Plant Anatomy

3.1 Plant Tissues — Meristematic and Permanent

A tissue is a group of cells of common origin, similar structure, and performing one or more related functions. Plant tissues were first systematically classified by the Swiss botanist Carl von Nägeli (1858) and later refined by Julius von Sachs and Gottlieb Haberlandt. The fundamental division separates tissues that retain the capacity to divide — the meristems — from those that have differentiated and mostly lost dividing ability — the permanent tissues.

Nägeli — meristem concept, 1858 · Strasburger — plant cytology, 1884 (Zellbildung und Zelltheilung) · Haberlandt — physiological plant anatomy, 1884 · Hanstein — root meristem three-layer theory, 1868 · Schmidt — tunica-corpus theory of shoot apex, 1924 · Hofmeister — alternation of generations, 1851

3.1.1 Meristematic Tissues

Meristems are perpetually embryonic tissues. Their cells are isodiametric (cube-like), have dense cytoplasm, large nucleus, no vacuole (or very small vacuole), and thin cellulosic walls. They divide repeatedly, with one daughter cell remaining meristematic (self-perpetuation) and the other differentiating into a permanent tissue (differentiation).

Classification by position:

• Apical meristems — located at root and shoot apices; responsible for primary growth (increase in length). The shoot apical meristem (SAM) is the most studied; it is organised into a central zone (slowly dividing, stem cells), peripheral zone (rapidly dividing, leaf and lateral organ founder cells), and rib zone (contributes to the pith).
• Intercalary meristems — located at the base of internodes or leaf blades; retained from the apical meristem but isolated from it by permanent tissue. Typical of monocots (grasses). Responsible for regrowth after grazing.
• Lateral meristems — run longitudinally along the sides of axis; responsible for secondary growth (increase in girth). Two types: vascular cambium (between xylem and phloem; produces secondary xylem inward and secondary phloem outward) and cork cambium / phellogen (forms the periderm: phellem outward + phelloderm inward).

Hanstein’s three-layer (histogen) theory (1868) describes the root apex as having three histogen layers: outermost dermatogen (gives rise to epidermis), middle periblem (gives cortex), and inner plerome (gives stele). The root cap arises from the calyptrogen (an independent fourth layer, not universally accepted). In the shoot, the tunica-corpus theory of Schmidt (1924) is more widely accepted: the tunica (1–2 surface layers) divides only anticlinally (perpendicular to surface), maintaining the surface; the corpus (inner mass) divides in all planes, adding volume.

3.1.2 Permanent Tissues — Simple

Permanent tissues are made of cells that have lost (or nearly lost) the ability to divide and have differentiated for specific functions. Simple permanent tissues consist of a single type of cell.

Parenchyma is the most abundant tissue, made of thin-walled, isodiametric, living cells with large central vacuoles. It stores food (starch, oils), water, performs photosynthesis (chlorenchyma), and is the basis of wound-healing and regeneration. Loosely packed parenchyma in aquatic plants with large air spaces is called aerenchyma. When parenchyma stores latex it is called laticiferous tissue. Prosenchyma refers to elongated parenchyma with tapering ends.

Collenchyma is a living tissue with unevenly thickened cellulose walls. The thickening is deposited at cell corners (angular collenchyma — most common, e.g., Cucurbita hypodermis), along tangential walls (lamellar / plate collenchyma — e.g., Sambucus), or surrounding intercellular spaces (lacunar collenchyma). It provides flexible mechanical support in growing organs — typical in petioles, young stems, leaf veins. Absent in monocots (replaced by sclerenchyma in hypodermis).

Sclerenchyma is dead at maturity, with heavily lignified secondary walls. Two forms: fibres (long, narrow, pointed ends; great tensile strength; jute, hemp, flax — all bast fibres; cotton = surface fibre from seed coat; sisal = leaf fibre) and sclereids / stone cells (isodiametric or irregular; gritty texture of pear fruit; hard seed coats; coconut shell). Sclereids are further typed: brachysclereids (stone cells, isodiametric), macrosclereids (rod-shaped, palisade-like; seed coats of legumes), osteosclereids (bone-shaped), astrosclereids (star-shaped; tea leaves), trichosclereids (hair-like projections into intercellular spaces).

3.1.3 Permanent Tissues — Complex

Complex permanent tissues contain more than one cell type and act as a coordinated unit. The two primary complex tissues are xylem and phloem.

Xylem (Greek xylon = wood) conducts water and mineral salts upward and provides structural support. It consists of four cell types:

• Tracheids — elongated, spindle-shaped, dead cells with lignified walls and bordered pits. No perforation plates. Water passes through pits. Found in all vascular plants (gymnosperms have tracheids only; angiosperms have both). Cedrus deodara wood = tracheids only (no vessels) — key HP exam fact.
• Vessels (tracheae) — dead cells arranged end-to-end with perforation plates (fully dissolved cross-walls). More efficient conductors than tracheids. Unique to angiosperms (and a few non-angiosperm groups: Gnetum, Ephedra, Selaginella). Vessel members: scalariform, reticulate, annular, spiral, pitted thickening patterns reflect increasing efficiency.
• Xylem fibres (libriform fibres) — dead, elongated, heavily lignified; mechanical support.
• Xylem parenchyma — living; stores starch and fats; forms tyloses in heartwood.

Phloem (Greek phloios = bark) conducts photosynthates (mainly sucrose) bidirectionally (predominantly downward in most contexts). Four cell types:

• Sieve tubes — elongated living cells (retain plasma membrane and cytoplasm but lack nucleus at maturity, lack tonoplast). End walls form sieve plates with open pores (callose-lined). Interconnected sieve tube members form sieve tube elements.
• Companion cells — nucleate parenchyma cells adjacent to sieve tube members; connected via plasmodesmata; supply enzymes and proteins to enucleate sieve tubes; control loading/unloading of sugars. Derived from same mother cell as adjacent sieve tube.
• Phloem fibres (bast fibres) — dead sclerenchyma; commercial source of jute (Corchorus), hemp (Cannabis), flax (Linum), ramie (Boehmeria).
• Phloem parenchyma — stores starch, fats; abundant in gymnosperms (sieve cells instead of sieve tubes in gymnosperms, associated with albuminous cells rather than companion cells).

3.2 Tissue Systems

The concept of tissue systems was introduced by Sachs (1875) who grouped tissues by their position in the organ rather than by cell type. The three tissue systems run continuously through the entire plant body.

3.2.1 Epidermal Tissue System

Forms the outermost protective layer of the primary plant body. Components:

• Epidermis — single layer of compact, living cells; no intercellular spaces; secretes waxy cuticle (suberin or cutin) that reduces water loss. Cuticle absent in roots. In xerophytes the cuticle can be very thick; in aquatic plants it is absent or thin.
• Stomata — pores formed by two guard cells (kidney-shaped in dicots; dumb-bell / bar-bell shaped in grasses). The pore (ostiole) opens when guard cells become turgid (potassium-ion pump mechanism). Subsidiary cells (accessory cells) surround the guard cells. The whole unit = stomatal apparatus.
• Trichomes — hair-like outgrowths of epidermal cells. Unicellular or multicellular; glandular or non-glandular. Functions: reduce water loss, trap insects (in carnivorous plants), secrete oils (in Rosa, Pelargonium). Root hairs are unicellular non-glandular trichomes that increase root surface area enormously.
• Bulliform cells — enlarged, bubble-like epidermal cells in monocot (grass) leaves arranged in longitudinal rows on the adaxial surface. When the leaf loses water and wilts, bulliform cells collapse, causing the leaf blade to roll up and reduce transpiring surface. Xerophyte adaptation.

Stomata classification (Metcalfe & Chalk system):

• Anomocytic (irregular-celled) — no subsidiary cells; guard cells surrounded by ordinary epidermal cells. Ranunculaceae, Capparaceae.
• Anisocytic (unequal-celled) — 3 subsidiary cells, 1 smaller than the other two. Solanaceae, Cruciferae.
• Paracytic (parallel-celled) — 2 subsidiary cells with long axis parallel to guard cells. Rubiaceae, grasses.
• Diacytic (cross-celled) — 2 subsidiary cells with their common wall perpendicular to the long axis of guard cells. Caryophyllaceae, Acanthaceae.
• Tetracytic — 4 subsidiary cells (2 lateral + 2 polar). Monocots.
• Gramineous type — dumb-bell-shaped guard cells + 2 subsidiary cells. Poaceae.

3.2.2 Ground Tissue System

All tissues except the epidermis and vascular bundles constitute the ground tissue system. In stems it forms the cortex (between epidermis and vascular ring) and pith (inside the vascular ring). In roots it forms the cortex and the pericycle. In leaves it is the mesophyll. In monocot stems the ground tissue is undivided (no distinct cortex and pith) and vascular bundles are scattered within it. Primarily parenchymatous; cortex may be collenchymatous in young stems.

3.2.3 Vascular Tissue System

Consists of xylem and phloem organised into vascular bundles. Bundle types by arrangement:

• Collateral — xylem and phloem on the same radius, with xylem adaxial and phloem abaxial. Most common. Open (cambium present, secondary growth possible; dicot stem) or closed (no cambium; monocot stem).
• Bicollateral — phloem on both sides of xylem (external + internal phloem). Cucurbita (pumpkin) stem is the textbook example. Also in Solanaceae, Convolvulaceae. The internal (intraxylary) phloem is significant — it allows nutrient supply to both sides.
• Concentric — one tissue surrounds the other. Amphicribal / hadrocentric: xylem in centre, phloem surrounds it (ferns). Amphivasal / leptocentric: phloem in centre, xylem surrounds it (some monocot rhizomes, Iris, Acorus).
• Radial — xylem and phloem in separate alternating radii (not on same radius). Found in roots only. Not called a “bundle” in roots; xylem and phloem are exarch (protoxylem outermost).

3.3 Anatomy of Stem (Dicot & Monocot)

A transverse section (T.S.) of a stem shows the tissue arrangement from outside to inside. The fundamental difference between dicot and monocot stems lies in the distribution of vascular bundles and the nature of the hypodermis.

3.3.1 Dicot Stem (e.g., Helianthus annuus, sunflower)

From outside inward: Epidermis (single layer, cuticle, multicellular epidermal hairs) → Hypodermis (2–4 layers of collenchyma; provides mechanical support to young growing stem; this is the key dicot feature) → General cortex (parenchyma, may have chlorenchyma) → Endodermis (innermost cortical layer; contains starch granules = starch sheath; Casparian strips not well-developed in stem) → Pericycle (1–2 layers; sclerenchyma in patches over each vascular bundle — “bundle cap”) → Vascular bundles arranged in a ring (eustele); each bundle is open collateral with a cambium strip between xylem and phloem → Pith (large parenchymatous; medullary rays connect pith to cortex).

3.3.2 Monocot Stem (e.g., Zea mays, maize)

From outside inward: Epidermis (single layer, thick cuticle, no stomata in most) → Hypodermis (2–3 layers of sclerenchyma; provides rigidity; this is the key monocot stem feature) → Ground tissue (no clear differentiation into cortex and pith; the entire central region is parenchymatous with scattered vascular bundles) → Vascular bundles scattered throughout the ground tissue; more numerous and smaller toward periphery, fewer and larger toward centre. Each bundle is closed collateral (no cambium) and surrounded by a bundle sheath of sclerenchyma (thick-walled protective layer). No pith as a distinct region. No endodermis in stem.

Special cases — anomalous stems: Cucurbita stem has bicollateral vascular bundles (phloem on both sides of xylem — both external and internal/intraxylary phloem). Dracaena (a monocot) shows anomalous secondary thickening from a secondary thickening meristem (STM) in the cortex; it is not a true lateral meristem.

3.4 Anatomy of Root (Dicot & Monocot)

A T.S. of a young root reveals a fundamentally different organisation from the stem: there is no epidermis with cuticle but a rhizodermis / piliferous layer with root hairs; and the vascular arrangement is radial (xylem and phloem in separate alternating radii, never forming a joint bundle with each other). The xylem is always exarch.

3.4.1 Dicot Root (e.g., Mustard, Ranunculus)

From outside inward: Epiblema (rhizodermis) — single layer; no cuticle; unicellular root hairs (trichoblasts) greatly increase absorbing surface → Cortex — many layers of parenchyma; stores starch; has intercellular spaces for aeration → Endodermis — single innermost layer of cortex; cells have Casparian strips (bands of suberin deposited on radial and transverse walls); controls ion transport into the stele; passage cells (thin-walled cells without Casparian strips opposite protoxylem poles, allowing some apoplastic flow) → Pericycle — 1–2 layers of parenchyma or sclerenchyma; origin of lateral roots and vascular cambium (in secondary growth) → Vascular tissue — tetrarch (4 xylem poles / protoxylem groups alternating with 4 phloem groups); xylem bundles meet at centre = solid xylem core (typically no pith or very little) → Pith — absent or very small (parenchymatous) in dicot root.

3.4.2 Monocot Root (e.g., Zea mays, grass)

Similar in basic plan to dicot root but with important differences: Polyarch condition (many xylem poles — typically 6 or more, sometimes >12 in large monocots). Large pith is present at the centre (unlike dicot root). Cortex is proportionally thicker. Sclerenchyma is more common in the pericycle. Endodermis is typically well-developed with thick Casparian strips and often develops tertiary thickening (U-shaped thick walls — except opposite protoxylem). Passage cells (thin-walled endodermal cells opposite protoxylem) allow some apoplastic flux.

3.5 Anatomy of Leaf — Dorsiventral, Isobilateral, Centric

Leaf anatomy is described from a transverse section cut through the lamina. Three fundamental patterns are recognised based on mesophyll differentiation and stomate distribution.

3.5.1 Dorsiventral Leaf (Bifacial Leaf — typical dicot)

The upper (adaxial) and lower (abaxial) surfaces are structurally and functionally different — hence dorsiventral. From top (adaxial) to bottom (abaxial):

• Upper epidermis — single layer; thick cuticle; no chloroplasts; fewer or no stomata on adaxial surface (sunflower exception: stomata on both surfaces).
• Palisade mesophyll — 1–3 layers of elongated, chloroplast-rich cells immediately below upper epidermis; tightly packed; major site of photosynthesis; cells arranged perpendicular to leaf surface to maximise light capture.
• Spongy mesophyll — irregular, loosely packed cells with large intercellular air spaces; fewer chloroplasts than palisade; major function is gas exchange; connects to substomatal chambers below stomata.
• Lower epidermis — single layer; thin cuticle; stomata mainly on lower surface (hypostomatous; reduces transpiration by avoiding direct sunlight on stomata).
• Vascular bundles (midrib and veins) — each vein enclosed in a bundle sheath of parenchyma (C3 plants) or Kranz parenchyma (C4 plants with wreath anatomy, e.g., Saccharum). Xylem is adaxial (toward upper epidermis); phloem is abaxial (toward lower epidermis).

3.5.2 Isobilateral Leaf (Isolateral / Equifacial — typical monocot, grasses)

Both surfaces are structurally similar. Mesophyll is undifferentiated — no distinction into palisade and spongy layers; all mesophyll cells are compact and similar. Stomata on both surfaces (amphistomatous). Large bulliform cells (motor cells) present in the upper epidermis in longitudinal bands — they are large, colourless, thin-walled; when the leaf desiccates, these cells lose turgor and the leaf rolls up to reduce transpiration. This is the grass leaf xerophytic adaptation — a key HP exam topic (alpine grasses roll leaves in response to drought and cold-dry wind).

3.5.3 Centric Leaf (Terete / Cylindrical leaf)

Cross-section is circular; both adaxial and abaxial surfaces are identical. The mesophyll is undifferentiated and radiates outward from the central vascular bundle. Found in xerophytes and halophytes such as Pinus (pine needle), Opuntia, and many succulents. In Pinus needles: thick cuticle, hypodermis (sclerenchyma), transfusion tissue around the vascular bundle (unique gymnosperm feature), resin canals (schizogenous) in the mesophyll, sunken stomata with thickened outer walls (xerophyte adaptations).

3.6 Secondary Growth

Secondary growth increases the girth of a plant axis via the activity of two lateral meristems: the vascular cambium and the cork cambium (phellogen). It occurs primarily in dicots and gymnosperms; absent in most monocots. The German botanist Hugo von Mohl (1851) described cambial activity in detail; Sanio (1872) described the cambium as a ring, and Haberlandt systematised the anatomy.

3.6.1 Vascular Cambium

In a young dicot stem, cambium exists only within the vascular bundles as fascicular cambium (intrafascicular cambium) — the strip of meristematic cells between xylem and phloem. As secondary growth begins, parenchyma cells of the medullary rays between vascular bundles de-differentiate to form interfascicular cambium. Together, fascicular + interfascicular cambium form a complete cambial ring.

The cambial ring is made of two types of initials: fusiform initials (elongated; give rise to axial system — vessel members, fibres, tracheids, sieve tube elements) and ray initials (isodiametric; give rise to vascular rays that run radially — facilitate radial transport).

Direction of secondary tissue deposition: Cambium cuts off cells inward (toward pith) → these differentiate into secondary xylem (wood). Cambium cuts off cells outward (toward cortex) → these differentiate into secondary phloem. Secondary xylem accumulates in far greater quantity (roughly 10:1 ratio) because more cells are laid down inward. Secondary phloem is typically narrow and may be crushed by expanding wood.

3.6.2 Annual Rings

In temperate climates (and in tropical regions with distinct dry seasons) the cambium is seasonally active. In spring/early season it produces spring wood (early wood): wide-lumen, thin-walled vessels that conduct large volumes of water to meet the demand of new leaves. In autumn/late season it produces autumn wood (late wood): narrow-lumen, thick-walled vessels with high density. One spring + one autumn zone = one annual ring. In cross-section, annual rings appear as alternating light (spring wood) and dark (autumn wood) bands. Counting rings gives the tree’s age — dendrochronology.

3.6.3 Heartwood and Sapwood

As secondary xylem accumulates, the innermost, older wood loses conductive function. Xylem parenchyma cells deposit resins, tannins, gums, oils, and coloured compounds (terpenes, flavonoids) into the central wood cells. The parenchyma also sends protrusions into vessel lumens through pit membranes — these are tyloses (singular: tylosis). Tyloses block the vessels permanently. This central non-conducting zone is the heartwood (duramen); it is typically darker, harder, and denser. The outer, conducting zone of younger wood is the sapwood (alburnum); it is lighter, softer, and moist.

3.6.4 Cork Cambium (Phellogen) and Periderm

As the vascular cambium produces secondary xylem and phloem, the girth of the axis increases. The original epidermis cannot stretch indefinitely and eventually ruptures. To replace it, the cork cambium (phellogen) arises from the outer cortex (or pericycle or old phloem depending on the species) — first in the outer cortex (in most dicots), then progressively deeper in older stems.

Phellogen produces:

• Phellem (cork) outward — dead, suberised cells; waterproof; protective; commercial cork from Quercus suber (cork oak).
• Phelloderm inward — living parenchyma cells; photosynthetic in young stems.

The three-layered unit (phellem + phellogen + phelloderm) = periderm. The periderm replaces the epidermis as the outer covering of mature stems and roots.

Lenticels are raised, lens-shaped areas in the periderm where the phellem is loose and has large intercellular spaces. They allow gas exchange between the internal tissues and atmosphere (replaced the role of stomata). Found prominently on potato tubers, apple bark, cherry bark.

Bark (in common usage) = all tissues outside the vascular cambium = secondary phloem + periderm. In commercial timber usage, bark is stripped before processing.

3.7 The Angiosperm Life Cycle — Embryology Overview

Angiosperms exhibit a diplontic life cycle in which the diploid sporophyte generation is dominant and the haploid gametophyte generation is extremely reduced (to a few cells wholly dependent on the sporophyte). The Hofmeister-Strasburger principle of alternation of generations applies: sporophyte (2n) → meiosis → spores (n) → gametophyte (n) → gametes (n) → fertilisation → sporophyte (2n).

Camerarius — demonstrated sex in plants, 1694 · Hofmeister — alternation of generations in plants, 1851 · Strasburger — plant cytology + fertilisation, 1884 · Nawaschin (Navashin) — double fertilisation discovered, 1898 (in Lilium and Fritillaria) · Hanstein — root meristem histogen theory, 1868

In the angiosperm life cycle, the flower is the reproductive structure. The stamen (androecium) produces the male gametophyte (pollen grain = reduced to 3 cells in most angiosperms), and the carpel (gynoecium) houses the female gametophyte (embryo sac = 7 cells, 8 nuclei). A key angiosperm synapomorphy is double fertilisation: both male gametes participate in fertilisation events (one fertilises the egg; the other fuses with the polar nuclei).

3.8 Microsporangium, Megasporangium & Embryo Sac

3.8.1 Microsporangium (Pollen Sac)

The anther is typically dithecous (two-lobed, four microsporangia / pollen sacs in two pairs — one pair per lobe). A single microsporangium in T.S. shows four distinct wall layers (from outside to inside):

1. Epidermis (exothecium) — outermost protective layer of the anther.
2. Endothecium — single layer of fibrous-banded cells; radial thickenings of cellulose; help in anther dehiscence by differential drying (hygroscopic mechanism) — the fibrous bands shrink unevenly, splitting the anther along the stomium (a line of weakness between the two microsporangia of one lobe). This layer causes dehiscence — a favourite exam question.
3. Middle layers (1–3) — ephemeral; crushed as the anther matures; may transfer nutrients to the tapetum.
4. Tapetum — innermost layer, directly surrounding the microspore mother cells; nutritive; binucleate in many species (polyploid); rich in RNA and proteins; two types: secretory (glandular) tapetum (remains in place, secretes nutrients) and amoeboid (periplasmodial) tapetum (cells lose their walls, cytoplasm flows among microspores). The tapetum contributes to the exine formation of the pollen wall via sporopollenin deposition and also produces ubisch bodies (orbicules) on its inner tangential wall.

Microsporogenesis: Microspore mother cells (MMCs, 2n) within the microsporangium undergo meiosis to produce a tetrad of microspores (n). The microspores separate from the tetrad and each develops into a pollen grain (immature male gametophyte). The pollen grain wall has two layers: outer exine (made of sporopollenin — most resistant biological substance; responsible for pollen grain persistence in geological record — basis of palynology) and inner intine (cellulose-pectin; forms the pollen tube). Exine has apertures (colpi — elongated; pores — rounded) through which the pollen tube emerges.

Microgametogenesis: The microspore nucleus divides mitotically → large vegetative (tube) cell and small generative cell. In most angiosperms pollen is shed at this 2-celled stage; the generative cell divides again (in pollen tube or in pollen grain before shedding) → two male gametes. Mature pollen = 3-celled in some species (e.g., grasses, Capsella) or 2-celled at shedding (generative divides in tube).

3.8.2 Megasporangium and Megasporogenesis

The ovule is the megasporangium + integuments — it develops into the seed after fertilisation. Parts of the ovule:

• Funicle — stalk attaching ovule to placenta; vascular strand runs through it.
• Hilum — junction of funicle and ovule body (visible as a scar on seed).
• Integuments — protective layers. Unitegmic (1 integument; typical of Asteraceae, Solanaceae) or bitegmic (2 integuments; most common in angiosperms). Outer integument → testa; inner integument → tegmen.
• Micropyle — small pore at apex of integuments; entry of pollen tube and water absorption in seeds.
• Nucellus — the megasporangium tissue proper; encloses the embryo sac; nutritive; degenerates as embryo sac grows (in most species) but persists as perisperm in some (e.g., Piper nigrum — black pepper; Phoenix dactylifera — date).
• Chalaza — basal region where integuments merge with nucellus; opposite end to micropyle.
• Raphe — ridge on anatropous ovule formed by fusion of funicle with integument along its length.

Types of ovule by curvature: Orthotropous (atropous) — straight, micropyle up (pepper, Polygonum); anatropous — completely inverted (micropyle at base, near hilum; most common in angiosperms, e.g., sunflower, legumes); campylotropous — curved (legumes); hemitropous — half-turned; circinotropous — funicle coils around ovule (cactus).

Megasporogenesis: A single archesporial cell in the nucellus differentiates into the megaspore mother cell (MMC, 2n). MMC undergoes meiosis (megasporogenesis) to produce a linear tetrad of 4 megaspores (n). In the Polygonum (monosporic) type (most common — ~70% of angiosperms): three of the four megaspores degenerate (usually the three micropylar ones); the chalazal megaspore survives and develops into the embryo sac.

3.8.3 Embryo Sac (Female Gametophyte — Polygonum Type)

The surviving functional megaspore undergoes 3 successive mitotic divisions (without cytokinesis between the first two) → 8 nuclei → cell walls form → 7 cells, 8 nuclei. This is the Polygonum-type embryo sac (monosporic, 8-nucleate, 7-celled).

Organisation of the Polygonum embryo sac:

• Egg cell (oosphere) — 1 cell at the micropylar end; larger; the female gamete; fertilised by one male gamete → zygote (2n).
• Synergids — 2 cells flanking the egg at the micropylar end; have filiform apparatus (finger-like projections into the micropyle end) that guide the pollen tube and control its entry/discharge; produce chemotropic signals (e.g., defensins, LURE peptides in Torenia). One synergid degenerates upon pollen tube discharge. Together = egg apparatus (egg + 2 synergids).
• Central cell — 1 large central cell with 2 polar nuclei (one from each pole — the two nuclei that migrated to the centre from the micropylar and chalazal poles); may fuse before fertilisation to form the secondary nucleus (2n) or remain separate; fertilised by second male gamete → primary endosperm cell (3n) → endosperm.
• Antipodal cells — 3 cells at the chalazal end (opposite to egg); ephemeral; may proliferate in some species (Triticum — wheat has ~40 antipodals at maturity). Function unclear; possibly nutritive. They degenerate after fertilisation.

3.9 Pollination, Fertilisation & Endosperm

3.9.1 Pollination

Pollination is the transfer of pollen from the anther to the stigma of the same or different flower. It is a prerequisite for fertilisation but not fertilisation itself (a commonly confused point). Two broad types:

Self-pollination (autogamy) — pollen transfers to stigma of the same flower or genetically identical flower on the same plant. Promoted by cleistogamy (flower never opens; obligate selfing, e.g., Commelina benghalensis underground flowers, Viola ground flowers). Also by homogamy (anthers and stigma mature simultaneously).

Cross-pollination (allogamy) — pollen transfer between genetically different plants. Promotes genetic variability. Mechanisms (agents) of cross-pollination:

Outbreeding devices (mechanisms that prevent self-pollination and promote cross-pollination): unisexuality (dioecious: Papaya, Cannabis; monoecious: maize — male tassel + female cob); dichogamy (protandry — anthers mature before stigma, e.g., Salvia; protogyny — stigma matures before anthers, e.g., Mirabilis); herkogamy (physical barrier between anther and stigma); self-incompatibility (SI — genetic mechanism preventing pollen tube germination on same-plant stigma; gametophytic SI: Nicotiana; sporophytic SI: Brassicaceae); heterostyly (pin + thrum morphs in Primula).

3.9.2 Double Fertilisation

Double fertilisation is the unique angiosperm process discovered by Nawaschin (Navashin) in 1898 working with Lilium and Fritillaria. Events:

1. Pollen grain lands on stigma; pollen tube germinates from the vegetative cell growing through the style toward the ovule via chemotropism (LURE defensin-like peptides secreted by synergids, discovered in Torenia fournieri).
2. Pollen tube enters ovule through micropyle (porogamy, most common) or through chalaza (chalazogamy, e.g., Casuarina) or through integuments (mesogamy).
3. Pollen tube tip enters one synergid, which degenerates; two male gametes are discharged.
4. Syngamy: Male gamete 1 + egg cell → zygote (2n) → develops into embryo.
5. Triple fusion: Male gamete 2 + 2 polar nuclei (or secondary nucleus) → primary endosperm nucleus (PEN; 3n, triploid) → develops into endosperm.

Why “double” fertilisation? Two separate fertilisation events occur simultaneously. Both require male gametes (hence “double”). The second event (triple fusion) is technically a fusion of one sperm + two nuclei = three nuclei total merging, hence also called triple fusion. The entire process from pollen landing to double fertilisation is sometimes called siphonogamy (pollen tube fertilisation, characteristic of seed plants).

3.9.3 Embryo Development

The zygote undergoes a period of rest (dormancy) then divides. In Capsella bursa-pastoris (shepherd’s purse — the classic textbook embryo development model):

1. Proembryo stage — zygote divides transversely into a terminal apical cell (smaller, dense) and a basal cell (larger, vacuolated). Basal cell gives rise to the suspensor (a chain of cells that pushes the embryo into the endosperm and absorbs nutrients). Apical cell develops into the embryo proper.
2. Globular stage — apical cell undergoes organised divisions → roughly spherical proembryo. Radial symmetry.
3. Heart stage — two cotyledon primordia emerge → heart shape. Bilateral symmetry appears. Embryo axis differentiates into plumule (future shoot) and radicle (future root).
4. Torpedo stage — cotyledons elongate; hypocotyl (between radicle and cotyledon attachment) becomes prominent. Protoderm, ground meristem, procambium are distinct.
5. Mature embryo — in dicots: 2 cotyledons, plumule, radicle, hypocotyl. In monocots: 1 cotyledon (scutellum in grasses); plumule protected by coleoptile; radicle protected by coleorhiza.

Seed = fertilised and mature ovule. Components: seed coat (testa from outer integument + tegmen from inner integument) + endosperm (3n, may be consumed by embryo before maturity → non-endospermic seed, e.g., pea, bean; or retained → endospermic seed, e.g., castor, maize) + embryo.

3.9.4 Apomixis, Polyembryony, Parthenocarpy

Apomixis = seed formation without fertilisation. Types: agamospermy (embryo from unfertilised egg or nucellar cells); vegetative reproduction considered separate. Nucellar polyembryony: somatic embryos develop from nucellar (2n) cells alongside the sexually produced embryo → multiple embryos in one seed. Examples: Citrus (often 5–10 embryos per seed, most from nucellar cells), Mangifera indica (mango — polyembryony common in certain varieties). Advantage: produces true-breeding (clonal) seedlings — important in horticulture.

Polyembryony = presence of more than one embryo in a single seed. Causes: nucellar polyembryony (most common), cleavage polyembryony (zygote splits; as in some orchids), adventive polyembryony.

Parthenocarpy = fruit development without fertilisation (seeds absent or non-viable). Examples: banana (Musa paradisiaca), seedless grape, seedless watermelon (induced by colchicine-produced triploids), some papaya. May be natural or induced (by application of auxins, gibberellins).

3.10 Quick-Reference Tables

End of Chapter 3 · Plant Anatomy. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.