Botany वनस्पतिशास्त्र

Quick summary

Plant taxonomy is the science of identifying, naming and classifying plants. It gives every plant a correct scientific name and a place in a system of classification. This topic covers the aims of taxonomy, types of classification systems, the Bentham and Hooker system, the rules of botanical nomenclature (ICN) and the use of herbaria and botanical gardens.

Key points

• Taxonomy (वर्गीकरणशास्त्र) has three parts: identification, nomenclature (naming) and classification.
• The word "taxonomy" was coined by A. P. de Candolle (1813).
• Systems of classification:
• Artificial – based on one or a few easily seen characters (e.g. Linnaeus' sexual system, based on number of stamens; 24 classes).
• Natural – based on many characters showing overall similarity (e.g. Bentham and Hooker, Genera Plantarum, 1862–1883).
• Phylogenetic – based on evolutionary relationships (e.g. Engler and Prantl; Hutchinson).
• Bentham and Hooker's system is widely used in Indian herbaria and for flowering plants in Indian syllabi.
• It divides seed plants into three classes: Dicotyledons, Gymnospermae and Monocotyledons.
• Dicots are divided into Polypetalae, Gamopetalae and Monochlamydeae.
• Polypetalae has series Thalamiflorae, Disciflorae and Calyciflorae; Gamopetalae has Inferae, Heteromerae and Bicarpellatae.
• Limitation: gymnosperms are placed between dicots and monocots; it is not phylogenetic.
• Modern trend: the APG (Angiosperm Phylogeny Group) system uses DNA sequence data.

Important terms

• Binomial nomenclature – two-word scientific name (genus + specific epithet), started by Linnaeus in Species Plantarum (1753).
• ICN – International Code of Nomenclature for algae, fungi and plants (earlier ICBN).
• Type specimen – the specimen on which a name is based. Holotype is the single specimen chosen by the author.
• Priority – the earliest validly published name is the correct name.
• Author citation – name of the scientist who first described the species, written after the name (e.g. Mangifera indica L., where L. = Linnaeus).
• Herbarium – collection of pressed, dried, mounted and labelled plant specimens.
• Flora – book listing and describing the plants of an area.
• Taxonomic key – tool with pairs of contrasting statements (couplets) used for identification.

Key facts

• Main rules of ICN: names in Latin; one correct name for each taxon; principle of priority (starting point for most plants: Species Plantarum, 1 May 1753); type method; names are italicised or underlined.
• Standard herbarium sheet size: about 29 × 42 cm.
• Important herbaria and gardens: Royal Botanic Gardens, Kew (England); Central National Herbarium at the Acharya Jagadish Chandra Bose Indian Botanic Garden, Howrah; Botanical Survey of India (BSI) has its headquarters in Kolkata (founded 1890).
• Steps of herbarium preparation: collection, pressing, drying, poisoning (to prevent insects and fungi), mounting, labelling, storing.

5 important questions

Q1. What is a herbarium?

A herbarium is a store-house of pressed, dried and labelled plant specimens mounted on standard sheets and arranged by a system of classification. It helps in identification and research.

Q2. State the principle of priority.

When a plant has more than one name, the earliest validly published name is accepted as the correct name. For most plants, the starting date is the publication of Linnaeus' Species Plantarum (1753).

Q3. Give an outline of Bentham and Hooker's system of classification.

• Published in Genera Plantarum (3 volumes, 1862–1883).
• Seed plants → Dicotyledons, Gymnospermae, Monocotyledons.
• Dicots → Polypetalae (Thalamiflorae, Disciflorae, Calyciflorae), Gamopetalae (Inferae, Heteromerae, Bicarpellatae), Monochlamydeae.
• Monocots divided into seven series.
• Total about 202 natural orders (families).
• Merits: easy, practical. Demerits: not phylogenetic; gymnosperms placed between dicots and monocots.

Q4. Explain the main rules of botanical nomenclature.

• Each species has a binomial: genus (capital) + specific epithet (small).
• Names in Latin or Latinised form; printed in italics.
• One correct name; principle of priority.
• Type method for each name.
• Author citation after the name.
• Later homonyms are rejected.

Q5. Describe the method of preparing herbarium specimens.

• Collect healthy specimens with flowers/fruits; note field data.
• Press between blotting papers; change papers until dry.
• Poison with mercuric chloride or other chemicals.
• Mount on herbarium sheet with glue or tape.
• Attach label: name, family, place, date, collector.
• Store in cabinets by family order.

Exam tip

Learn the Bentham and Hooker outline as a branching chart. Quote dates: Species Plantarum 1753, Genera Plantarum 1862–1883.

Quick summary

Classical taxonomy uses mainly external morphology. Modern taxonomy adds evidence from anatomy, embryology, chromosomes, chemistry, DNA and computers, so that classification shows true relationships (phylogeny). This topic covers numerical taxonomy, cytotaxonomy, chemotaxonomy, molecular taxonomy (DNA barcoding), cladistics and the APG system.

Key points

• Alpha taxonomy is based on morphology alone; omega taxonomy uses all available data. Modern trends move us from alpha towards omega.
• Anatomy and embryology in taxonomy: stomata types, wood structure, embryo sac type, endosperm type help place doubtful taxa.
• Cytotaxonomy: uses chromosome number, size, shape (karyotype) and behaviour in meiosis. Example: polyploid series in wheat (2n = 14, 28, 42).
• Chemotaxonomy: uses plant chemicals such as alkaloids, flavonoids, betalains, essential oils and proteins. Example: betalain pigments occur in Caryophyllales (beet, Amaranthus, cacti) instead of anthocyanins.
• Numerical taxonomy (taximetrics): Sneath and Sokal (1963). Many characters (often 50 or more) are scored and given equal weight; similarity between Operational Taxonomic Units (OTUs) is calculated by computer and shown as a tree-like phenogram (dendrogram).
• Cladistics (Willi Hennig): groups taxa by shared derived characters (synapomorphies); result is a cladogram. Only monophyletic groups are accepted.
• Molecular taxonomy: compares DNA sequences (chloroplast genes rbcL, matK; nuclear ITS). It gives large, objective data sets.
• DNA barcoding: a short standard gene region identifies species; for land plants, ***rbcL* + *matK*** is the core barcode.
• APG system (Angiosperm Phylogeny Group, 1998; APG IV 2016) is based mainly on DNA. It recognises orders and families, groups such as monocots and eudicots, and basal angiosperms (e.g. Amborella, water lilies).
• Digital tools: online floras, herbarium databases and identification keys help modern taxonomists.

Important terms

• OTU – Operational Taxonomic Unit, the basic unit compared in numerical taxonomy.
• Phenogram – tree showing overall similarity between OTUs.
• Cladogram – branching diagram showing shared derived characters.
• Monophyletic group – a group with a common ancestor and all its descendants.
• Karyotype – the complete set of chromosomes of a species described by number, size and shape.
• DNA barcode – a short standard DNA sequence used to identify a species.

Key facts

5 important questions

Q1. What is chemotaxonomy?

Chemotaxonomy is the use of chemical constituents of plants – alkaloids, flavonoids, pigments, proteins – to solve problems of classification. For example, betalain pigments mark most families of Caryophyllales.

Q2. What is DNA barcoding?

It is the identification of a species using a short, standard DNA region. In land plants the chloroplast genes rbcL and matK are used together as the core barcode.

Q3. Explain the principles and steps of numerical taxonomy.

• Sneath and Sokal (1963); equal weight to all characters.
• Steps: choose OTUs → record many characters → code them → calculate similarity coefficients → cluster analysis → phenogram.
• Merits: objective, uses computers; demerits: needs many characters, ignores evolution.

Q4. Discuss the role of cytology and chemistry in taxonomy.

• Chromosome number, karyotype, polyploidy, meiotic pairing with examples.
• Chemical characters: primary metabolites (proteins, serology) and secondary metabolites.
• Examples and limitations.

Q5. Write a note on molecular systematics and the APG system.

• DNA sequences as characters; cladistic analysis.
• APG history (1998, 2003, 2009, 2016).
• Main groups: basal angiosperms, magnoliids, monocots, eudicots.
• Changes from Bentham and Hooker (e.g. families merged or split).

Exam tip

For "modern trends" questions, write a short paragraph on each branch with one example. Never forget the year 1963 (Sneath and Sokal) and the rbcL + matK barcode.

Quick summary

A flower is a modified shoot meant for sexual reproduction. An inflorescence is the arrangement of flowers on the floral axis. This topic covers the parts of a typical flower, floral terms, aestivation, placentation, types of inflorescence, and how to write a floral formula and draw a floral diagram.

Key points

• A flower (फूल) has four whorls on the thalamus: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels).
• Calyx and corolla are accessory whorls; androecium and gynoecium are essential whorls.
• When calyx and corolla look alike, they are called perianth (tepals), e.g. lily.
• Symmetry: actinomorphic (radial, e.g. mustard), zygomorphic (bilateral, e.g. pea), asymmetric (e.g. Canna).
• Position of ovary: hypogynous (superior ovary, e.g. mustard), perigynous (half-inferior, e.g. rose), epigynous (inferior ovary, e.g. sunflower, guava).
• Aestivation is the arrangement of sepals or petals in a bud: valvate (Calotropis), twisted (cotton, china rose), imbricate (Cassia), vexillary or papilionaceous (pea).
• Placentation is the arrangement of ovules in the ovary: marginal (pea), axile (china rose, tomato), parietal (mustard), free central (Dianthus), basal (sunflower, marigold).
• Racemose inflorescence: main axis keeps growing; flowers in acropetal order (older at the base). Types: raceme, spike, catkin, spadix, corymb, umbel, head (capitulum).
• Cymose inflorescence: main axis ends in a flower; flowers in basipetal order. Types: monochasial (helicoid, scorpioid), dichasial, polychasial.
• Special inflorescences: cyathium (Euphorbia), hypanthodium (Ficus), verticillaster (Ocimum, tulsi).

Important terms

• Bract – reduced leaf at the base of a flower stalk.
• Bisexual / unisexual – flower with both or only one type of essential whorl.
• Epipetalous – stamens attached to petals (e.g. brinjal).
• Monadelphous – stamens fused into one bundle (china rose); diadelphous – two bundles (pea, 9 + 1).
• Syngenesious – anthers fused, filaments free (sunflower).
• Didynamous – four stamens, two long and two short (tulsi); tetradynamous – six stamens, four long and two short (mustard).
• Syncarpous / apocarpous – carpels fused / free.

Key facts

Floral formula symbols: Br – bracteate; ⊕ – actinomorphic; % – zygomorphic; ⚥ – bisexual; ♂ male; ♀ female; K – calyx; C – corolla; P – perianth; A – androecium; G – gynoecium; G with a line below it – superior ovary; G with a line above it – inferior ovary; brackets ( ) – fusion; a number above or ∞ – many.

• Floral formula of mustard (Brassicaceae): ⊕ ⚥ K2+2 C4 A2+4 G(2), ovary superior.
• Floral diagram: drawn as seen from above; the mother axis is shown as a dot at the top; whorls drawn as concentric circles from outside (calyx) to inside (gynoecium) with a T.S. of the ovary in the centre.

5 important questions

Q1. Differentiate between racemose and cymose inflorescence.

In racemose, the main axis grows on and flowers open in acropetal order, older at the base. In cymose, the main axis ends in a flower and flowers open in basipetal order.

Q2. What is placentation? Name its types.

Placentation is the arrangement of ovules inside the ovary. Types: marginal, axile, parietal, free central, basal, and superficial (water lily).

Q3. Describe the types of aestivation with examples.

• Valvate: margins just touch – Calotropis.
• Twisted: one margin overlaps the next regularly – china rose, cotton.
• Imbricate: irregular overlap – Cassia, gulmohur.
• Vexillary: large standard covers two wings, which cover keel – pea, bean.
• Draw a small sketch of each.

Q4. Explain the types of racemose inflorescence.

• Raceme: stalked flowers on long axis – mustard.
• Spike: sessile flowers – Achyranthes.
• Spadix: fleshy axis with spathe – banana, Colocasia.
• Catkin: drooping spike of unisexual flowers – mulberry.
• Corymb, umbel (coriander, onion) and capitulum/head (sunflower).

Q5. What is a floral formula? Write the floral formula of a pea flower and explain it.

• Short way of expressing floral characters with symbols.
• Pea (Fabaceae): % ⚥ K(5) C1+2+(2) A(9)+1 G1, ovary superior.
• Zygomorphic, bisexual; 5 fused sepals; vexillary corolla (standard, 2 wings, 2 fused keel petals).
• Diadelphous stamens; monocarpellary, superior ovary with marginal placentation.

Exam tip

Practise drawing one floral diagram (mustard or pea) and writing its formula. Examiners give marks for correct symbols and bracket use.

Quick summary

After fertilisation, the ovary develops into a fruit and the ovules into seeds. Fruits protect seeds and help in their dispersal. This topic covers the structure of a fruit, the classification of fruits (simple, aggregate and composite) with examples, the structure of dicot and monocot seeds, and the agents of seed and fruit dispersal.

Key points

• A true fruit develops only from the ovary (mango, tomato). A false fruit (pseudocarp) has other floral parts too – thalamus in apple and strawberry, whole inflorescence in fig.
• Parthenocarpy – fruit formed without fertilisation, usually seedless (banana, seedless grapes).
• Pericarp (fruit wall) has three layers: epicarp, mesocarp, endocarp.
• Drupe – stony endocarp: mango, coconut (fibrous mesocarp).
• Berry – fleshy throughout: tomato, grape, guava, banana.
• Pepo – cucumber, watermelon. Hesperidium – orange (juicy hairs from endocarp). Pome – apple (fleshy thalamus).
• Simple fleshy fruits:
• Dehiscent: legume (pea, bean), capsule (cotton, lady's finger), siliqua (mustard).
• Indehiscent: caryopsis (wheat, rice, maize; pericarp fused with seed coat), cypsela (sunflower), nut (cashew).
• Schizocarpic: lomentum (Mimosa), cremocarp (coriander).
• Simple dry fruits:
• Aggregate fruit – from many free carpels of one flower: custard apple, strawberry, Michelia.
• Composite (multiple) fruit – from a whole inflorescence: sorosis (pineapple, jackfruit), syconus (fig).
• Dicot seed (gram, bean): seed coat (testa and tegmen), hilum, micropyle, embryo with two cotyledons, plumule, radicle; usually non-endospermic.
• Monocot seed (maize grain): endosperm large with aleurone layer, single cotyledon called scutellum, coleoptile over plumule and coleorhiza over radicle.
• Dispersal agents: wind (light seeds, wings – Moringa; hairs – Calotropis, cotton; pappus – Tridax), water (coconut – fibrous buoyant mesocarp; lotus), animals (hooks – Xanthium, Achyranthes; fleshy edible fruits), explosive mechanism (balsam, castor).

Important terms

• Pericarp – wall of the fruit developed from the ovary wall.
• Pseudocarp – false fruit formed with non-ovarian parts.
• Caryopsis – one-seeded dry fruit with pericarp fused to the seed coat.
• Scutellum – shield-shaped cotyledon of grasses.
• Pappus – modified calyx of hairs in Asteraceae that helps wind dispersal.

Key facts

5 important questions

Q1. What is a false fruit? Give two examples.

A false fruit is a fruit in which parts other than the ovary, such as the thalamus or the inflorescence axis, form the fleshy part. Examples: apple, strawberry, fig.

Q2. Distinguish between a legume and a siliqua.

A legume comes from a monocarpellary ovary and splits along both sutures (pea). A siliqua comes from a bicarpellary ovary with a false septum (replum) and opens from below upwards (mustard).

Q3. Classify fruits with suitable examples.

Simple (fleshy and dry: dehiscent, indehiscent, schizocarpic), aggregate and composite fruits; at least two examples each and labelled sketches of mango, legume and pineapple.

Q4. Compare the structure of a dicot seed and a monocot seed.

Gram/bean seed vs maize grain: seed coat, cotyledons, endosperm, scutellum, coleoptile, coleorhiza – labelled L.S. diagrams.

Q5. Describe the methods of seed and fruit dispersal.

Wind, water, animals, explosive mechanism – adaptations with two examples each; importance of dispersal (less competition, new habitats).

Exam tip

Questions like "edible part of mango, coconut, apple, wheat" are common one-mark items. Learn the table and one dispersal example for each agent.

Quick summary

University syllabi ask students to describe important dicot families: their vegetative and floral characters, floral formula, floral diagram and economic importance. This topic covers five commonly prescribed families – Malvaceae, Fabaceae, Asteraceae, Solanaceae and Lamiaceae – following Bentham and Hooker's system, with their position in the APG system.

Key points

• Malvaceae (cotton family; Thalamiflorae): mucilage, stellate hairs; epicalyx present; calyx valvate; corolla twisted; stamens many, monadelphous (staminal tube), anthers monothecous (one-celled), kidney-shaped; superior ovary with axile placentation; capsule. Examples: Hibiscus, cotton (Gossypium), lady's finger.
• Fabaceae / Papilionaceae (pea family; Calyciflorae): leaves pinnately compound with stipules; root nodules; papilionaceous corolla – 1 standard, 2 wings, 2 keel petals; vexillary (descendingly imbricate) aestivation; stamens 10, diadelphous (9) + 1; monocarpellary ovary, marginal placentation; fruit legume. Examples: pea, gram, groundnut, soybean.
• Asteraceae / Compositae (sunflower family; Inferae): inflorescence a capitulum (head) with ray and disc florets surrounded by an involucre; calyx modified into pappus; stamens 5, syngenesious (anthers fused, filaments free), epipetalous; bicarpellary, inferior ovary, one ovule, basal placentation; fruit cypsela. Examples: sunflower, marigold, Tridax.
• Solanaceae (potato family; Bicarpellatae): calyx persistent; corolla rotate or funnel-shaped; stamens 5, epipetalous; bicarpellary, superior ovary, obliquely placed carpels, swollen axile placentae; berry or capsule. Examples: potato, tomato, brinjal, chilli, Datura.
• Lamiaceae / Labiatae (mint family; Bicarpellatae): square stem, opposite-decussate aromatic leaves; verticillaster inflorescence; bilabiate corolla (2/3); stamens 4, didynamous; ovary four-lobed with gynobasic style; fruit of 4 nutlets. Examples: tulsi (Ocimum), mint, Leucas.

Important terms

• Epicalyx – whorl of bracteoles outside the calyx.
• Monadelphous – all filaments fused in one bundle.
• Syngenesious – anthers fused but filaments free.
• Didynamous – four stamens, two long and two short.
• Gynobasic style – style arising from the base of the lobed ovary.
• Floral formula – short code of a flower's parts, e.g. Br, ⊕ or %, ⚥, K, C, A, G.

Key facts

(Brackets mean the parts are fused; in A5 of Solanaceae the stamens are free from each other but fused to the petals.)

5 important questions

Q1. What is a papilionaceous corolla?

A butterfly-like corolla of five petals: one large posterior standard (vexillum), two lateral wings (alae) and two anterior keel petals fused along one edge (carina). It is typical of Fabaceae.

Q2. Give two distinguishing characters of Lamiaceae.

Square stem with opposite aromatic leaves, and a four-lobed ovary with a gynobasic style (also bilabiate corolla and didynamous stamens).

Q3. Describe the family Malvaceae with economic importance.

Vegetative characters, inflorescence, flower (epicalyx, twisted petals, monadelphous monothecous stamens, axile placentation), floral formula and diagram; uses: cotton fibre, lady's finger, ornamental Hibiscus.

Q4. Describe Asteraceae and explain why it is called the most advanced dicot family.

Capitulum, florets, pappus, syngenesious stamens, inferior ovary, cypsela; advanced because of head inflorescence, reduced calyx for dispersal, efficient pollination, huge number of species. Economic importance: sunflower oil, safflower, ornamentals, weeds.

Q5. Compare Solanaceae and Lamiaceae.

Stem, leaves, inflorescence, corolla, stamens, ovary, style and fruit in a table; one floral diagram each.

Exam tip

In practicals, always open a flower and check: free or fused petals, number and fusion of stamens, and ovary position – these three features usually give the family. Write the floral formula correctly.

Quick summary

Monocot families are recognised by parallel venation, trimerous flowers and a single cotyledon. This topic covers three families commonly prescribed in B.Sc. syllabi – Liliaceae, Musaceae and Poaceae (Gramineae) – with their vegetative and floral characters, floral formulae, special features (such as the grass spikelet) and economic importance.

Key points

• General monocot features: fibrous adventitious roots; scattered closed vascular bundles; leaves with sheathing base and parallel venation; flowers usually trimerous; perianth (tepals) instead of separate calyx and corolla.
• Liliaceae (lily family; Bentham and Hooker's Coronarieae): herbs with bulbs, rhizomes or corms; flowers actinomorphic, bisexual; perianth 6 in two whorls of 3 (petaloid); stamens 6, often epiphyllous (on tepals); ovary superior, tricarpellary, syncarpous, trilocular, axile placentation; fruit capsule or berry.
• Liliaceae examples: lily, Gloriosa (glory lily), Asparagus, Aloe, onion and garlic (now often placed in Amaryllidaceae in APG). Uses: food (onion, garlic, Asparagus), medicine (Aloe, Gloriosa gives colchicine), ornamentals.
• Musaceae (banana family; Epigynae/Scitamineae): giant herbs with an underground rhizome and a pseudostem formed by leaf sheaths; huge leaves with pinnate-parallel venation; flowers in spadix-like inflorescence covered by large coloured spathes; flowers zygomorphic, unisexual or bisexual; perianth 6; stamens 5 fertile + 1 staminode; ovary inferior, tricarpellary, axile; fruit a berry (parthenocarpic in cultivated banana).
• Poaceae / Gramineae (grass family; Glumaceae): stem a culm with hollow internodes and solid nodes; leaves with sheath, ligule and long blade; inflorescence of spikelets arranged in spikes or panicles.
• Grass floret: enclosed by lemma and palea; perianth reduced to 2–3 lodicules; stamens usually 3 (6 in rice) with versatile anthers; ovary monocarpellary (with 2–3 feathery stigmas), superior, one ovule, basal; fruit caryopsis.
• Each spikelet has two sterile glumes at the base and one or more florets.
• Poaceae is the most important family for humans: rice, wheat, maize, jowar, bajra, sugarcane, bamboo, fodder grasses.

Important terms

• Tepal – a perianth member when sepals and petals look alike.
• Pseudostem – false stem made of rolled leaf sheaths (banana).
• Spathe – large bract covering an inflorescence.
• Ligule – membranous outgrowth at the junction of leaf sheath and blade in grasses.
• Lemma and palea – outer and inner bracts enclosing a grass floret.
• Lodicules – tiny scales (reduced perianth) that swell to open the floret.

Key facts

5 important questions

Q1. What is a spikelet?

A spikelet is the unit of the grass inflorescence. It has two sterile glumes at the base and one or more florets on a short axis (rachilla); each floret is enclosed by a lemma and a palea.

Q2. Why is the banana stem called a pseudostem?

Because the aerial "stem" is formed by tightly rolled leaf sheaths. The true stem is an underground rhizome; it grows up through the sheaths only at flowering.

Q3. Describe the family Poaceae with its economic importance.

Culm, leaf with ligule, spikelet structure, floret (lemma, palea, lodicules, 3 stamens, feathery stigmas), caryopsis; floral formula and diagram; cereals, millets, sugarcane, bamboo, fodder.

Q4. Give the distinguishing features of Liliaceae.

Bulbous habit, trimerous actinomorphic flowers, 6 petaloid tepals, 6 epiphyllous stamens, superior trilocular ovary with axile placentation; examples and uses; note its splitting under APG.

Q5. Describe Musaceae with a floral diagram.

Rhizome and pseudostem, leaves, spathe-covered inflorescence, zygomorphic flowers, 5 stamens + staminode, inferior ovary, berry; uses – fruit, fibre (Musa textilis, Manila hemp), leaves as plates.

Exam tip

For grasses, examiners look for the labelled spikelet diagram – glumes, lemma, palea, lodicules, stamens and feathery stigma. Write "caryopsis" for the fruit, not "grain".

Quick summary

All primary organs of a plant are formed by groups of always-dividing cells at the tips of the shoot and root, called apical meristems. Different theories explain how these meristems are organised. This topic covers the apical cell theory, histogen theory, tunica–corpus theory, the organisation of the root apex and the quiescent centre.

Key points

• Meristem – a tissue of small, thin-walled, densely cytoplasmic cells with large nuclei and no intercellular spaces, able to divide.
• By position: apical (tips), intercalary (bases of internodes, grasses), lateral (cambium, cork cambium).
• Apical cell theory (Hofmeister; supported by Nägeli, 19th century): a single large apical cell at the tip forms the whole plant body. True for many algae, bryophytes and pteridophytes (e.g. Equisetum), not for seed plants.
• Dermatogen → epidermis,
• Periblem → cortex,
• Plerome → stele (vascular tissue and pith).
• Histogen theory (Hanstein, 1868): the apex has three layers (histogens):

The theory fits the root of many plants but not the shoot, because the layers do not always give these fixed tissues.

• Tunica–corpus theory (A. Schmidt, 1924) for the shoot apex: an outer tunica of one or more layers that divide only anticlinally (adding surface), and an inner corpus whose cells divide in all planes (adding volume). It is based on planes of division, not on fate of tissues.
• Cytohistological zonation in gymnosperm and angiosperm shoot apices: central mother cell zone, peripheral zone (forms leaves) and rib meristem (forms pith).
• Root apex is covered by a root cap produced by the calyptrogen (in monocots) or by a common initial layer (in many dicots).
• Körper–Kappe theory (Schüepp, 1917) for roots: T-divisions in the root apex.
• Quiescent centre (F. A. L. Clowes, 1956): a group of cells at the root tip that divide very slowly; it is a reserve that can replace damaged active initials.
• Leaves start as leaf primordia on the flanks of the shoot apex; the order of their origin decides phyllotaxy.

Important terms

• Anticlinal division – wall formed at right angles to the surface; increases surface area.
• Periclinal division – wall parallel to the surface; increases the number of layers.
• Histogen – a layer of the apex that forms a definite tissue.
• Calyptrogen – separate initial layer forming the root cap in monocots.
• Quiescent centre – slowly dividing central region of the root apex.

Key facts

5 important questions

Q1. What is the quiescent centre?

It is a group of cells at the centre of the root apex that divide very rarely. Discovered by Clowes (1956), it acts as a reserve of initials when active cells are damaged.

Q2. Name the three histogens and what they form.

Dermatogen forms the epidermis, periblem forms the cortex and plerome forms the central stele.

Q3. Explain the tunica–corpus theory with a diagram.

• Schmidt (1924); tunica layers divide anticlinally; corpus divides in all planes.
• Number of tunica layers varies (1–5).
• Merits over the histogen theory. Labelled L.S. of shoot apex.

Q4. Describe the organisation of the root apex.

• Root cap, calyptrogen (monocots), histogens.
• Körper–Kappe concept, quiescent centre.
• Diagram of root tip L.S.

Q5. Compare the theories of shoot apex organisation.

Apical cell, histogen and tunica–corpus theories in a table: basis, layers, merits, limits; add cytohistological zonation.

Exam tip

Learn the five names and years in the table. In a long answer, always end with why tunica–corpus is preferred for the shoot (it is based on division planes, not fixed tissue fate).

Quick summary

Secondary growth is the increase in thickness (girth) of stems and roots due to the activity of lateral meristems: the vascular cambium and the cork cambium. It occurs in dicots and gymnosperms, but usually not in monocots. This topic covers the formation of the cambium ring, secondary xylem and phloem, annual rings, heartwood and sapwood, periderm and lenticels.

Key points

• Primary growth increases length (apical meristems). Secondary growth (द्वितीयक वाढ) increases girth.
• In a dicot stem, the cambium inside the vascular bundles is intrafascicular cambium.
• Parenchyma cells of medullary rays between bundles become meristematic to form interfascicular cambium.
• Both join to form a complete vascular cambium ring.
• Cambium cuts off cells on both sides: more towards the inside as secondary xylem (wood) and fewer towards the outside as secondary phloem.
• Primary xylem gets pushed towards the centre; primary phloem gets crushed outside.
• Cambium also forms secondary medullary rays for radial conduction.
• Cork cambium (phellogen) arises in the cortex. It forms cork (phellem) outside and secondary cortex (phelloderm) inside.
• Phellem + phellogen + phelloderm = periderm.
• Bark = all dead tissues outside the vascular cambium (periderm plus secondary phloem in a wide sense).
• In the dicot root, cambium arises from conjunctive parenchyma and pericycle opposite the protoxylem; it is first wavy and later becomes circular.

Important terms

• Spring wood (early wood) – light, wide vessels; formed when cambium is very active.
• Autumn wood (late wood) – dark, narrow vessels; formed when activity is low.
• Annual ring (growth ring) – one band of spring wood plus autumn wood formed in one year.
• Dendrochronology – estimating the age of a tree by counting annual rings.
• Heartwood (duramen) – dark, central, dead wood filled with tannins, resins and tyloses; gives strength and is durable.
• Sapwood (alburnum) – light, outer wood that conducts water.
• Tyloses – balloon-like ingrowths of parenchyma into vessels, blocking them.
• Lenticels – lens-shaped openings in bark for gas exchange.

Key facts

• Clear annual rings form in temperate climates; in tropical regions with no sharp seasons, rings are often not distinct.
• Anomalous secondary growth (unusual pattern) is seen in some plants, e.g. Dracaena (a monocot that shows secondary growth), Boerhaavia, Bignonia.

5 important questions

Q1. What are annual rings?

An annual ring is a pair of spring wood and autumn wood formed in one year by the vascular cambium. Counting these rings gives the approximate age of the tree.

Q2. Differentiate between heartwood and sapwood.

Heartwood is the dark, central, non-functional wood with tannins and tyloses; it is hard and durable. Sapwood is the light, outer, living-cell-containing wood that conducts water and minerals.

Q3. Describe secondary growth in a dicot stem.

• Intrafascicular and interfascicular cambium join to form a ring.
• Cambium forms secondary xylem inside and secondary phloem outside.
• Secondary medullary rays formed.
• Annual rings formed due to seasonal activity.
• Cork cambium forms periderm; lenticels develop.
• Diagram stages: young stem, cambium ring, T.S. of old stem showing rings, pith, rays, phloem, periderm.

Q4. Explain the formation and structure of periderm.

• Phellogen arises from outer cortex (or epidermis/pericycle in some).
• Outer cells form phellem (cork): dead, suberised, compact.
• Inner cells form phelloderm: living, parenchymatous.
• Function: protection, prevents water loss.
• Lenticels: loose complementary cells allow gas exchange.

Q5. Explain secondary growth in a dicot root.

• Cambium forms from conjunctive parenchyma below phloem.
• Pericycle opposite protoxylem also becomes meristematic.
• Wavy ring becomes circular.
• Secondary xylem and phloem formed; broad medullary rays opposite protoxylem.
• Cork cambium from pericycle forms periderm; cortex sheds.

Exam tip

Draw stage-wise diagrams of a dicot stem (cambium ring formation to old stem). Remember: cambium gives more xylem than phloem.

Quick summary

Embryology of angiosperms deals with the formation of male and female gametophytes, pollination, fertilisation and the development of the endosperm and embryo. Angiosperms show a special process called double fertilisation. This topic covers microsporogenesis, megasporogenesis, embryo sac types, double fertilisation, endosperm, embryo development and polyembryony.

Key points

• Stamen = anther + filament. A typical anther is dithecous with four microsporangia (pollen sacs).
• Anther wall (outside to inside): epidermis, endothecium (helps dehiscence), middle layers, tapetum (nourishes developing pollen).
• Microsporogenesis: each microspore mother cell (2n) divides by meiosis to form a tetrad of four haploid microspores (pollen grains).
• Pollen wall: outer exine of sporopollenin (very resistant) with germ pores, and inner intine of cellulose and pectin.
• Male gametophyte: pollen divides into a large vegetative (tube) cell and a small generative cell; the generative cell forms two male gametes (in the pollen grain or in the pollen tube).
• Ovule parts: funicle, hilum, integuments, micropyle, chalaza, nucellus and embryo sac.
• Megasporogenesis: megaspore mother cell (2n) undergoes meiosis to give four megaspores; usually three degenerate and the chalazal one is functional.
• Polygonum type (monosporic) embryo sac is the most common: 8 nuclei in 7 cells – egg apparatus (egg + two synergids) at the micropylar end, three antipodals at the chalazal end, and a central cell with two polar nuclei.
• Double fertilisation (discovered by S. G. Nawaschin, 1898): one male gamete fuses with the egg (syngamy) → zygote (2n); the other fuses with the two polar nuclei (triple fusion) → primary endosperm nucleus (3n).
• Endosperm types: nuclear (coconut water), cellular, helobial.
• Dicot embryo development (Crucifer / Onagrad type, e.g. Capsella): zygote → proembryo with suspensor → globular → heart-shaped → mature embryo with two cotyledons.

Important terms

• Pollination – transfer of pollen to the stigma. Self (autogamy, geitonogamy) and cross (xenogamy).
• Filiform apparatus – finger-like wall thickenings of synergids that guide the pollen tube.
• Porogamy / chalazogamy / mesogamy – pollen tube entering through micropyle / chalaza / integuments.
• Perisperm – remaining nucellus in a seed (black pepper, beet).
• Polyembryony – more than one embryo in a seed (citrus, mango).
• Apomixis – seed formation without fertilisation.
• Parthenocarpy – fruit formation without fertilisation (seedless banana, grapes).

Key facts

• Capsella bursa-pastoris (shepherd's purse) is the standard type for dicot embryo; grass embryo has a scutellum (single cotyledon) and coleoptile/coleorhiza.

5 important questions

Q1. What is triple fusion?

Triple fusion is the fusion of the second male gamete with the two polar nuclei (or the secondary nucleus) in the central cell. It forms the triploid primary endosperm nucleus, which develops into the endosperm.

Q2. What is the role of the tapetum?

The tapetum is the innermost layer of the anther wall. It nourishes developing pollen grains and contributes materials to the pollen wall, including sporopollenin and pollenkitt.

Q3. Describe the structure of a mature embryo sac (Polygonum type).

• Monosporic, 8-nucleate, 7-celled.
• Micropylar end: egg apparatus (egg cell + two synergids with filiform apparatus).
• Chalazal end: three antipodal cells.
• Centre: large central cell with two polar nuclei.
• Labels: micropyle, synergids, filiform apparatus, egg, polar nuclei, central cell, antipodals, chalaza.

Q4. Explain double fertilisation and its significance.

• Pollen tube enters the embryo sac through a synergid and releases two male gametes.
• Syngamy: male gamete + egg → zygote (2n).
• Triple fusion: male gamete + two polar nuclei → PEN (3n).
• Significance: forms nutritive endosperm only when fertilisation occurs (saves energy); unique to angiosperms; gives vigour.

Q5. Describe the development of a dicot embryo.

• Zygote divides transversely: basal cell (forms suspensor and hypophysis) and terminal cell (forms embryo proper).
• Quadrant and octant stages → globular embryo.
• Dermatogen forms epidermis.
• Heart-shaped stage as cotyledons start.
• Mature embryo: two cotyledons, plumule, radicle, hypocotyl.

Exam tip

Remember ploidy of each structure (table above). Draw a labelled anther T.S., ovule L.S. and embryo sac.

Quick summary

Plants absorb water from the soil through roots, carry it up through the xylem and lose most of it as water vapour from leaves. This topic explains diffusion, osmosis, water potential, plasmolysis, imbibition, absorption of water, ascent of sap and transpiration.

Key points

• Diffusion – net movement of molecules from higher to lower concentration.
• Osmosis (परासरण) – movement of water through a semi-permeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution).
• Endosmosis is entry of water into a cell; exosmosis is exit of water.
• Water potential (Ψw) is the chemical potential of water. Pure water at atmospheric pressure has Ψw = 0; adding solutes makes it negative. Water always moves from higher (less negative) to lower (more negative) Ψw.
• Water potential has two main components: solute potential (Ψs) (always negative) and pressure potential (Ψp) (usually positive in turgid cells).
• Plasmolysis – shrinking of protoplasm away from the cell wall when a cell is placed in a hypertonic solution. Reversing it in water is deplasmolysis.
• Imbibition – absorption of water by solid hydrophilic substances (dry seeds, wood) causing swelling.
• Absorption of water is mainly through root hairs. Active absorption uses energy (root pressure); passive absorption is due to transpiration pull (the main way).
• Water moves across the root by the apoplast pathway (through cell walls and spaces) and the symplast pathway (through cytoplasm and plasmodesmata). At the endodermis the Casparian strip forces water into the symplast.
• Ascent of sap is best explained by the cohesion–tension (transpiration pull) theory of Dixon and Joly (1894).
• Transpiration – loss of water as vapour from aerial parts; mostly through stomata (stomatal), some through cuticle (cuticular) and lenticels (lenticular).

Important terms

• Turgor pressure – pressure of cell contents against the cell wall.
• Root pressure – positive pressure in xylem due to active absorption; seen as guttation (water drops at leaf tips through hydathodes) and bleeding.
• Cohesion – attraction between water molecules.
• Adhesion – attraction between water and xylem walls.
• Guard cells – kidney-shaped cells (dumb-bell shaped in grasses) around a stoma.
• Antitranspirant – substance that reduces transpiration (e.g. phenyl mercuric acetate, abscisic acid).

Formulas

• Water potential: Ψw = Ψs + Ψp (in some texts also + Ψm, matric potential).
• Units: pascals (Pa), usually megapascals (MPa); 1 MPa = 10 bar.
• Example: a cell with Ψs = −0.8 MPa and Ψp = +0.3 MPa has Ψw = −0.8 + 0.3 = −0.5 MPa.
• If this cell is placed next to a cell with Ψw = −0.7 MPa, water moves from the first cell (−0.5) to the second (−0.7).

5 important questions

Q1. Define plasmolysis.

Plasmolysis is the shrinkage of the protoplast away from the cell wall due to exosmosis when a living plant cell is kept in a hypertonic solution.

Q2. A cell has solute potential −1.2 MPa and pressure potential 0.4 MPa. Find its water potential.

Ψw = Ψs + Ψp = −1.2 + 0.4 = −0.8 MPa. Water will enter this cell from any neighbour whose water potential is higher (less negative) than −0.8 MPa.

Q3. Explain the cohesion–tension theory of ascent of sap.

• Proposed by Dixon and Joly.
• Continuous water column from root to leaf in xylem.
• Cohesion keeps the column unbroken; adhesion holds it to xylem walls.
• Transpiration from leaf cells creates tension (negative pressure).
• This pull is transmitted down the column, lifting water.
• Evidence: high tensile strength of water; negative pressure in xylem; reduced flow when transpiration stops.

Q4. Describe the mechanism of stomatal opening and closing.

• Guard cells have thick inner walls and thin outer walls.
• Opening: K⁺ ions enter guard cells (proton pump), malate forms; Ψ falls, water enters, guard cells become turgid and bow outward.
• Closing: K⁺ leaves, water leaves, guard cells become flaccid.
• Abscisic acid causes closure during water stress.
• Older starch–sugar hypothesis (Steward) also mentioned in syllabi.

Q5. Give the factors affecting transpiration and its significance.

• External: light, temperature, humidity, wind, soil water.
• Internal: number and position of stomata, leaf area, cuticle thickness.
• Significance: pull for ascent of sap, cooling, mineral distribution.
• Called a "necessary evil" (Curtis) because it also wastes much water.

Exam tip

Practise one numerical on Ψw = Ψs + Ψp and remember that water flows towards the more negative value.