Plant Development, Anatomy and Embryology BBO4T08

Syllabus topics (official RTMNU)

1. Tissue: Definition, Characteristics of Meristematic tissue; Classification of meristems based on position.
2. Simple Permanent Tissue – Structure and Role of Parenchyma, Collenchyma, and Sclerenchyma
3. Complex Permanent Tissue – Structure and Role of Xylem and Phloem
4. Apical meristem of shoot and root: Histogen theory, Tunica-Corpus theory, Newman's theory
5. Cambium: Types and functions.

Tissue: Definition, Characteristics of Meristematic tissue; Classification of meristems based on position

Tissue: A group of cells that are similar (or dissimilar) in structure and origin and perform a common function. The term was given by N. Grew. Plant tissues are of two main types: meristematic (dividing) and permanent (non-dividing, differentiated).

Meristematic tissue (meristem): A group of immature cells that keep the power of cell division. Term coined by Nägeli (1858).

• Cells are small, living, isodiametric (round, oval or polygonal).
• Cell wall is thin, made of cellulose (primary wall only).
• Dense cytoplasm with a large, prominent nucleus.
• Vacuoles absent or very small.
• Cells are compactly arranged; no intercellular spaces.
• Ergastic (reserve) substances absent; high metabolic rate.

Classification based on position:

Simple Permanent Tissue – Structure and Role of Parenchyma, Collenchyma, and Sclerenchyma

Simple permanent tissue is made of only one type of cell, similar in structure and function.

1. Parenchyma: Living, thin-walled, isodiametric cells with intercellular spaces; large central vacuole. Found in cortex, pith, mesophyll, fruit pulp.

• Role: storage of food, photosynthesis (as chlorenchyma), buoyancy and gas exchange (as aerenchyma in hydrophytes), wound healing, gives rise to secondary meristem.

2. Collenchyma: Living cells with walls unevenly thickened with cellulose and pectin, mostly at corners. Found below the epidermis (hypodermis) of dicot stems and petioles; absent in monocots and roots.

• Types: angular (thickening at corners), lamellar (on tangential walls), lacunar (around intercellular spaces).
• Role: mechanical support with flexibility to young stems and leaves; may contain chloroplasts and photosynthesize.

3. Sclerenchyma: Dead cells with thick, lignified walls having simple pits; lumen very narrow.

• Fibres: long, narrow, pointed ends; e.g., jute, flax, hemp.
• Sclereids (stone cells): short, isodiametric or irregular; e.g., pear pulp (grit), coconut shell, seed coats of legumes.
• Role: mechanical strength and rigidity; protection.

Complex Permanent Tissue – Structure and Role of Xylem and Phloem

Complex tissue is made of more than one type of cell working together as a unit. Xylem and phloem together form the vascular tissue.

Xylem (term by Nägeli) – conducts water and minerals upward and gives mechanical support. Elements:

1. Tracheids: dead, elongated, tapering cells with lignified walls and bordered pits; main conducting element in pteridophytes and gymnosperms.
2. Vessels (tracheae): dead, tube-like rows of cells whose end walls are dissolved (perforation plates); characteristic of angiosperms.
3. Xylem fibres: dead, give support.
4. Xylem parenchyma: the only living element; stores food and helps lateral conduction.

Phloem – conducts organic food (mainly sucrose) from leaves to other parts. Elements:

1. Sieve tubes: living, enucleate at maturity, end walls with sieve plates; plugged in winter by callose.
2. Companion cells: living, nucleated, connected to sieve tubes by plasmodesmata; control sieve tube activity (absent in pteridophytes and gymnosperms, where albuminous cells occur).
3. Phloem parenchyma: living, storage; absent in most monocots.
4. Phloem fibres (bast fibres): the only dead element; e.g., jute.

Apical meristem of shoot and root: Histogen theory, Tunica-Corpus theory, Newman's theory

1. Histogen theory – Hanstein (1868): The apex has three distinct zones (histogens), each giving rise to a definite tissue.

• Dermatogen (outermost) → epidermis.
• Periblem (middle) → cortex.
• Plerome (innermost) → stele (vascular tissue, pericycle, pith).
• In roots an extra histogen, calyptrogen, forms the root cap (in monocots).
• Limitation: in the shoot apex these zones are not clearly separable, so the theory fits root apex better.

2. Tunica-Corpus theory – A. Schmidt (1924): For the shoot apex only, based on the plane of cell division.

• Tunica: outer one to several layers with anticlinal divisions only (increase in surface area) → epidermis (and some outer tissues).
• Corpus: inner mass of cells dividing in all planes (increase in volume) → cortex and stele.

3. Newman's theory (1965) – Apical cell group / continuing meristematic residue: Newman classified apices by the number and behaviour of initials: monoplex (single apical cell, e.g., pteridophytes), simplex (one superficial layer of initials dividing in all planes, e.g., many gymnosperms) and duplex (two zones: outer layer with anticlinal divisions and inner zone dividing in all planes, e.g., angiosperms). Newman's view stresses that growth comes from a continuing meristematic residue rather than from fixed histogens. (For the root apex, the related Körper-Kappe theory of Schüepp, 1917, explains growth by T-shaped divisions.)

Cambium: Types and functions

Cambium is a lateral meristem responsible for secondary growth (increase in thickness).

• Vascular cambium: made of fusiform initials (long, give rise to axial xylem and phloem elements) and ray initials (short, give rise to vascular rays).
  • Intrafascicular cambium – present inside vascular bundles (primary in origin).
  • Interfascicular cambium – formed from medullary ray parenchyma between bundles (secondary in origin).
• Cork cambium (phellogen): develops in cortex or epidermis; forms cork (phellem) outside and phelloderm inside.
• Based on arrangement: storied (initials in horizontal tiers) and non-storied (overlapping initials).

Functions: produces secondary xylem inside and secondary phloem outside; forms vascular rays for radial conduction; cork cambium forms protective periderm; helps in wound healing.

5 likely exam questions

Long answer

1. Describe the structure and functions of the complex permanent tissues xylem and phloem with suitable diagrams.
2. Explain the organisation of shoot and root apex with reference to Histogen theory, Tunica-Corpus theory and Newman's theory.

Short answer / short notes

3. Write the characteristics of meristematic tissue and classify meristems based on position.
4. Differentiate between parenchyma, collenchyma and sclerenchyma.
5. Write a short note on types and functions of cambium.

Syllabus topics (official RTMNU)

1. Types of vascular bundles: Radial, Conjoint and Concentric.
2. Normal Primary structure of root: Dicot (Sunflower) and Monocot (Maize)
3. Normal Primary structure of stem: Dicot (Sunflower) and Monocot (Maize)
4. Normal secondary growth in dicot stem: Sunflower
5. Anomalous Secondary growth in stem: Dicot (Bignonia) and Monocot (Dracaena)

Types of vascular bundles: Radial, Conjoint and Concentric

A vascular bundle is a strand of xylem and phloem (with or without cambium) running through the plant body.

• Radial: xylem and phloem lie on different radii, alternating with each other. Typical of roots.
• Conjoint: xylem and phloem lie on the same radius, together in one bundle. Typical of stems and leaves.
  • Collateral: phloem towards outside, xylem towards inside. Open if cambium present (dicot stem, e.g., sunflower); closed if absent (monocot stem, e.g., maize).
  • Bicollateral: phloem on both outer and inner sides of xylem, with outer and inner cambium; e.g., Cucurbitaceae (Cucurbita).
• Concentric: one tissue completely surrounds the other.
  • Amphicribral (hadrocentric): phloem surrounds xylem; e.g., ferns (Pteris, Polypodium).
  • Amphivasal (leptocentric): xylem surrounds phloem; e.g., Dracaena, Yucca (secondary bundles).

Normal Primary structure of root: Dicot (Sunflower) and Monocot (Maize)

T.S. of dicot root (Helianthus) – outline circular; from outside to inside:

1. Epiblema (piliferous layer): single layer, no cuticle, bears unicellular root hairs.
2. Cortex: many layers of thin-walled parenchyma with intercellular spaces; stores food.
3. Endodermis: barrel-shaped cells with Casparian strips; passage cells opposite protoxylem.
4. Pericycle: single layer; origin of lateral roots and (later) cambium.
5. Vascular bundles: radial, exarch, xylem usually tetrarch (2–6 groups).
6. Conjunctive tissue: parenchyma between xylem and phloem.
7. Pith: very small or absent.

T.S. of monocot root (Zea mays) – similar plan, but: xylem polyarch (many groups, often more than 6), large well-developed pith, endodermis cells with thick walls (passage cells present), conjunctive tissue often sclerenchymatous, and no secondary growth.

Normal Primary structure of stem: Dicot (Sunflower) and Monocot (Maize)

T.S. of young dicot stem (Helianthus):

1. Epidermis: single layer with cuticle and multicellular hairs.
2. Hypodermis: 3–5 layers of collenchyma.
3. General cortex: parenchyma.
4. Endodermis: wavy layer rich in starch – starch sheath.
5. Pericycle: patches of sclerenchyma (bundle caps) alternating with parenchyma.
6. Vascular bundles: arranged in a ring; conjoint, collateral, open, endarch.
7. Medullary rays between bundles; large parenchymatous pith.

T.S. of monocot stem (Zea mays):

• Epidermis with cuticle, no hairs; hypodermis of sclerenchyma.
• Ground tissue not differentiated into cortex, endodermis, pericycle and pith.
• Vascular bundles scattered, smaller and crowded towards periphery, larger in centre.
• Bundles conjoint, collateral, closed, endarch, surrounded by a sclerenchymatous bundle sheath; skull-like appearance with two metaxylem vessels and a lysigenous water cavity below protoxylem; phloem parenchyma absent.

Normal secondary growth in dicot stem: Sunflower

Secondary growth is the increase in girth due to the activity of lateral meristems (vascular cambium and cork cambium).

1. Formation of cambium ring: intrafascicular cambium is present in bundles; parenchyma of medullary rays between bundles becomes meristematic to form interfascicular cambium. Both join to form a complete cambium ring.
2. Activity of cambium: cells divide periclinally; cells cut towards inside become secondary xylem, towards outside secondary phloem. More xylem than phloem is produced. Primary phloem is crushed outward; primary xylem stays near pith.
3. Secondary medullary rays: ray initials form narrow bands of parenchyma through secondary xylem and phloem for radial conduction.
4. Periderm formation: phellogen arises in outer cortex and forms cork outside and phelloderm inside; lenticels develop.

Anomalous Secondary growth in stem: Dicot (Bignonia) and Monocot (Dracaena)

Anomalous secondary growth is secondary growth that deviates from the normal pattern (abnormal cambium position, activity or extra cambia).

1. Bignonia (a liana/climber):

• Normal cambium ring forms, but at four points the cambium produces less xylem and more phloem.
• As a result the xylem shows four furrows filled with phloem, giving a cross-shaped (ridged and furrowed) xylem cylinder.
• Cambium segments in furrows get cut off and move outward in steps (ladder-like); the phloem wedges become wider.
• Significance: flexibility to climbing stem and protection of phloem.

2. Dracaena (a monocot tree):

• Monocots normally lack cambium; in Dracaena a secondary cambium arises from parenchyma of the outer cortex (outside the primary bundles).
• Towards inside it forms secondary vascular bundles embedded in lignified conjunctive tissue; towards outside it forms a little parenchyma.
• Secondary bundles are amphivasal (leptocentric) – xylem surrounding phloem; primary bundles remain collateral and closed.
• Significance: increase in thickness of a monocot tree; e.g., also in Yucca, Agave.

5 likely exam questions

Long answer

1. Describe normal secondary growth in dicot stem (Sunflower) with labelled diagrams.
2. Explain anomalous secondary growth in Bignonia and Dracaena stem with suitable diagrams.

Short answer / short notes

3. Classify vascular bundles with one example of each type.
4. Give differences between the primary structure of dicot root (Sunflower) and monocot root (Maize).
5. Describe the structure of a vascular bundle of maize stem.

Syllabus topics (official RTMNU)

1. Periderm: Composition, functions, Lenticel, Bark and Commercial cork
2. Secondary structures: Growth rings, Sap wood, Heart wood and Tyloses
3. Anatomy of leaf: Dicot (Nerium) and Monocot (Maize)
4. Nodal anatomy: Leaf trace, leaf gap, branch trace and branch gap.

Periderm: Composition, functions, Lenticel, Bark and Commercial cork

When the stem grows in girth, the epidermis ruptures and is replaced by a protective tissue called periderm.

Composition (3 parts):

1. Phellogen (cork cambium): secondary lateral meristem arising usually in outer cortex (sometimes epidermis, hypodermis or pericycle). Divides periclinally.
2. Phellem (cork): formed towards outside; dead, compactly arranged cells with walls impregnated with suberin; impervious to water and gases.
3. Phelloderm (secondary cortex): formed towards inside; living parenchyma, may store food or have chloroplasts.

Functions: protection against water loss, injury, insects and pathogens; heat insulation; healing of wounds; gaseous exchange through lenticels.

Lenticel: a small raised, lens-shaped pore on the bark of woody stems. Below a stoma, phellogen produces loosely arranged, rounded cells with large intercellular spaces called complementary cells (filling tissue), which push out and rupture the epidermis. Function: gaseous exchange and transpiration (lenticular transpiration). Absent in climbers like Vitis.

Bark: non-technical term for all tissues outside the vascular cambium (secondary phloem + periderm + any tissue outside). Technically, dead tissue outside the innermost phellogen is rhytidome.

• Ring bark: phellogen forms complete rings; bark peels as sheets – e.g., Betula (birch), Eucalyptus.
• Scale bark: phellogen forms in overlapping patches; bark peels as scales – e.g., Pinus, Psidium (guava), mango.
• Early (soft) bark forms in early season; late (hard) bark at end of season.

Commercial cork: obtained from the cork oak Quercus suber (Mediterranean region). The first cork (virgin cork) is removed when the tree is about 20 years old; the new phellogen then forms thicker, more uniform cork that is stripped every 8–10 years. Uses: bottle stoppers, insulation, shoe soles, floats, sports goods, linoleum.

Secondary structures: Growth rings, Sap wood, Heart wood and Tyloses

Growth rings (annual rings): In temperate regions cambium activity changes with season.

• Spring wood (early wood): formed when cambium is very active; vessels wide, thin-walled; light coloured, low density.
• Autumn wood (late wood): formed when cambium is less active; vessels narrow, thick-walled, more fibres; dark, dense.
• One spring wood + one autumn wood = one annual ring. Counting rings gives the age of the tree – dendrochronology. Rings are not distinct in tropical trees with uniform climate.

Tyloses: balloon-like outgrowths of adjacent xylem parenchyma or ray cells that enter the lumen of vessels through pits. They block the vessels; may get filled with tannins, resins or gums. Tyloses are common in heartwood and make it non-conducting and durable. Also formed after injury or infection to stop spread of pathogens. Example: Vitis, Quercus.

Anatomy of leaf: Dicot (Nerium) and Monocot (Maize)

V.S. of Nerium leaf (dorsiventral, xerophytic):

• Multiple epidermis (2–3 layered) on both surfaces with a thick cuticle – reduces transpiration.
• Lower epidermis has deep depressions called stomatal crypts (pits) lined with hairs; stomata are present only inside the crypts (sunken stomata).
• Mesophyll differentiated into palisade (2–3 layers below upper epidermis; a palisade layer may also occur above lower epidermis) and spongy parenchyma in the middle.
• Midrib has a large bicollateral vascular bundle (phloem on both sides of xylem) with collenchyma; calcium oxalate crystals (druses) present.

V.S. of Zea mays leaf (isobilateral):

• Upper and lower epidermis with cuticle; stomata on both surfaces (amphistomatic), with dumb-bell shaped guard cells.
• Upper epidermis has large, thin-walled bulliform (motor) cells that help in rolling of leaf during water stress.
• Mesophyll not differentiated into palisade and spongy.
• Parallel venation: many closed, collateral vascular bundles each surrounded by a bundle sheath of large chloroplast-rich cells – Kranz anatomy (C₄ plant). Sclerenchyma patches above and below larger bundles.

Nodal anatomy: Leaf trace, leaf gap, branch trace and branch gap

Node is the part of the stem from which leaves and branches arise; the vascular connection between stem and leaf is studied in nodal anatomy.

• Leaf trace: the vascular bundle(s) that branch off from the stele of the stem and enter the leaf base.
• Leaf gap (lacuna): the parenchymatous region left in the vascular cylinder of the stem just above the point where the leaf trace departs.
• Branch trace: vascular strands (usually two) that leave the stem stele and enter an axillary branch.
• Branch gap: the parenchymatous gap in the stem stele above the departing branch traces; it usually merges with the leaf gap of the subtending leaf.

Types of nodes (Sinnott, 1914) based on number of leaf gaps per leaf:

1. Unilacunar: one gap per leaf; e.g., Calotropis, many Ericaceae.
2. Trilacunar: three gaps per leaf; e.g., Salix, many Rosaceae.
3. Multilacunar: many gaps; e.g., Polygonum, Aralia, many monocots with sheathing leaf bases.

5 likely exam questions

Long answer

1. What is periderm? Describe its composition, development and functions. Add a note on lenticels and commercial cork.
2. Describe the internal structure of the leaf of Nerium and Maize with labelled diagrams and give the differences between them.

Short answer / short notes

3. Differentiate between sap wood and heart wood.
4. What are tyloses? Give their significance.
5. Write a short note on leaf trace and leaf gap.

Syllabus topics (official RTMNU)

1. Pollination: Types and Significance.
2. Anther: T.S. of mature anther, microsporogenesis; structure of pollen grain and development of male gametophyte.
3. Ovule: Types of ovule, Structure of anatropous ovule, megasporogenesis, Development of female gametophyte (Polygonum type)
4. Fertilization: Double fertilization and triple fusion, endosperm and its types, structure of dicot embryo (Onagrad type) and monocot embryo.

Pollination: Types and Significance

Pollination is the transfer of pollen grains from the anther to the stigma of a flower.

Types:

1. Self-pollination (autogamy): pollen reaches the stigma of the same flower. Devices: bisexuality, homogamy (anther and stigma mature together), cleistogamy (flowers never open – e.g., Commelina, Viola, Oxalis).
2. Geitonogamy: pollen from another flower of the same plant; functionally cross, genetically self-pollination.
3. Cross-pollination (xenogamy/allogamy): pollen from a flower of a different plant of the same species. Devices: dicliny (unisexual flowers), dichogamy – protandry (anthers mature first, e.g., sunflower) or protogyny (stigma matures first, e.g., Magnolia), herkogamy (physical barrier, e.g., Calotropis), heterostyly (e.g., Primula), self-incompatibility.

Agents of cross-pollination:

Significance: brings male and female gametes together for fertilization and seed/fruit formation; self-pollination maintains pure lines; cross-pollination produces genetic variation, hybrid vigour and better adaptation, and is used in plant breeding.

Anther: T.S. of mature anther, microsporogenesis; structure of pollen grain and development of male gametophyte

T.S. of mature anther: a typical anther is dithecous and tetrasporangiate (four pollen sacs), joined by a connective with a vascular strand. Wall layers from outside:

1. Epidermis: protective.
2. Endothecium: cells with fibrous thickenings; helps in dehiscence. Thin-walled cells at the junction of two sacs form the stomium.
3. Middle layers: 1–3 layers, degenerate at maturity.
4. Tapetum: innermost, cells often bi- or multinucleate; nourishes developing pollen; forms Ubisch bodies, sporopollenin and pollenkitt. Types: secretory (glandular) and amoeboid (plasmodial).

Microsporogenesis: formation of microspores from microspore mother cells (MMC). Sporogenous tissue in each pollen sac develops into diploid MMCs (2n). Each MMC undergoes meiosis to form a tetrad of four haploid microspores (n). Tetrads may be tetrahedral (most common in dicots), isobilateral (common in monocots), decussate, linear or T-shaped. Microspores separate and become pollen grains.

Structure of pollen grain: wall has two layers:

• Exine: outer, thick, made of sporopollenin (most resistant organic material); sculptured; has thin areas called germ pores.
• Intine: inner, thin, of cellulose and pectin; forms the pollen tube.

Development of male gametophyte:

1. The microspore enlarges, a vacuole forms and the nucleus moves to the wall.
2. First mitosis (unequal) forms a large vegetative (tube) cell and a small generative cell. Pollen is mostly shed at this 2-celled stage (about 60% of angiosperms).
3. The generative cell divides (in pollen or in pollen tube) to form two male gametes – 3-celled stage.
4. On stigma, intine comes out through a germ pore as the pollen tube, carrying the tube nucleus and two male gametes.

Ovule: Types of ovule, Structure of anatropous ovule, megasporogenesis, Development of female gametophyte (Polygonum type)

Types of ovule (based on the position of micropyle, chalaza and funicle):

• Orthotropous (atropous): straight; micropyle, chalaza and funicle in one line – Polygonum, Piper.
• Anatropous: inverted, micropyle close to funicle – most common (about 80% of families).
• Hemianatropous: body at right angle to funicle – Ranunculus.
• Campylotropous: body curved, embryo sac slightly curved – Brassicaceae (mustard), pea.
• Amphitropous: body and embryo sac horse-shoe shaped – Lemna, Alisma.
• Circinotropous: funicle coiled around the ovule – Opuntia.

Structure of anatropous ovule: attached to placenta by a stalk, the funicle; point of attachment is the hilum. The funicle fuses with the ovule body to form a ridge, the raphe. Body has the nucellus (main mass) covered by one or two integuments, leaving an opening, the micropyle. The basal region opposite micropyle is the chalaza. The embryo sac (female gametophyte) lies in the nucellus near the micropyle.

Megasporogenesis: a hypodermal archesporial cell in nucellus forms the megaspore mother cell (MMC, 2n), which undergoes meiosis to form a linear tetrad of four haploid megaspores. Usually the chalazal megaspore is functional and the three micropylar ones degenerate (monosporic development).

Development of female gametophyte – Polygonum type (monosporic, 8-nucleate, 7-celled; found in about 70% of angiosperms):

1. Functional megaspore enlarges; its nucleus divides by mitosis to form 2 nuclei that move to opposite poles.
2. Two more mitotic divisions give 4 nuclei at each pole (total 8, free nuclear).
3. One nucleus from each pole moves to the centre – the two polar nuclei; they fuse to form the secondary nucleus (2n).
4. Three micropylar nuclei form the egg apparatus: one egg and two synergids (with filiform apparatus).
5. Three chalazal nuclei form three antipodal cells.

Fertilization: Double fertilization and triple fusion, endosperm and its types, structure of dicot embryo (Onagrad type) and monocot embryo

The pollen tube usually enters the ovule through the micropyle (porogamy), enters one synergid and releases the two male gametes.

• Syngamy: one male gamete (n) fuses with the egg (n) → zygote (2n) → embryo.
• Triple fusion: the second male gamete (n) fuses with the two polar nuclei / secondary nucleus → primary endosperm nucleus (3n) → endosperm.
• Since two fusions occur in one embryo sac, it is called double fertilization, discovered by S. G. Nawaschin (1898) in Lilium and Fritillaria. It is a unique feature of angiosperms.

Endosperm is the triploid nutritive tissue for the developing embryo. Types (based on development):

1. Nuclear: free nuclear divisions first, walls formed later – most common; e.g., coconut water is free-nuclear endosperm, maize, wheat.
2. Cellular: every nuclear division followed by wall formation – e.g., Petunia, Datura, balsam.
3. Helobial: first division forms a large micropylar and small chalazal chamber; nuclear divisions in micropylar chamber – common in monocots (Helobiales), e.g., Eremurus.

Dicot embryo – Onagrad (Crucifer) type, e.g., Capsella: zygote divides transversely into a terminal cell and a basal cell. The terminal cell forms the octant and then the embryo proper (two cotyledons, plumule, hypocotyl). The basal cell forms a suspensor; its uppermost cell, the hypophysis, forms the radicle tip and root cap. Mature dicot embryo: embryonal axis with plumule (epicotyl) at one end, radicle at the other, and two cotyledons attached laterally.

Monocot embryo (e.g., grass/maize): one large shield-shaped cotyledon called scutellum, attached laterally to the axis. The plumule is enclosed in a sheath called coleoptile and the radicle with root cap in a sheath called coleorhiza. A small outgrowth opposite the scutellum, the epiblast, may be present.

5 likely exam questions

Long answer

1. Describe the structure of a mature anther and explain microsporogenesis and development of male gametophyte.
2. What is double fertilization? Describe the process of fertilization and add notes on types of endosperm and the structure of dicot and monocot embryo.

Short answer / short notes

3. Describe the structure of an anatropous ovule with a labelled diagram.
4. Explain the development of Polygonum type of embryo sac.
5. Write a note on the significance of pollination and contrivances of cross-pollination.