Class 12 Biology Plan Anatomy Notes and Important Questions

Class 12 Plan Anatomy Notes (Biology)

Class 12 Biology | Unit 1: Plant Anatomy Plant Anatomy Class 12 Biology • Unit 1 (Botany) Plant anatomy is the branch of botany that studies the internal structure of different plant parts (root, stem, leaf) by cutting thin sections and observing them under a microscope.

Key points

• Class 12 Biology | Unit 1: Plant Anatomy
• Stinging hairs - e.g. Urtica (sisno).
• Digestive glands - in insectivorous plants (Drosera, Nepenthes).
• Hydathodes - water glands at leaf margins; help in guttation. Internal glands (embedded in other tissues):
• Oil glands - in pericarp (peel) of citrus fruits.
• Resin ducts - resins and gums in gymnosperms (e.g. pine). Anatomy of Root, Stem & Leaf Exam tip: Diagrams of T.S. of monocot/dicot root and stem and V.S. of leaf, and their differences, are very frequently asked (4 marks). Always label neatly. Internal Structure of Dicot Root (e.g. gram, bean, sunflower) - T.S. shows the following from outside to inside: 1) Epiblema (piliferous layer) - single outer layer of thin-walled cells; no cuticle and no stomata; bears unicellular root hairs for absorption of water. 2) Cortex - many layers of thin-walled parenchyma with intercellular spaces; stores food and passes water inward. 3) Endodermis - innermost layer of cortex; barrel-shaped cells with Casparian strips (suberin) on radial and transverse walls, forming a water-tight jacket. Thin-walled passage cells lie opposite the protoxylem. 4) Pericycle - single layer inside endodermis; gives rise to lateral roots, and during secondary growth to part of vascular cambium and cork cambium. 5) Vascular bundles - radial (xylem and phloem on different radii, alternating); xylem is exarch; usually 2-4 (diarch to tetrarch) bundles. 6) Pith - small or absent.

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Class 12 Biology | Unit 1: Plant Anatomy Plant Anatomy Class 12 Biology • Unit 1 (Botany) Plant anatomy is the branch of botany that studies the internal structure of different plant parts (root, stem, leaf) by cutting thin sections and observing them under a microscope. Types of sections – T.S. (Transverse section) cut at right angle to the long axis. Used for root and stem. – L.S. (Longitudinal section) cut along the length of the organ. – V.S. (Vertical section) used for the leaf, because a leaf is flat; the cut is taken vertically through the lamina. Tissue A tissue is a group of cells that are similar in origin, structure and function and work together to do a particular job. Plant tissues are of two main types: i) Meristematic tissue cells that keep on dividing continuously. ii) Permanent tissue cells that have lost the power of division, either temporarily or permanently. Flow chart: Plant tissue Meristematic (dividing) + Permanent (non-dividing). Permanent tissue Simple, Complex and Secretory. 1. Meristematic Tissue Meristematic tissues are young and immature tissues whose cells are either dividing or are capable of dividing. The word comes from Greek "meristos" = divisible. Characteristics – Cells are young, living and actively dividing. – Cells are small and usually isodiametric (round, oval or polygonal). – Cell wall is thin, elastic and made of cellulose; secondary wall is absent. – Cells are compactly arranged with no (or very few) intercellular spaces.

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Class 12 Biology | Unit 1: Plant Anatomy – Cytoplasm is dense with a large, prominent nucleus. – Vacuoles are absent or very small. – Plastids are present only as proplastids; no stored food or waste material. – Rate of metabolism is very high (active state of protoplasm). Classification of meristematic tissue Meristems are classified on four bases: 1) On the basis of origin / development 2) On the basis of position 3) On the basis of function 4) On the basis of plane of division Exam tip: "Classification of meristematic tissue" is a common long question. Learn all four bases with examples. 1) On the basis of origin a) Promeristem (primordial or embryonic meristem) – The earliest and youngest group of meristematic cells. – Occupies a very small area at the tip of stem and root (growing regions). – All other advanced meristems are derived from it. b) Primary meristem – Derived from the promeristem; lies just below it. – Found at tips of root, stem and appendages; may be apical or intercalary. – Its cells divide and mature to form primary permanent tissues. – Responsible for growth in length (height) and formation of root and shoot. – Fascicular cambium of dicot stem is also a primary meristem. c) Secondary meristem – Develops when some primary permanent tissue regains the power of division (dedifferentiation). – Always lateral in position. – Examples: interfascicular cambium, cork cambium (phellogen), cambium of dicot root. – Forms secondary tissues (secondary xylem, secondary phloem, cork) increases the diameter (girth) of root and stem; also helps in healing wounds.

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Class 12 Biology | Unit 1: Plant Anatomy 2) On the basis of position a) Apical meristem – Present at the apices (tips) or growing points of main and lateral shoots and roots. – Includes both promeristem and primary meristem. – Increases the length of the plant and gives rise to primary permanent tissue. – In cryptogams the promeristem has a single apical cell; in phanerogams the apex has a group of cells (tunica and corpus). b) Intercalary meristem – Lies in between regions of permanent tissue. – It is a part of the apical meristem that got separated during growth of the organ. – Found at base of leaf (pine), base of internodes (grasses, bamboo) and base of node (Mentha). – Short-lived; forms primary permanent tissue. Helps grasses regrow after grazing or mowing. c) Lateral meristem – Lies along the lateral sides of roots and stems, parallel to the long axis. – Responsible for increase in thickness of the plant part. – Examples: vascular cambium and cork cambium. – Gives rise to secondary permanent tissue. Apical meristem (tip) length Intercalary meristem (base of internode) Intercalary meristem Lateral meristem (sides) thickness Fig: Position of meristems in a plant

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Class 12 Biology | Unit 1: Plant Anatomy 3) On the basis of function (Proposed by Haberlandt) - the apical meristem shows three functional zones: a) Protoderm – The outermost layer of the meristem. – Divides mainly anticlinally and forms the epidermis (epiblema in root). b) Procambium – Meristem that later forms vascular tissue i.e. primary xylem and primary phloem (and cambium). c) Ground meristem – Forms the major part of the apical meristem; made of large, thin-walled cells. – Differentiates into the ground tissue system: hypodermis, cortex, endodermis, pericycle, pith and medullary rays. 4) On the basis of plane of division a) Mass meristem – Divides in all planes and forms a mass of cells. e.g. endosperm, early embryo. b) Rib (file) meristem – Divides in a single plane and forms a row (file) of cells. e.g. cortex, pith. c) Plate meristem – Divides in two planes and forms a flat plate of cells. e.g. epidermis, epiblema, leaf lamina. Mass meristem (all planes) Rib meristem (one plane) Plate meristem (two planes) Fig: Mass, rib and plate meristem Shoot Apex and Root Apex Shoot apex – The growing region at the tip of the stem. – A dome-shaped mass of actively dividing cells. – Covered and protected by leaf primordia (young leaves).

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Class 12 Biology | Unit 1: Plant Anatomy – Gives rise to leaves, axillary buds, floral buds and brings about the growth of the stem. Two theories explain the organization of the shoot apex: a) Histogen theory - Hanstein (1868) The apex is made of three distinct zones called histogens: i) Dermatogen - single outermost layer forms the epidermis of stem. ii) Periblem - middle region, inner to dermatogen forms the cortex (including endodermis). iii) Plerome - central region, inner to periblem forms the stele: pericycle, vascular bundles, medullary rays and pith. Easy link: Dermatogen ≈ protoderm, periblem ≈ ground meristem, plerome ≈ procambium + pith. b) Tunica-Corpus theory - A. Schmidt (1924) The shoot apex, protected by leaf primordia, has two zones: an outer mantle-like tunica and an inner cell mass called corpus. i) Tunica – Outer zone of one or more peripheral layers of cells. – Cells are smaller than those of corpus. – Divides anticlinally (perpendicular to surface) increases surface area. – The outermost layer forms the epidermis; other layers help form leaf primordia and hypodermal cells. ii) Corpus – Central, undifferentiated, multilayered mass of cells enclosed by tunica. – Divides in all directions increases volume. – Forms cortex, vascular tissue and pith.

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Class 12 Biology | Unit 1: Plant Anatomy Tunica (outer 1-2 layers) Corpus (inner mass) Leaf primordium Fig: L.S. of shoot apex (tunica-corpus) Root apex – Found at the tip of main root and its branches; it is sub-terminal, i.e. covered externally by the root cap. – Its regions (histogens) are: i) Protoderm / dermatogen - outermost region; forms the epiblema. In dicots it also forms the root cap (dermocalyptrogen). ii) Calyptrogen - a separate layer found only in monocots; forms the root cap. iii) Procambium (plerome) - central region; forms the vascular bundles. iv) Ground meristem (periblem) - forms the cortex and endodermis. v) Quiescent centre - a central zone where cells divide very slowly. It acts as a reserve of cells to replace damaged meristem cells. Protoderm epiblema Ground meristem cortex Procambium vascular tissue Quiescent centre Root cap (calyptrogen) Fig: L.S. of root apex Difference: Tunica vs Corpus Tunica Corpus Outer zone of cells. Inner mass of cells. One to several layers. Many layers.

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Class 12 Biology | Unit 1: Plant Anatomy Tunica Corpus Cells are relatively smaller. Cells are relatively bigger. Gives rise to epidermis (protoderm). Gives rise to cortex, vascular bundles, medullary rays and pith. Divides anticlinally increase in surface Divides in all planes increase in volume. area. Difference: Shoot apex vs Root apex Shoot apex Root apex Terminal (apical); protected by young leaves. Sub-terminal; protected by root cap. Bears lateral appendages as leaf primordia. Does not bear lateral appendages. Quiescent centre is absent. Quiescent centre is present. Distinct nodes and internodes are present. Nodes and internodes are absent. Cells may be green and photosynthetic. Cells are non-green, non-photosynthetic. 2. Permanent Tissue Permanent tissues are made of cells that have lost the power of division (completely or temporarily) after differentiation and maturation, and have attained a definite shape, size and function. Characteristics – Cells may be living or dead. – Cell walls may be thin or thick. – Intercellular spaces are usually present. – Cells have large vacuoles. – Cells may be loosely or compactly arranged. Types (on the basis of nature of cells) A) Simple permanent tissue - made of only one type of cells. e.g. parenchyma, collenchyma, sclerenchyma. B) Complex permanent tissue - made of more than one type of cells working as a unit. e.g. xylem, phloem. C) Secretory (special) tissue - secretes substances. e.g. laticiferous and glandular tissue.

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Class 12 Biology | Unit 1: Plant Anatomy A) Simple Permanent Tissue 1) Parenchyma – Living cells with thin cellulose walls; no lignin. – More or less isodiametric; cells may be oval, round, rectangular, polygonal or irregular. – Intercellular spaces are present; large central vacuole. – Most common and least specialized tissue; forms cortex, pith, mesophyll and fleshy parts. Types of parenchyma i) Chlorenchyma - parenchyma with chloroplasts; found in mesophyll of leaves and cortex of young stems. Function: photosynthesis. ii) Aerenchyma - parenchyma with large air spaces; found in aquatic plants like Hydrilla, Nymphaea. Function: buoyancy (floating) and storage of gases. iii) Prosenchyma - elongated, thick-walled, narrow cells with tapering ends; found in pericycle. Function: mechanical support. iv) Storage parenchyma - stores food (starch), e.g. potato tuber. v) Epidermal, xylem and phloem parenchyma - found in epidermis, xylem and phloem respectively. Functions – Storage of food, water and waste; photosynthesis (chlorenchyma); buoyancy (aerenchyma); gives turgidity; helps in wound healing as it can become meristematic. 2) Collenchyma – Living tissue made of somewhat elongated cells. – Walls have extra thickening of cellulose, hemicellulose and pectin, mostly at corners; no lignin. – Cell wall has simple pits; each cell has vacuolated cytoplasm with a nucleus; may have chloroplasts. – Found in hypodermis of dicot stem, petiole, pedicel and midrib of dicot leaves. – Absent in monocots and in roots. Types of collenchyma (on the basis of thickening) i) Angular - thickening at the angles/corners where cells meet. e.g. stem of Datura,

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Class 12 Biology | Unit 1: Plant Anatomy tomato. ii) Lamellar (plate) - thickening on the tangential walls. e.g. hypodermis of sunflower stem. iii) Lacunar (lacunate) - thickening on walls bordering intercellular spaces. e.g. Cucurbita, Salvia. iv) Annular - cells appear circular with uniformly thickened walls; a rare type. Angular Lamellar Lacunar Annular Fig: Types of collenchyma (shaded = thickening) Functions – Gives mechanical strength and elasticity (flexibility) to growing organs. – Prevents tearing of leaves when present at leaf margins. – Allows elongation of stem as it is living and flexible. – Performs photosynthesis if chloroplasts are present; stores food. 3) Sclerenchyma (Gk. skleros = hard) – Made of dead cells with little or no protoplasm. – Walls are very thick and highly lignified, with a narrow cell cavity (lumen) and simple pits. – Compactly arranged without intercellular spaces; middle lamella is conspicuous. – Being lignified, it is the hardest and purely mechanical tissue; does not decompose easily. – Resists forces of pulling and bending and protects soft inner parts. – Found in hypodermis of monocot stem, xerophytes, xylem, phloem, seed coats and nut shells. Based on shape and size, it is of two types: fibres and sclereids. a) Sclerenchyma fibres – Elongated, narrow, spindle-shaped cells with tapering ends. – Develop in groups along the long axis; ends interlock with neighbouring fibres very strong tissue.

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Class 12 Biology | Unit 1: Plant Anatomy – Usually 1-3 mm long; walls uniformly thickened, lignified, with simple oblique pits. – Commercially important: raw material for textiles, sacks, bags, twine and ropes (e.g. jute, flax, hemp). Types of fibres (on the basis of origin): i) Surface fibres - from seed coat or fruit wall. e.g. cotton (seed hairs), coir (mesocarp of coconut). ii) Wood (xylem) fibres - found in xylem. iii) Bast fibres - found in phloem/pericycle. e.g. jute, flax, hemp. b) Sclereids (stone cells) – Short, broad, highly thickened dead cells; broader than fibres with very narrow lumen. – Found in cortex and pith of gymnosperms and dicots, seed coats and stony fruits; give hardness (e.g. gritty pulp of pear, shell of walnut). Types of sclereids: 1) Brachysclereids - isodiametric stone cells. e.g. pulp of pear. 2) Macrosclereids - rod-shaped (columnar). e.g. seed coat of legumes. 3) Osteosclereids - bone-shaped with swollen ends. e.g. seed coat of pea. 4) Astrosclereids - star-shaped, branched. e.g. leaf/petiole of Nymphaea, tea leaf. 5) Filiform (trichosclereids) - long, hair-like. e.g. leaf of Olea. Fibre (long, tapering ends) Stone cell Astrosclereid Fig: Sclerenchyma fibre and sclereids Difference: Parenchyma, Collenchyma and Sclerenchyma Parenchyma Collenchyma Sclerenchyma Living cells. Living cells. Dead cells. Thin cellulose wall. Wall thickened at corners Wall uniformly thick and (cellulose + pectin). lignified. Intercellular spaces present. Few or no spaces. No intercellular spaces.

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Class 12 Biology | Unit 1: Plant Anatomy Parenchyma Collenchyma Sclerenchyma Isodiametric cells. Somewhat elongated. Long (fibres) or short (sclereids). Storage, photosynthesis. Mechanical strength + Mechanical strength flexibility. (rigidity). B) Complex Permanent Tissue Made of more than one type of cells working together as a unit. These are the vascular tissues: xylem and phloem. 1) Xylem Conducts water and minerals upward from root to leaves and gives mechanical support. Its elements: i) Tracheids - elongated dead cells with tapering ends and lignified walls. ii) Vessels - long tubes formed by cells joined end to end with dissolved end walls; dead. Main water-conducting element in angiosperms. iii) Xylem fibres (wood fibres) - dead; give support. iv) Xylem parenchyma - the only living element; stores food and helps in radial conduction. First formed xylem = protoxylem (narrow); later formed = metaxylem (wider). Endarch: protoxylem towards centre (stem). Exarch: protoxylem towards periphery (root). 2) Phloem Conducts food (sugars) from leaves to other parts. Its elements: i) Sieve tubes - living but without nucleus; end walls perforated as sieve plates. ii) Companion cells - living, nucleated; control sieve tube activity (absent in pteridophytes and gymnosperms). iii) Phloem parenchyma - living; stores food. iv) Phloem (bast) fibres - the only dead element; gives support.

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Class 12 Biology | Unit 1: Plant Anatomy Tracheid Vessel Sieve tube + companion cell Companion cell (nucleus) Sieve plate Fig: Elements of xylem and phloem C) Secretory Tissue – Tissue concerned with secretion of various chemicals like tannins, resins, gums, enzymes, hormones, essential oils, nectar, latex etc. – Some secretions are useful to the plant itself; many others are not used by the plant but are economically useful to humans. 1) Laticiferous tissue (laticifers) – Thin-walled, much elongated and branched structures with many nuclei. – Contain a fluid called latex - milky or coloured; it is a colloidal solution of proteins, lipids, starch, alkaloids, enzymes, rubber etc. – Found scattered in parenchyma. e.g. the sticky milky juice that oozes when a banana stem, papaya or fig is cut. Laticifers are of two types: i) Latex cells (non-articulated) - single independent cells that do not fuse; much elongated and branched. e.g. Calotropis, Nerium, Euphorbia, Ficus. ii) Latex vessels (articulated) - formed by fusion of many cells end to end whose walls dissolve, forming a network. e.g. Hevea (rubber), Papaver, papaya. Uses of latex – Storage of food; protection by sealing wounds and discouraging animals; storage of waste; commercial products like rubber. 2) Glandular tissue – Glands are special structures containing secretory or excretory materials. – They may be a single cell or a group of many cells. External glands (on epidermis): • Nectar glands (nectaries) - secrete nectar in flowers.

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• Class 12 Biology | Unit 1: Plant Anatomy
• Stinging hairs - e.g. Urtica (sisno).
• Digestive glands - in insectivorous plants (Drosera, Nepenthes).
• Hydathodes - water glands at leaf margins; help in guttation. Internal glands (embedded in other tissues):
• Oil glands - in pericarp (peel) of citrus fruits.
• Resin ducts - resins and gums in gymnosperms (e.g. pine). Anatomy of Root, Stem & Leaf Exam tip: Diagrams of T.S. of monocot/dicot root and stem and V.S. of leaf, and their differences, are very frequently asked (4 marks). Always label neatly. Internal Structure of Dicot Root (e.g. gram, bean, sunflower) - T.S. shows the following from outside to inside: 1) Epiblema (piliferous layer) - single outer layer of thin-walled cells; no cuticle and no stomata; bears unicellular root hairs for absorption of water. 2) Cortex - many layers of thin-walled parenchyma with intercellular spaces; stores food and passes water inward. 3) Endodermis - innermost layer of cortex; barrel-shaped cells with Casparian strips (suberin) on radial and transverse walls, forming a water-tight jacket. Thin-walled passage cells lie opposite the protoxylem. 4) Pericycle - single layer inside endodermis; gives rise to lateral roots, and during secondary growth to part of vascular cambium and cork cambium. 5) Vascular bundles - radial (xylem and phloem on different radii, alternating); xylem is exarch; usually 2-4 (diarch to tetrarch) bundles. 6) Pith - small or absent.

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Class 12 Biology | Unit 1: Plant Anatomy Root hair Epiblema Cortex (parenchyma) Endodermis Pericycle Xylem (exarch) Phloem (radial) Fig: T.S. of dicot root Internal Structure of Monocot Root (e.g. maize) - the basic layers are the same as dicot root, but: – Cortex is wide; endodermis has Casparian strips and passage cells. – Pericycle forms only lateral roots (no secondary growth). – Vascular bundles are radial, exarch and polyarch (more than 6). – Pith is large and well developed. Difference: Dicot root vs Monocot root Dicot root Monocot root Xylem bundles 2-4 (rarely 6): diarch to Xylem bundles many (more than 6): tetrarch. polyarch. Pith is small or absent. Pith is large and well developed. Pericycle forms lateral roots, cambium and Pericycle forms lateral roots only. cork cambium. Secondary growth occurs. Secondary growth absent. Xylem vessels angular. Xylem vessels round/oval. Internal Structure of Dicot Stem (e.g. sunflower) - T.S. shows: 1) Epidermis - single layer with cuticle; bears multicellular hairs and few stomata. 2) Hypodermis - 3-5 layers of collenchyma; gives mechanical strength. 3) Cortex - few layers of parenchyma with intercellular spaces.

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Class 12 Biology | Unit 1: Plant Anatomy 4) Endodermis - wavy innermost cortical layer rich in starch grains, so called starch sheath. 5) Pericycle - semilunar patches of sclerenchyma (bundle cap) alternating with parenchyma. 6) Vascular bundles - arranged in a ring, wedge-shaped, conjoint (xylem and phloem on same radius), collateral (phloem outside, xylem inside), open (cambium present) and endarch. 7) Medullary rays - parenchyma between bundles; help in radial conduction. 8) Pith - large, central parenchyma; stores food. Epidermal hair Epidermis + cuticle Hypodermis (collenchyma) Cortex Endodermis (starch sheath) Pericycle (bundle cap) Phloem Cambium Xylem (endarch) Medullary ray Pith Fig: T.S. of dicot stem (diagrammatic) Internal Structure of Monocot Stem (e.g. maize) - T.S. shows: 1) Epidermis - single layer with thick cuticle; no hairs. 2) Hypodermis - 2-3 layers of sclerenchyma. 3) Ground tissue - undifferentiated parenchyma; no distinct cortex, endodermis, pericycle or pith. 4) Vascular bundles - many, scattered, oval; smaller and crowded near periphery, larger towards centre; conjoint, collateral, closed (no cambium) and endarch. 5) Each bundle is surrounded by a sclerenchymatous bundle sheath; has a lysigenous water cavity near protoxylem; phloem parenchyma is absent.

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• Class 12 Biology | Unit 1: Plant Anatomy Epidermis + thick cuticle Hypodermis (sclerenchyma) Scattered vascular bundles Ground tissue (parenchyma) (small near edge, large at centre; no pith, no medullary rays) Fig: T.S. of monocot stem (diagrammatic) Difference: Dicot stem vs Monocot stem Dicot stem Monocot stem Hypodermis is collenchymatous. Hypodermis is sclerenchymatous. Ground tissue differentiated into cortex, Ground tissue undifferentiated. endodermis, pericycle, pith. Vascular bundles in a ring. Vascular bundles scattered. Bundles open (cambium present). Bundles closed (no cambium). Bundle sheath absent; wedge-shaped Sclerenchymatous bundle sheath; oval bundles. bundles. Medullary rays and pith present. Medullary rays and pith absent. Secondary growth occurs. Secondary growth absent. Internal Structure of Dicot Leaf Dicot leaf is dorsiventral (upper and lower surfaces differ). e.g. mango, sunflower. V.S. shows: 1) Upper epidermis - single layer with thick cuticle; stomata few or absent. 2) Lower epidermis - thinner cuticle with many stomata; so transpiration occurs mostly from the lower surface. 3) Mesophyll - differentiated into two layers:
• Palisade layer - below upper epidermis; elongated, columnar, compactly arranged cells rich in chloroplasts.
• Spongy layer - towards lower epidermis; irregular, loosely arranged cells with

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Class 12 Biology | Unit 1: Plant Anatomy large air spaces. 4) Vascular bundles - conjoint, collateral, closed; xylem towards upper epidermis and phloem towards lower epidermis; each surrounded by a parenchymatous bundle sheath. Collenchyma patches lie above and below the midrib. 5) Air cavity (sub-stomatal chamber) is present above each stoma. Cuticle (thick) Upper epidermis Palisade (columnar cells) Spongy (air spaces) Xylem (upper side) Phloem (lower side) Sub-stomatal cavity Stoma Lower epidermis Fig: V.S. of dicot leaf Internal Structure of Monocot Leaf Monocot leaf is isobilateral (both surfaces alike). e.g. maize, grass. V.S. shows: 1) Upper and lower epidermis - similar thickness with cuticle; stomata equal on both surfaces. 2) Bulliform (motor) cells - large, empty cells in upper epidermis; help in rolling of leaf during drought to reduce water loss. 3) Mesophyll - not differentiated into palisade and spongy; only spongy-like cells. 4) Vascular bundles - many, parallel, conjoint, collateral, closed; each surrounded by bundle sheath; sclerenchyma patches above and below larger bundles. Protoxylem has spiral and annular vessels.

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Class 12 Biology | Unit 1: Plant Anatomy Upper epidermis Bulliform (motor) cells Mesophyll (undifferentiated) Bundle sheath Xylem Phloem Sclerenchyma patch Lower epidermis Fig: V.S. of monocot leaf Difference: Dicot leaf vs Monocot leaf Dicot leaf Monocot leaf Dorsiventral. Isobilateral. Upper cuticle thicker than lower. Both surfaces have equal cuticle. More stomata on lower surface. Stomata equal on both surfaces. Mesophyll has palisade + spongy layers. Mesophyll undifferentiated (only spongy). Bulliform cells absent. Bulliform cells present. Reticulate venation; bundles of different Parallel venation; bundles mostly similar. sizes. Bundle sheath parenchymatous. Sclerenchyma patches with bundles. Secondary Growth in Dicot Stem – Primary growth - increase in length (height) due to apical meristem. – Secondary growth - increase in diameter (girth) of dicot stem and root due to formation of secondary tissues by the vascular cambium and cork cambium (lateral meristems). – Occurs in dicots and gymnosperms. It is absent in monocots, pteridophytes and leaves because their bundles are closed (no cambium). Remember: Open bundle = cambium present (secondary growth possible). Closed bundle = cambium absent.

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Class 12 Biology | Unit 1: Plant Anatomy 1) Formation of cambium ring – In a dicot stem the vascular bundles are arranged in a ring and are conjoint, collateral and open. – The cambium present between xylem and phloem inside a bundle is called fascicular (intrafascicular) cambium - it is primary. – Medullary ray cells lying in line with the fascicular cambium become meristematic and form interfascicular cambium - it is secondary. – Both fascicular and interfascicular cambium join to form a complete cambium ring. Fascicular Interfascicular (a) Cambium in bundles + forming between bundles (dashed) (b) Complete cambium ring Fig: Formation of cambium ring 2) Activity of cambium ring – The cambium ring divides and produces new cells on both sides. – Cells formed towards the inside become secondary xylem - vessels, tracheids, many wood fibres and wood parenchyma. – Cells formed towards the outside become secondary phloem - sieve tubes, companion cells, bast fibres and phloem parenchyma. – Secondary xylem is always more than secondary phloem because the cambium is more active on the inner side. – The primary phloem is pushed outward and gets crushed, while primary xylem remains intact around the pith. – The cambium also forms radially elongated parenchyma called secondary medullary rays, which carry food, water and minerals radially.

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Class 12 Biology | Unit 1: Plant Anatomy 3) Annual rings (growth rings) The activity of cambium is not uniform throughout the year. Spring wood (early wood) Autumn wood (late wood) Formed in spring/summer. Formed in autumn/winter. Cambium is very active. Cambium is less active. Vessels with wide cavities. Vessels with narrow cavities. Light in colour, less dense. Dark in colour, more dense. – In spring, metabolic activity is high and there is a high demand for water and minerals, so wider vessels are formed. – One spring wood + one autumn wood formed in a year appear together as a concentric ring called an annual ring. – Significance: counting annual rings gives the age of the tree (dendrochronology). Rings are clear in temperate regions but not distinct in tropical plants with uniform climate. 4) Heartwood and sapwood – In old trees, the older (central) secondary xylem gets filled with tannins, resins, gums and its vessels are blocked by tyloses. It becomes dark and is called heartwood (duramen). It does not conduct water but gives mechanical support and is durable. – The outer (peripheral) secondary xylem is light in colour, has living cells and conducts water and minerals. It is called sapwood (alburnum). Heartwood Sapwood Central, older part of wood. Peripheral, younger part of wood. Dark in colour. Light in colour. Vessels blocked by tyloses; dead. Vessels open; contains living cells. Does not conduct water. Conducts water and minerals. Hard, durable, resistant to decay. Soft, less durable.

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Class 12 Biology | Unit 1: Plant Anatomy 5) Cork cambium and periderm – As the stem thickens, the epidermis breaks. So a new meristem, cork cambium (phellogen), develops in the outer cortex. – It forms cork (phellem) on the outside and secondary cortex (phelloderm) on the inside. – Phellem + phellogen + phelloderm = periderm. All tissues outside the vascular cambium together form the bark. – Lenticels - small lens-shaped openings in the periderm for gaseous exchange. Cork (phellem) Cork cambium (phellogen) Secondary phloem Vascular cambium ring Annual rings (secondary xylem) Secondary medullary rays Primary xylem Pith Fig: T.S. of dicot stem after secondary growth Significance of secondary growth – Increases girth, giving strength and support to tall trees. – Adds new xylem and phloem every year more conduction of water and food. – Forms protective cork (bark); provides commercially useful wood (timber) and cork. Important Questions 1. Classify meristematic tissue on different bases with examples. (Long Q) 2. Differentiate between tunica and corpus. 3. Differentiate between shoot apex and root apex. 4. Describe the types of collenchyma / sclerenchyma. 5. Draw a labelled diagram of T.S. of dicot/monocot stem or root and give differences. (4 marks) 6. Compare the internal structure of dicot and monocot leaf.

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Class 12 Biology | Unit 1: Plant Anatomy 7. Describe secondary growth in dicot stem. What are annual rings? 8. Differentiate between heartwood and sapwood.