Anatomy of Flowering Plants | ISC Class 11 Biology Notes

This note covers plant tissues, regions of growth, supporting and conducting cells, protective coverings, tissue systems, and the internal structure of roots, stems and leaves in flowering plants.

What are plant tissues, and how do growing cells become specialised?

Anatomy is the study of internal structure. A tissue is a group of cells working together to perform a function. Cells form tissues, and tissues form organs such as roots, stems and leaves. Different organs have different internal arrangements.

Flowering plants, or angiosperms, include monocotyledons and dicotyledons. A cotyledon is a seed leaf: monocotyledons have one and dicotyledons have two. The shortened terms monocot and dicot refer to these groups, whose internal structures also differ.

What distinguishes meristematic cells?

Meristematic tissue consists of actively dividing cells. Its cells are small, with thin walls, a large prominent nucleus, the cell structure containing genetic material, and dense cytoplasm, the cellular material outside the nucleus. They contain many organelles, specialised structures within cells.

Vacuoles, membrane-bound storage compartments, are generally absent. Cells are tightly packed with little or no intercellular space, meaning space between neighbouring cells. These features accompany continuous, rapid division. New cells are added where meristems remain active.

How does differentiation produce permanent tissue?

1. Meristematic cells divide and add new cells to the plant body.
2. Some newly formed cells remain meristematic and continue dividing.
3. Other cells lose the ability to divide and undergo changes in their structure and function.
4. These cells become specialised for functions such as support, transport or storage, forming permanent tissues.

Differentiation is the process through which cells mature and become specialised. Permanent tissues contain cells that have undergone this specialisation. A simple tissue has one type of cell; a complex tissue contains different cell types working together.

Specialisation does not mean every cell has died. Many permanent tissues contain living cells. Under certain conditions, living differentiated cells can regain the ability to divide. This process is called dedifferentiation.

How are meristems classified by location and origin?

Meristems can be classified using two different criteria. Location describes where a growing region occurs in the plant. Origin describes how that meristem arose. A location name and an origin name therefore answer different questions about the same tissue.

Where do the three positional types occur?

A node is a point on a stem where leaves or branches arise. An internode is the stem region between two nodes. An apex is a tip; thus, apical meristems occupy root and shoot tips.

Primary growth principally lengthens the plant axis. Secondary growth increases the girth of organs through lateral meristem activity. These terms describe growth and should be distinguished from the classification of meristems by origin.

What are primary and secondary meristems?

An embryo is the young developing plant within a seed. A primary meristem originates from embryonic meristematic tissue and retains the ability to divide. A secondary meristem arises when previously differentiated living cells regain that ability. The distinction concerns developmental origin, rather than simply whether a meristem lies at a tip or side.

Promeristem is the earliest meristematic tissue in an embryo or growing apex, from which primary meristems develop. It is also called primordial meristem. Recognising this initial tissue helps trace the origin of the later growth regions.

Cambium is meristematic tissue producing new cells. Vascular cambium forms secondary conducting tissues; cork cambium forms protective tissues. Vascular bundles are associations of water-conducting and food-conducting tissues. Interfascicular cambium, which develops between these bundles, and cork cambium can arise from fully differentiated living cells by dedifferentiation.

Cells produced by such meristems can subsequently lose the capacity to divide and mature for particular functions. This is redifferentiation. Differentiation, dedifferentiation and redifferentiation describe changes in cell state, rather than three positions within an organ.

How do the simple permanent tissues differ?

The three simple permanent tissues are parenchyma, collenchyma and sclerenchyma. They differ in wall thickness, whether their cells remain living, and their contribution to storage or support. Identifying the tissue requires several features together, rather than the organ name alone.

The epidermis is the outermost layer of the primary plant body. Ground tissue comprises tissues other than the epidermis and vascular bundles. Photosynthesis is the formation of food using light energy.

What are their structures and functions?

Pectin, a wall substance contributing flexibility, is deposited at the corners of collenchyma cells. This tissue allows plant parts to bend without breaking. A tendril is a slender climbing structure that coils around a support.

Lignin is a wall-strengthening substance. Its deposition makes sclerenchyma walls hard and strong. Coconut husk and walnut shell illustrate supporting tissues associated with hard plant coverings. Keep the qualification that most sclerenchyma cells are dead.

How can parenchyma perform different roles?

Parenchyma mainly stores food, but in green parts it also carries out photosynthesis. Chloroplasts are the cell organelles in which photosynthesis occurs. Specialised parenchyma in aquatic plants forms air spaces that help the plants float.

Parenchyma containing chloroplasts is called chlorenchyma. Parenchyma with extensive air spaces is called aerenchyma. These names describe specialisations of parenchyma; they do not add two separate basic simple-tissue categories.

Two forms of sclerenchyma are fibres, elongated supporting cells, and sclereids, shorter cells of varied shape with thick, lignified walls. A lignified wall is one containing deposited lignin. Both forms illustrate the relationship between wall structure and mechanical support.

How does xylem structure support water transport?

Xylem is the complex tissue that transports water and minerals from roots to other plant parts and provides strength. Its four components are tracheids, vessels, xylem parenchyma and xylem fibres. Their different structures allow conduction, storage and support within one tissue.

What does each xylem component do?

Vessel elements are the individual cells forming a vessel. Their perforated end walls provide openings between successive elements. Pits are regions where secondary wall thickening is absent; they occur in the walls of conducting cells and allow lateral movement of water.

Protoplasm means the living contents of a cell. During differentiation, tracheary elements, the water-conducting cells of xylem, lose their protoplasm and develop strong secondary walls. The primary wall is the first-formed cell wall. A secondary wall is additional wall material deposited inside it.

Xylem parenchyma is the living component of mature xylem. Tracheids, vessels and xylem fibres are primarily sclerenchymatous, meaning that they have the structural character of sclerenchyma. Do not describe xylem as a tissue made entirely of dead cells.

What do protoxylem and metaxylem mean?

Protoxylem is the first-formed primary xylem; metaxylem develops later. In an endarch arrangement, protoxylem lies towards the centre and metaxylem towards the outside. In an exarch arrangement, protoxylem lies towards the outside and metaxylem towards the centre.

The typical dicot stem has endarch protoxylem. Root sections show the reverse orientation. Identify the position relative to the organ's centre: merely saying that protoxylem is “inside” without naming the reference point can make the arrangement unclear.

How does phloem transport food through living tissues?

Phloem is the complex conducting tissue that transports food from leaves to other parts of the plant. It is mostly made up of living cells. Its components include sieve tubes, companion cells, phloem parenchyma and phloem fibres.

How do the components work together?

Sieve tubes consist of long, tubular cells joined end to end by perforated walls. Their individual cells are called sieve-tube elements. The perforated end walls are sieve plates. This arrangement connects successive elements along the pathway of food transport.

Companion cells are specialised parenchyma cells associated with sieve tubes. They regulate the cellular functions of sieve-tube cells and monitor the loading and unloading of sugars. Here, loading means entry of sugars into sieve tubes, and unloading means their removal.

Phloem parenchyma consists of living cells that store food materials. Phloem fibres are primarily sclerenchymatous supporting cells that provide strength. Thus, food transport and mechanical support are associated within phloem, but are performed by different components.

A complete description of either complex tissue includes its components as well as its overall function. “Transport tissue” alone does not distinguish xylem from phloem. Likewise, the presence of fibres does not turn either complex tissue into a simple tissue.

Different cell types cooperate within each conducting tissue. The word complex refers to this combination of cell types, rather than simply to the thickness of the walls or the distance through which a substance travels.

How does the epidermal tissue system protect the plant?

The epidermal tissue system forms the outermost covering of the plant body. It includes epidermal cells, stomata and epidermal appendages such as hairs. The epidermis is the outermost layer of the primary plant body and is usually single-layered.

Its elongated, compactly arranged cells form a continuous layer. They are parenchymatous, with a small amount of cytoplasm lining the cell wall and a large vacuole. The outer surface is often covered by a thick waxy cuticle, which prevents water loss. Roots lack a cuticle.

What makes up the stomatal apparatus?

Stomata are structures in the leaf epidermis that regulate gaseous exchange and transpiration, the loss of water as vapour from plant surfaces. Each stoma has two guard cells enclosing a stomatal pore, the opening through which exchange occurs.

Guard cells are bean-shaped, but in grasses they are dumb-bell-shaped. Their outer walls, away from the pore, are thin; the inner walls facing the pore are highly thickened. Guard cells possess chloroplasts and regulate opening and closing of the pore.

Sometimes nearby epidermal cells become specialised in shape and size. These are subsidiary cells. The pore or aperture, guard cells and surrounding subsidiary cells together constitute the stomatal apparatus.

The two drawings compare bean-shaped and dumb-bell-shaped guard cells. Labels identify epidermal cells, subsidiary cells, chloroplasts, guard cells and the central stomatal pore.

See Fig. 6.1 in your NCERT textbook

How do root hairs differ from shoot hairs?

Root hairs are unicellular elongations of epidermal cells, meaning each is an extension of one cell. They help absorb water and minerals from soil. Epidermal hairs on the stem are called trichomes.

Trichomes in the shoot system are usually multicellular, meaning made of several cells. They may be branched or unbranched, soft or stiff, and may even be secretory, releasing substances. Trichomes help prevent water loss through transpiration.

Note: Keep the distinctions precise: the epidermis is usually single-layered, subsidiary cells occur sometimes, and shoot trichomes are usually multicellular. These qualifications are part of the descriptions.

How are ground tissue and vascular bundles organised?

A tissue system groups tissues according to their structure and location. The three systems are epidermal, ground and vascular. Ground tissue, also called fundamental tissue, comprises all tissues except the epidermis and vascular bundles.

Ground tissue includes parenchyma, collenchyma and sclerenchyma. In primary roots and stems, parenchymatous cells are usually present in the cortex, pericycle, pith and medullary rays. These terms identify regions within organs, rather than additional basic cell types.

What do the regional names mean?

The cortex is the tissue region between the outer covering and the inner boundary called the endodermis. The endodermis is the innermost cortical layer. The pericycle lies immediately inside it. The pith is the central tissue region.

Medullary rays are radially arranged parenchymatous tissues between vascular bundles in a dicot stem. In leaves, ground tissue containing chloroplasts is called mesophyll. The word radial refers to a direction along a radius from an organ's centre.

How are vascular bundles classified?

The vascular tissue system contains xylem and phloem. Together these tissues form vascular bundles. Their arrangement and the presence or absence of cambium provide two separate ways of describing bundles.

Conjoint bundles usually have phloem only on the outer side of xylem. A bundle can be both conjoint and open because those words describe different features. “Radial” does not mean that entire bundles form a ring around the pith.

The radial drawing shows alternating xylem and phloem patches. The conjoint closed drawing places phloem outside xylem. The conjoint open drawing adds a labelled cambium band between them.

See Fig. 6.2 in your NCERT textbook

How can dicot and monocot roots be recognised internally?

A transverse section, abbreviated T.S., is a cut across the long axis of an organ. Sections through mature root regions reveal the sequence of tissues from the surface towards the centre. The sunflower root illustrates the typical dicot arrangement.

How are tissues arranged in a dicot root?

1. The outermost epiblema, the root epidermis, bears unicellular root hairs projecting from many of its cells.
2. The cortex contains several layers of thin-walled parenchyma with intercellular spaces.
3. The endodermis is a single layer of barrel-shaped cells without intercellular spaces, forming the innermost cortex.
4. The pericycle lies next to the endodermis and comprises a few layers of thick-walled parenchymatous cells.
5. Inside are alternating xylem and phloem patches, with a small or inconspicuous central pith.

Endodermal walls contain Casparian strips, deposits of suberin, a waxy, water-impermeable material. These occur on radial and tangential walls. Radial walls lie along radii; tangential walls lie parallel to the circumference of the root section.

Lateral roots, roots branching from an existing root, and vascular cambium during secondary growth are initiated in the pericycle. Parenchyma between xylem and phloem is called conjunctive tissue. A cambium ring later develops between the conducting tissues.

The stele comprises all tissues inside the endodermis, including the pericycle, vascular tissues and pith. Thus, the endodermis marks the boundary outside the stele; the cortex is not included in this central region.

Which features distinguish monocot roots?

Polyarch describes the many xylem groups characteristic of a monocot root. The usual count of more than six must not be changed into an exact fixed count. Combine this feature with the large pith when comparing root sections.

Both cellular sections label root hair, epidermis, cortex, endodermis, pericycle, phloem, protoxylem, metaxylem and pith. The monocot drawing shows numerous xylem groups around a conspicuous central pith; the dicot drawing has a much smaller central region.

See Fig. 6.3 in your NCERT textbook

What is the internal structure of a young dicot stem?

A typical young dicot stem has an outer epidermis, a differentiated cortex and vascular bundles arranged in a ring. Within this arrangement, tissues differ in both cell type and position. The centre contains a conspicuous parenchymatous pith.

Which tissues occur outside the vascular bundles?

The epidermis is the outer protective layer. It has a thin cuticle and may bear trichomes and a few stomata. The cortex comprises several layers between the epidermis and pericycle and has three sub-zones.

The outer cortical sub-zone is the hypodermis, the tissue immediately beneath the epidermis. In a young dicot stem, it consists of a few collenchymatous layers that provide mechanical strength. Rounded, thin-walled parenchyma with conspicuous intercellular spaces forms the cortical layers below.

The innermost cortex is the endodermis. Its cells are rich in starch grains, so this layer is also called the starch sheath. Starch is a stored food carbohydrate. The pericycle lies inside the endodermis, above the phloem, as semi-lunar sclerenchyma patches.

What lies within the vascular region?

A large number of vascular bundles form a ring. Each bundle is conjoint and open, with endarch protoxylem. Cambium separates phloem from xylem, and the first-formed xylem faces towards the centre of the stem.

Between the bundles, a few layers of radially arranged parenchyma form the medullary rays. The pith consists of many rounded parenchymatous cells with large intercellular spaces. The ring of bundles therefore surrounds the central pith rather than occupying the entire ground tissue.

A circular section and enlarged cellular sector show the epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles, medullary rays and pith. The enlarged bundle labels phloem, cambium, metaxylem and protoxylem, with protoxylem towards the pith.

See Fig. 6.4a in your NCERT textbook

For recognition, use the ring arrangement together with open bundles and a collenchymatous hypodermis. A description based on the epidermis alone cannot establish that the specimen is a dicot stem, because an epidermal covering occurs in other organs too.

How does a monocot stem differ from a dicot stem?

A monocot stem has a large, conspicuous parenchymatous ground tissue containing numerous scattered vascular bundles. Its hypodermis is sclerenchymatous. Each vascular bundle is surrounded by a bundle sheath, a layer of cells enclosing the bundle; here the sheath is sclerenchymatous.

What features identify its vascular bundles?

Monocot stem bundles are conjoint and closed. They lack cambium, so the bundles do not form secondary conducting tissues. Peripheral bundles, near the outside, are generally smaller than those located centrally. This size difference is a general pattern, not an exceptionless rule.

Phloem parenchyma is absent in the monocot stem. Water-containing cavities occur within the vascular bundles. When identifying the organ, combine these details with the scattered arrangement and sclerenchymatous bundle sheath rather than relying on a single feature.

The circular section shows vascular bundles scattered through ground tissue. The enlarged cellular sector labels epidermis, hypodermis, vascular bundles, phloem, xylem and ground tissue. The distribution contrasts with the ring in the dicot drawing.

See Fig. 6.4b in your NCERT textbook

“Scattered” describes the positions of complete bundles throughout the ground tissue. “Conjoint” describes the relationship of xylem and phloem within each bundle. These descriptions are compatible because they refer to different levels of organisation.

Similarly, “closed” refers specifically to the absence of cambium within a bundle. It does not mean that conducting cells are sealed, nor that substances cannot move through the tissue. The definition concerns secondary tissue formation.

How is a dorsiventral dicot leaf organised?

A dorsiventral leaf has distinguishable upper and lower internal organisation. A vertical section, abbreviated V.S., passes through the thickness of its lamina, the leaf blade. The three main components are epidermis, mesophyll and the vascular system.

How do the two surfaces and mesophyll differ?

The upper epidermis is the adaxial epidermis; the lower is the abaxial epidermis. Both have a conspicuous cuticle. The abaxial epidermis generally bears more stomata than the adaxial epidermis. The upper epidermis may even lack stomata.

The mesophyll lies between the two epidermal surfaces. It consists of parenchyma containing chloroplasts and carries out photosynthesis. It is differentiated into palisade parenchyma and spongy parenchyma, which differ in cell shape and arrangement.

Palisade parenchyma occurs towards the adaxial surface. Its elongated cells stand vertically and parallel to one another. Spongy parenchyma lies below the palisade cells and extends to the lower epidermis. Its oval or rounded cells are loosely arranged, leaving numerous large spaces and air cavities.

Where are the conducting tissues?

The vascular system occurs in the veins and midrib, the main central vein. Venation means the arrangement of veins in a leaf. In the reticulate venation of dicot leaves, the veins form a network and vary in thickness.

The size of a vascular bundle depends on the size of its vein. Thick-walled bundle sheath cells surround the bundles. In the leaf section, xylem faces towards the upper epidermis and phloem towards the lower epidermis.

The cellular drawing labels adaxial and abaxial epidermis, palisade and spongy mesophyll, air cavity, stoma and sub-stomatal cavity, the space beneath a stoma. A bundle sheath encloses xylem above phloem in the vascular bundle.

See Fig. 6.5a in your NCERT textbook

Use the upper-to-lower sequence to organise a description: upper epidermis, palisade tissue, spongy tissue and lower epidermis. Place the vascular bundle within the mesophyll, and preserve the differences between the closely arranged palisade cells and loosely arranged spongy cells.

How is an isobilateral monocot leaf adapted to water stress?

An isobilateral leaf has a more similar organisation on its two sides. Like a dorsiventral leaf, it contains epidermis, mesophyll and vascular tissues. However, stomata occur on both epidermal surfaces, and the mesophyll is not differentiated into palisade and spongy parenchyma.

What are bulliform cells?

In grasses, certain adaxial epidermal cells along the veins become large, empty and colourless. These are bulliform cells. Their water condition is associated with whether the leaf surface remains exposed or curls inward.

A turgid cell is swollen with absorbed water; a flaccid cell has lost this firmness. Water stress means a shortage of water affecting the plant. The sequence linking water stress to leaf curling is:

1. When bulliform cells have absorbed water, they become turgid.
2. With bulliform cells turgid, the leaf surface remains exposed.
3. Under water stress, the bulliform cells become flaccid.
4. They make the leaves curl inwards, minimising water loss.

How does venation relate to bundle size?

Parallel venation is an arrangement in which veins run parallel through the leaf. In monocot leaf sections, this is reflected in the near-similar sizes of vascular bundles, except in the main veins. “Near-similar” must not be replaced with “identical”.

The drawing labels adaxial epidermis, abaxial epidermis, mesophyll, xylem, phloem, stoma and sub-stomatal cavity. The mesophyll is drawn without the separate palisade and spongy layers visible in the dicot leaf diagram.

See Fig. 6.5b in your NCERT textbook

The useful comparison is therefore structural: differentiated mesophyll in a dorsiventral leaf, undifferentiated mesophyll in an isobilateral leaf, and different stomatal distributions. Bulliform cells add a particular feature of grass leaves; they should not be described as a universal feature of every leaf.

Glossary

• Anatomy — The study of internal structure, including the arrangement of cells and tissues within organs.
• Meristem — A region of actively dividing cells that adds new cells to the plant body.
• Differentiation — The process through which cells mature and acquire structures suited to particular functions.
• Dedifferentiation — Regaining the capacity to divide by living cells that had previously become differentiated.
• Parenchyma — Simple living tissue with thin-walled cells, involved in storage and photosynthesis in green parts.
• Collenchyma — Living supporting tissue with unevenly thickened cell corners, providing strength together with flexibility.
• Sclerenchyma — Supporting tissue with thick, lignified walls, most of whose cells are dead.
• Stomatal apparatus — The stomatal aperture, its guard cells and the surrounding specialised subsidiary cells considered together.
• Vascular bundle — An association of xylem and phloem forming part of the plant's conducting tissue system.
• Endodermis — The innermost cortical layer, separating the cortex from the tissues situated further inside.
• Pericycle — Tissue immediately inside the endodermis, involved in lateral root initiation in roots.
• Casparian strips — Deposits of water-impermeable suberin on the radial and tangential walls of root endodermal cells.
• Stele — All tissues inside the endodermis, including pericycle, vascular tissues and the central pith.
• Mesophyll — Chloroplast-containing ground tissue between the epidermal surfaces of a leaf, carrying out photosynthesis.
• Bulliform cells — Large, empty, colourless adaxial epidermal cells in grasses, associated with leaf curling during water stress.

Common errors and misconceptions

• Misconception: All permanent tissues consist of dead cells. Correct: Parenchyma and collenchyma are living tissues, and phloem is mostly living. Permanent tissue describes specialised cells, rather than a universal absence of living contents.
• Misconception: Primary and apical mean the same thing. Correct: Primary classifies a meristem by origin, whereas apical classifies it by location at a root or shoot tip.
• Misconception: A closed bundle cannot transport substances. Correct: Closed means that cambium is absent and the bundle cannot form secondary tissues; its xylem and phloem remain conducting tissues.
• Misconception: All epidermal cells contain chloroplasts. Correct: Guard cells possess chloroplasts, but this feature must not be extended to every epidermal cell.
• Misconception: A monocot root and stem both have scattered vascular bundles. Correct: The root has a radial arrangement, whereas the stem has scattered conjoint bundles within its ground tissue.
• Misconception: Every dicot root has exactly four xylem groups. Correct: Dicot roots usually have two to four xylem and phloem patches; the description is a usual range, not a fixed count.
• Misconception: The upper surface of a dicot leaf never bears stomata. Correct: The lower surface generally bears more stomata; the upper epidermis may lack them.
• Misconception: All monocot leaf bundles have identical sizes. Correct: Their sizes are near-similar, except in the main veins. Bulliform cells should specifically be associated with grass leaves.

Exam-style questions with model answers

Q1. Define a simple permanent tissue and a complex permanent tissue, giving one example of each. [2 marks]

1. A simple permanent tissue contains one type of cell; parenchyma is an example.
2. A complex permanent tissue contains different cell types working together; xylem is an example.

Q2. Classify meristems by location. For each of the three types, give its position and contribution to growth. [3 marks]

1. Apical meristems occur at root and shoot tips. They principally contribute to elongation along the plant axis during primary growth.
2. Intercalary meristems occur at the base of an internode or just above a node, as in grasses, helping growth resume after cutting.
3. Lateral meristems occur along the sides of organs. Vascular cambium and cork cambium contribute to increasing the girth of the organs in which they are active.

Q3. Name the four components of xylem and give the function of each component. [4 marks]

1. Tracheids are elongated conducting cells that carry water and minerals; their thick walls also contribute mechanical support to the plant.
2. Vessels form tubular pathways from successive vessel elements, enabling conduction of water and minerals through the plant body.
3. Xylem parenchyma consists of living cells that store food and assist the lateral conduction of water.
4. Xylem fibres are thick-walled supporting cells whose main contribution is mechanical strength, rather than forming the principal water-conducting pathway.

Q4. A stem section shows scattered conjoint bundles, a sclerenchymatous sheath around each bundle, no cambium within the bundles, and absence of phloem parenchyma. Identify the stem and explain three of these observations supporting the identification. [4 marks]

1. The specimen is a monocot stem, identified from the combination of the anatomical features given in the question.
2. The vascular bundles are scattered, matching their distribution through the ground tissue in a monocot stem.
3. Each bundle has a sclerenchymatous sheath, a characteristic supporting covering of monocot stem bundles.
4. The absence of cambium means that the bundles are closed, another feature of the monocot stem arrangement.

Q5. Describe a typical dicot root from the outside inwards under these five points: epiblema, cortex, endodermis, pericycle, and conducting tissues with pith. [5 marks]

1. The epiblema is the outermost covering. Many of its cells project as unicellular root hairs, which help absorb water and minerals.
2. The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces between them.
3. The endodermis is a single layer of barrel-shaped cells without intercellular spaces. Its radial and tangential walls bear water-impermeable suberin deposits called Casparian strips.
4. The pericycle lies immediately inside the endodermis and contains a few layers of thick-walled parenchymatous cells. Lateral roots and vascular cambium are initiated here.
5. Xylem and phloem alternate radially, with usually two to four patches of each. Conjunctive tissue occurs between them, and the central pith is small or inconspicuous.

Q6. Compare a young dicot stem with a monocot stem for hypodermis, bundle distribution, cambium, tissues surrounding bundles, and ground tissue organisation. [5 marks]

1. The young dicot stem has a collenchymatous hypodermis, whereas the monocot stem has a sclerenchymatous hypodermis.
2. Dicot stem vascular bundles are arranged in a ring around the central pith, whereas numerous monocot stem bundles are scattered through the ground tissue.
3. Dicot bundles have cambium between xylem and phloem and are open. Monocot bundles lack cambium and are closed.
4. Dicot stems have medullary rays between bundles and pericycle patches above phloem. Each monocot stem bundle is enclosed by a sclerenchymatous sheath.
5. The dicot ground tissue is organised into distinct cortical, medullary-ray and pith regions. The monocot stem contains conspicuous parenchymatous ground tissue through which its bundles are scattered.

Q7. Describe a dorsiventral leaf under six points: epidermal surfaces, stomatal distribution, palisade tissue, spongy tissue, vascular bundle location and size, and bundle sheath with xylem-phloem orientation. [6 marks]

1. The upper adaxial epidermis and lower abaxial epidermis enclose the leaf tissues, and both surfaces have a conspicuous cuticle.
2. The abaxial epidermis generally bears more stomata than the adaxial epidermis, which may even lack stomata.
3. Palisade parenchyma occurs towards the upper surface. Its elongated cells stand vertically and parallel to one another.
4. Spongy parenchyma lies below the palisade tissue and extends to the lower epidermis. Its loosely arranged rounded or oval cells leave large spaces and air cavities.
5. Vascular bundles occur in veins and the midrib. Their sizes depend on vein sizes, which vary in the reticulate venation.
6. Thick-walled bundle sheath cells surround the bundles. Xylem lies towards the upper epidermis, while phloem lies towards the lower epidermis.

Q8. In a grass leaf, water stress makes the bulliform cells flaccid. State the resulting change in leaf shape and explain its significance. [2 marks]

1. Flaccid bulliform cells cause the grass leaf to curl inwards under the stated water-stress condition.
2. This inward curling minimises water loss from the leaf.

Key takeaways

• Meristems add cells through division; differentiation produces specialised permanent tissues, which may contain living cells.
• Classify meristems separately by location and origin: these criteria describe different aspects of a growth region.
• Parenchyma, collenchyma and sclerenchyma differ in wall structure, living condition, location and contribution to storage or support.
• Xylem conducts water and minerals; phloem transports food. Both combine several cell types within a complex tissue.
• Radial and conjoint describe conducting-tissue arrangement; open and closed describe the presence or absence of cambium.
• Dicot roots usually have fewer xylem groups and small pith; monocot roots usually have numerous groups and large pith.
• Young dicot stems have open bundles in a ring; monocot stems have scattered closed bundles with sclerenchymatous sheaths.
• Dorsiventral leaves have palisade and spongy mesophyll; isobilateral leaves lack this division, while grass bulliform cells help limit water loss.

Test yourself

What does dedifferentiation mean?

Living differentiated cells regain the capacity to divide under certain conditions, as in the formation of interfascicular cambium or cork cambium.

Why is collenchyma suited to a young supporting organ?

Its living cells have unevenly thickened corners, providing support and flexibility so that plant parts can bend without breaking.

What forms the stomatal apparatus?

The stomatal aperture, the two guard cells and surrounding subsidiary cells together form the stomatal apparatus.

How does an open bundle differ from a closed bundle?

An open bundle contains cambium between xylem and phloem and can form secondary tissues. A closed bundle lacks cambium.

Where does the stele begin in relation to the endodermis?

The stele includes all tissues inside the endodermis, including the pericycle, vascular tissues and pith.

Which layer is called the starch sheath in a dicot stem?

The endodermis is called the starch sheath because its cells are rich in starch grains.

What is the difference between endarch and exarch xylem?

Endarch xylem has protoxylem towards the centre; exarch xylem has protoxylem towards the outside of the organ.

How do bulliform cells respond to water stress?

They become flaccid and make grass leaves curl inwards, helping minimise water loss.