cuticle lies on the outside of the epidermis, and stomata are generally absent here. Keeping the upper surface free of pores makes sense for a leaf held horizontally: that surface receives direct sun and would lose water rapidly through open pores.
Between the upper and the lower epidermis there is chloroplast-containing photosynthetic tissue called mesophyll. The mesophyll is differentiated into palisade and spongy tissue.
Palisade parenchyma is present below the upper epidermis and consists of closely packed elongated cells. These cells contain abundant chloroplasts and help in photosynthesis. Their elongated shape, standing on end like a row of columns, lets light pass deep into the leaf while placing the maximum number of chloroplasts directly in its path.
Spongy parenchyma is present below the palisade tissue and consists of loosely arranged, irregularly shaped cells with intercellular spaces. The spongy parenchyma cells contain chloroplast and are in contact with the atmosphere through the stomata. The large air spaces here are the leaf’s internal circulation system: carbon dioxide entering through a stoma diffuses through them to every photosynthesising cell.
mesophyll (Palisade cells elongated and tightly packed just under the upper surface → maximum chloroplasts in the brightest light → most of the photosynthesis happens here. Spongy cells loosely arranged with wide air spaces above the stomata → gases diffuse freely inside the leaf → carbon dioxide reaches the palisade cells and oxygen and water vapour leave.)
The vascular system is made up of a number of vascular bundles of varying size, depending upon the venation. Each one is surrounded by a thin layer of parenchymatous cells called the bundle sheath. The vascular bundles are closed, and the xylem lies towards the upper epidermis while the phloem lies towards the lower epidermis. Cambium is absent, hence there is no secondary growth in the leaf.
The lower epidermis consists of a single layer of compactly arranged, rectangular, parenchymatous cells. A thin layer of cuticle is also present. The lower epidermis contains a large number of microscopic pores called stomata, and there is an air-space called the sub-stomatal chamber at each stoma. The whole section is shown in Fig. 8.23.
The bundle sheath is far more than packing tissue in some plants. In maize, sugarcane and several other tropical grasses, the sheath cells are large, thick-walled and crowded with chloroplasts, forming a ring around each bundle. This arrangement, visible in an ordinary leaf section, allows those plants to concentrate carbon dioxide around the photosynthetic machinery and keep photosynthesis efficient at high temperature — one reason such crops thrive in Indian summers.
Isobilateral Leaf
In this leaf both the surfaces are equally illuminated, as both surfaces can face the sun, and they show similar structure. The two surfaces are equally green. Generally, monocotyledonous plants have isobilateral leaves. A grass blade, for instance, stands more or less vertically, so neither face is consistently the shaded one.
A typical monocot leaf resembles a dicot leaf in its anatomical structure. However, it shows stomata on both the surfaces, and the mesophyll is not differentiated into palisade and spongy tissue. It has parallel veins, and these are conjoint, collateral and closed. The section is shown in Fig. 8.18.
Q5. Why does a mango leaf feel darker green on top and paler below, while a grass leaf looks equally green on both sides?
A. A mango leaf is dorsiventral. It is held horizontally, so only its upper surface faces the sun, and its mesophyll is differentiated accordingly — closely packed palisade cells crowded with chloroplasts lie just under the upper surface, while the looser spongy tissue with fewer chloroplasts lies below. That difference in chloroplast density is what you see as a difference in colour. A grass leaf is isobilateral. It stands nearly upright, both surfaces are equally illuminated, the mesophyll is not differentiated into palisade and spongy tissue, and stomata occur on both surfaces — so both faces look alike.
Dorsiventral (dicot) leaf vs Isobilateral (monocot) leaf | Feature | Dorsiventral leaf | Isobilateral leaf | |—|—|—| | Orientation | Commonly horizontal, distinct upper and lower surfaces | Both surfaces equally illuminated | | Colour of the two surfaces | Upper surface darker than lower | Both surfaces equally green | | Mesophyll | Differentiated into palisade and spongy parenchyma | Not differentiated | | Stomata | Mainly on the lower epidermis, generally absent above | Present on both surfaces | | Venation | Reticulate | Parallel | | Vascular bundles | Closed, xylem upward and phloem downward | Conjoint, collateral and closed |
Grass leaves carry large, thin-walled bulliform cells in the upper epidermis. When water is short, these cells lose turgor and shrink, and the blade rolls inward, hiding many of its stomata inside the roll and cutting water loss. Farmers use this as a field signal — rolled leaves in a paddy or maize crop at midday are an early warning of water stress, well before the plant shows any permanent damage.
from tissue to organ (Zoom in: each layer named in this section — epiblema, cortex, endodermis, palisade, spongy tissue — is one of the tissues studied earlier in the chapter. Zoom out: these layers make up the root, stem and leaf, and those organs together make the plant body whose growth in length and girth the previous sections described.)
This section supplies most of the diagram-based questions in board papers and NEET — labelling a T.S., naming the organ from a described section, and listing anatomical differences between monocots and dicots. Working from a fixed order of layers, outside inward, is far more dependable than trying to recall each section as a separate picture.