Anatomy of Flowering Plants Class 11 Notes

Meristematic tissue consists of cells that divide continuously and are the basis of growth. Cells are small, thin-walled, with dense cytoplasm and large nuclei; they lack vacuoles. Meristems are classified by position.

• Apical meristems: at root and shoot tips — responsible for primary (length) growth.
• Intercalary meristems: at the base of internodes (e.g. grass) — also contribute to primary growth.
• Lateral meristems: on the sides of stem and root — vascular cambium and cork cambium — responsible for secondary (girth) growth.

Every plant organ has three tissue systems arranged concentrically: epidermal tissue system (outermost — epidermis with cuticle and stomata), ground tissue system (all tissue between epidermis and vascular bundles — cortex, endodermis, pericycle, pith) and vascular tissue system (xylem and phloem).

Dicot Root (cross-section)

• Epiblema (epidermis) with root hairs.
• Cortex — multilayered parenchyma.
• Endodermis — innermost cortical layer with Casparian strips (suberin deposits forcing water through the symplast, controlling entry into the stele).
• Pericycle — gives rise to lateral roots and vascular cambium in secondary growth.
• Vascular bundles: 2–6 (diarch to hexarch), arranged radially. Xylem is exarch (protoxylem towards periphery, metaxylem towards centre) — a key diagnostic feature.
• Pith is small or absent.

Monocot Root (cross-section)

• Similar ground plan to dicot root but many vascular bundles (polyarch — more than 6).
• Large, well-developed pith at the centre.
• Metaxylem towards centre, protoxylem towards periphery (exarch), same as dicot.

Dicot Stem

• Epidermis with trichomes and cuticle.
• Hypodermis of collenchyma (mechanical support).
• Cortex: general parenchyma, inner with starch sheath.
• Endodermis with Casparian strips.
• Pericycle (semilunar bundles of sclerenchyma in sunflower).
• Vascular bundles — conjoint, collateral, open (vascular cambium present between xylem and phloem — allows secondary growth), arranged in a ring.
• Pith is well-developed.

Monocot Stem

• Hypodermis is sclerenchymatous.
• Vascular bundles — scattered throughout the ground tissue (no ring arrangement).
• Bundles are conjoint, collateral, closed (no vascular cambium — no secondary growth).
• Each bundle has a sclerenchymatous bundle sheath.
• Protoxylem with a water cavity (protoxylem lacuna or cavity).
• Ground tissue is not differentiated into cortex and pith.

A leaf has three main parts in cross-section: upper epidermis (with thick cuticle), mesophyll (ground tissue performing photosynthesis) and lower epidermis (with more stomata). Vascular bundles (veins) run through the mesophyll.

• Dorsiventral leaf (dicots): upper epidermis on dorsal side, lower on ventral. Mesophyll differentiated into palisade (closely packed, columnar, many chloroplasts — upper) and spongy (loosely arranged, large air spaces — lower). Stomata mainly on lower epidermis.
• Isobilateral leaf (monocots): both surfaces are similar. Mesophyll is undifferentiated — all parenchymatous. Stomata are equally distributed on both sides (amphistomatic).
• Guard cells in dicots are bean-shaped; in monocots (grasses) they are dumbbell-shaped.

Secondary growth increases the girth (thickness) of the stem and root. It is absent in monocots (no cambium). It occurs by two lateral meristems: vascular cambium and cork cambium (phellogen).

Vascular Cambium

• In dicot stems, the fascicular cambium (between xylem and phloem in open bundles) and interfascicular cambium (from medullary rays) join to form a complete ring.
• The ring produces secondary xylem inward and secondary phloem outward — secondary xylem accumulates far more, so trunk girth increases.
• Annual rings: in temperate regions, spring wood (large vessels, light) and autumn/fall wood (small vessels, dense) form distinct rings. Each ring ≈ one year.

Cork Cambium (Phellogen) and Bark

• Phellogen (cork cambium) forms from cortical cells. It produces cork (phellem) outward and secondary cortex (phelloderm) inward — together called periderm.
• Bark = all tissues outside the vascular cambium — secondary phloem + periderm. Older bark peels off as the trunk grows.
• Heartwood (centrally, older, dark, non-functional, filled with tannins and resins) vs sapwood (outer, lighter, functional xylem — conducts water).

Example: Why is the vascular bundle in a monocot stem described as 'closed' while that in a dicot stem is 'open'?

Solution: An 'open' vascular bundle contains a strip of vascular cambium between the xylem and phloem — this fascicular cambium can produce secondary xylem and phloem, allowing secondary growth. A 'closed' bundle lacks this cambium, so no secondary growth is possible. Monocot stems have closed bundles and therefore never increase in girth.

Example: How does the Casparian strip in the endodermis affect the pathway of water from soil to xylem?

Solution: Water enters root hairs and moves through the cortex via apoplast (cell walls) and symplast (cytoplasm). At the endodermis, the Casparian strip (suberin) blocks the apoplastic route, forcing all water to cross the plasma membrane and enter the symplast. This allows selective uptake of minerals and prevents uncontrolled flow of solutes into the stele — the endodermis acts as a checkpoint.

Exercises (NCERT Q1–Q7)

• 1) Draw illustrations to bring out the anatomical difference between (a) Monocot root and Dicot root (b) Monocot stem and Dicot stem
• 2) Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
• 3) The transverse section of a plant material shows the following anatomical features - (a) the vascular bundles are conjoint, scattered and surrounded by a sclerenchymatous bundle sheaths. (b) phloem parenchyma is absent. What will you identify it as?
• 4) What is stomatal apparatus? Explain the structure of stomata with a labelled diagram.
• 5) Name the three basic tissue systems in the flowering plants. Give the tissue names under each system.
• 6) How is the study of plant anatomy useful to us?
• 7) Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.

Answers at a glance

1. 1
   Q1 → (a) The dicot root, in transverse section, shows the epidermis, a wide parenchymatous cortex, the endodermis with Casparian strips, a few layers of thick-walled parenchymatous cells forming the pericycle, and radial vascular bundles, usually two to four, arranged in a star so that the pith is small or entirely absent, with a cambium present between the xylem and the phloem and the xylem exarch. The monocot root shows the same outer layers, epidermis, cortex, endodermis and pericycle, but with eight or more xylem bundles, that is polyarch, radiating from a large, well-defined pith at the centre, and with no cambium. So the two are told apart by the number of xylem bundles, two to four against more than six, by the size of the pith, absent against large, and by the presence or absence of the cambium. (b) The dicot stem shows the epidermis, a collenchymatous hypodermis forming the general cortex, the endodermis or starch sheath, a pericycle in the form of semi-lunar patches of sclerenchyma, conjoint and open vascular bundles arranged in a ring near the centre with a cambium between the xylem and the phloem, medullary rays radiating from the pith that occupies the central portion of the stem. The monocot stem shows the epidermis, a sclerenchymatous hypodermis forming the ground tissue, conjoint and closed vascular bundles that are scattered, with the peripheral bundles generally smaller than the centrally placed ones, each bundle surrounded by a sclerenchymatous bundle sheath and containing water cavities, with a large and conspicuous parenchymatous ground tissue and no cambium at all. So the contrasts to mark on the stem drawing are the arrangement of the bundles, a ring against scattered, and the cambium, present against absent, so open against closed bundles, together with the layered ground tissue of the dicot and the sclerenchymatous bundle sheaths of the monocot.
2. 2
   Q2 → Start from a transverse section of the young stem, stain it with safranin and fast green, and examine it first under low power, since the arrangement of the vascular bundles is visible at that magnification. The first character is the arrangement of the bundles: if the vascular bundles form a ring near the centre, all at the same distance from the centre, the stem is a dicot, whereas if the bundles are scattered through the ground tissue, with those at the periphery smaller and those towards the centre larger, it is a monocot. The second character is the cambium, which must then be looked for at high power: a strip of actively dividing cells between the xylem and the phloem of each bundle, giving a conjoint open bundle, indicates a dicot, and its absence, giving a conjoint closed bundle, indicates a monocot. The third character is the ground tissue: in a dicot it is differentiated into a hypodermis, a cortex and an endodermis around the ring of bundles, whereas in a monocot it is not layered in this way but is large and conspicuous. The fourth is the hypodermis, which is collenchymatous in a dicot and sclerenchymatous in a monocot. The fifth is the bundle sheath, since in a monocot each vascular bundle is completely enclosed in a sclerenchymatous sheath and has water-containing cavities within it, and the phloem parenchyma is absent. Thus a ring of bundles, a cambium, a collenchymatous hypodermis and no bundle sheaths identify a dicot stem, while scattered bundles, no cambium, a sclerenchymatous hypodermis and sclerenchymatous bundle sheaths with water cavities identify a monocot stem.
3. 3
   Q3 → The material is a monocot stem. Each part of the observation points to it. Conjoint means that the xylem and the phloem lie on the same radius in the bundle, side by side, and such bundles are common in stems and in leaves; a root is excluded, because in a root the xylem and the phloem lie on separate radii. Scattered means the bundles are not arranged in a ring, and this is the monocot condition, since a dicot stem has its vascular bundles arranged in a ring near the centre. The sclerenchymatous bundle sheath surrounding each bundle confirms this, for such a sheath is characteristic of the monocot stem, whereas a dicot stem has no sheath around each bundle and carries the pericycle as semi-lunar patches of sclerenchyma above the phloem. The absence of phloem parenchyma is a further confirmation and a refinement, since the phloem of a monocot stem is reduced to the sieve tubes and their companion cells, with no phloem parenchyma, whereas the phloem of a dicot stem contains phloem parenchyma; the same description also records water-containing cavities within the vascular bundles, a further monocot character. Thus the conjoint, scattered bundles with sclerenchymatous bundle sheaths and the absence of phloem parenchyma together identify the material as a monocot stem, and the further features to be expected in confirmation are a large well-defined pith, the absence of any cambium and a sclerenchymatous hypodermis.
4. 4
   Q4 → A stoma is a minute pore or opening in the epidermis of leaves and of green stems, through which gaseous exchange and transpiration occur, and the whole structure consisting of the stomatal aperture together with the two surrounding guard cells and the subsidiary cells around them is called the stomatal apparatus. In the surface view the stoma appears as a narrow slit or pore bounded by two guard cells lying side by side with their inner walls facing each other; the guard cells are kidney-shaped or dumb-bell-shaped and are the only cells of the epidermis that contain chloroplasts, and around them lie a few specialised epidermal cells called subsidiary cells, which form part of the apparatus and are shown in the diagram. In the transverse section the two guard cells project inwards, so that a chamber or pit is formed, opening outside through the stomatal pore and inside into the substomatal chamber, the intercellular space lying just below the stoma; the diagram should be labelled for the raised guard cells with their chloroplasts, the stomatal pore or slit, the substomatal chamber, the surrounding epidermal cells and the subsidiary cells. The structural basis of the working of the stoma is the differential thickness of the wall of a guard cell, the inner wall facing the pore being thick and the outer wall thin; when the guard cells lose water they become flaccid and the pore is closed, and when they take up water they become turgid, the thin outer walls expand and the thick inner walls are pulled apart, so the pore opens. The guard cells themselves possess chloroplasts, and it is they who regulate the opening and closing of the stomata.
5. 5
   Q5 → The flowering plant has three basic tissue systems. The epidermal or dermal system forms the outermost covering of the whole plant body and comprises the epidermal cells, which make up the epidermis itself, the stomata, which are the minute pores regulating gaseous exchange, and the epidermal appendages, the trichomes and hairs, which in the shoot system are usually multicellular and help in preventing water loss. The ground tissue system consists of all the tissues except the epidermis and the vascular bundles, and it is the largest of the three; it is made of the simple tissues, parenchyma, collenchyma and sclerenchyma, with parenchymatous cells in the cortex, pericycle, pith and medullary rays of the primary stems and roots, and with the thin-walled chloroplast-containing mesophyll in the leaves, and in a transverse section it is divided into the three zones of cortex, pericycle and pith. The vascular or conductive system is the innermost system and is the transport system of the plant, and it consists of two complex tissues: the xylem, composed of tracheids, vessels, xylem parenchyma and xylem fibres, of which the tracheids and the vessels are the conducting elements, and the phloem, composed of sieve tube elements, companion cells, phloem parenchyma and phloem fibres, of which the sieve tube elements and the companion cells are the conducting elements. Thus the three systems are the epidermal or dermal with the epidermal cells, the stomata and the trichomes, the ground with parenchyma, collenchyma and sclerenchyma, and the vascular with the xylem and the phloem.
6. 6
   Q6 → The study of plant anatomy is useful in several practical ways. The study of the root shows how the plant absorbs water and minerals and how it is anchored, and it is of direct use in agriculture, since knowing the extent and depth of the root system decides the depth of sowing and the spacing of crops, and the root also shows the first appearance of the vascular tissue. The study of the stem explains how water and food are conducted through the plant, and it is the basis of the wood technology industry, since the structure of the wood, and in particular whether the plant is a dicot or a gymnosperm and how the secondary growth occurred, decides its grain, its strength and its suitability for construction, furniture and paper, while the growth rings make the study of wood the basis of dating. The study of the leaf explains photosynthesis and transpiration, and the arrangement of the mesophyll, the stomata and the vascular bundles in the leaf is the clearest illustration of the three tissue systems in the plant body. The study of the flower and of the seed is the basis of taxonomy and plant breeding, since the characters of the calyx, corolla, androecium and gynoecium are constant within a group and so classify it, and the structure of the ovule, the embryo sac and the seed underlies all hybridisation and crop improvement. Above all, anatomical characters are among the least variable in a species, so they provide the evidence on which classification and evolutionary relationships, or phylogeny, are established, and they allow a plant to be identified even when its flowers and leaves are missing.
7. 7
   Q7 → A dorsiventral, or dicot, leaf has an internal structure that differs on its upper and lower surfaces, and the layers from the top surface downwards are as follows. The upper or adaxial epidermis is a single layer of compactly arranged cells covered by a conspicuous cuticle, it may even lack stomata, and in surface view the epidermal cells are polygonal and the guard cells kidney-shaped. Below it lies the adaxially placed palisade parenchyma, made up of elongated cells arranged vertically and parallel to one another, tightly packed with very little intercellular space and lying immediately beneath the upper epidermis, and this is the chief food-manufacturing tissue and the main site of photosynthesis. Below that lies the spongy parenchyma, several layers of rounded, loosely arranged, thin-walled cells with large intercellular spaces and fewer chloroplasts, the spaces being continuous with the stomatal pore, and this is the main site of respiration and the route of gaseous exchange. The vascular bundles, which are the veins, occur in the leaf as a larger midrib and smaller lateral veins, each surrounded by a layer of thick-walled bundle sheath cells, and within each bundle the xylem lies towards the upper surface and the phloem towards the lower one. Finally the lower or abaxial epidermis is a single layer of loosely arranged flat cells covered by a thinner cuticle than the upper surface, and it bears most of the stomata, which open into the substomatal chambers. Thus the two surfaces differ, the upper being cutinised and backed by palisade tissue with few or no stomata, and the lower bearing most of the stomata together with air spaces and spongy tissue, and this difference of the two surfaces is what makes the leaf dorsiventral, in contrast to the isobilateral leaf of a monocot which has the same structure on both surfaces together with bulliform cells.

MCQ Questions

1 Mark Questions

1. 1
   Define a tissue. (Ans: group of cells performing a common function)
2. 2
   What is the function of heartwood? (Ans: provides mechanical support)
3. 3
   Name the two types of complex tissues in plants. (Ans: xylem and phloem)
4. 4
   Write the function of sclerenchyma. (Ans: mechanical support and protection)

2 Mark Questions

1. 1
   Differentiate between simple and complex tissues with one example each.
2. 2
   What is phellogen? (Ans: cork cambium — the lateral meristem that produces bark)
3. 3
   Mention two functions of cork in woody stems.

3 Mark Questions

1. 1
   Describe the primary structure of a monocot root. (Ans: epiblema/hairs, cortex, endodermis with casparian thickenings, pericycle, polyarch xylem and phloem bundles, pith)
2. 2
   Explain the process of secondary growth in a dicot stem. (Ans: cambium ring activity produces secondary xylem (wood) inward and secondary phloem outward, forming bark and annual rings)
3. 3
   Differentiate between cork and bark. (Ans: cork is phellem produced by phellogen; bark is all tissue outside vascular cambium)

5 Mark Questions

1. 1
   Draw a labelled diagram of a transverse section of a dicot stem and describe the arrangement of epidermis, cortex, vascular bundles and pith. (Ans: hypodermis collenchyma, endodermis, pericycle, conjoint-colateral open bundles, interfascicular cambium)
2. 2
   Explain the formation of annual rings and their significance in determining the age of a tree. (Ans: one ring per year from spring/autumn wood difference; used for age/dendrochronology)