Plant Anatomy and Morphology
Plant anatomy is the study of internal structure; plant morphology is the study of external form. Together they explain how plants are organised from cell to organ.
A group of structurally similar or functionally related cells that perform a common task. Plant tissues are broadly classified as meristematic (actively dividing) or permanent (differentiated, no longer dividing).
Plant cells
A typical plant cell has:
- Cell wall — chiefly cellulose; rigid; secondary walls add lignin.
- Plasma membrane — selectively permeable lipid bilayer.
- Cytoplasm with nucleus, plastids (chloroplasts, leucoplasts, chromoplasts), mitochondria, endoplasmic reticulum, Golgi, ribosomes, vacuole.
- Plasmodesmata — cytoplasmic channels through walls.
Meristematic tissues
| Type | Position | Function |
|---|---|---|
| Apical meristem | Tips of roots and shoots | Primary growth (length) |
| Lateral meristem | Vascular cambium, cork cambium | Secondary growth (girth) |
| Intercalary meristem | Internodes (esp. grasses) | Internode elongation |
Permanent tissues
Simple permanent tissues
- Parenchyma — living, thin-walled; storage, photosynthesis (chlorenchyma), buoyancy (aerenchyma).
- Collenchyma — living, thickened at corners; flexible support in young stems.
- Sclerenchyma — dead, lignified; rigid support (fibres, sclereids).
Complex permanent tissues
- Xylem — conducts water and minerals upward; consists of tracheids, vessels (in angiosperms), xylem parenchyma, xylem fibres.
- Phloem — conducts photosynthate; consists of sieve elements, companion cells (in angiosperms), phloem parenchyma, phloem fibres.
Secretory tissues
- Resin ducts, latex vessels, glandular hairs, nectaries.
Anatomy of root
- Outer epidermis (with root hairs) → cortex (parenchyma, sometimes endodermis with Casparian strip) → stele (pericycle, vascular bundles).
- In dicots: vascular bundles are radial; xylem and phloem alternate; xylem forms a star pattern.
- In monocots: polyarch xylem with pith.
Anatomy of stem
- Outer epidermis (cuticle, stomata, sometimes hairs) → cortex (parenchyma, collenchyma) → vascular bundles → pith.
- Dicot stem: vascular bundles in a ring, open (with cambium) — supports secondary growth.
- Monocot stem: vascular bundles scattered, closed (no cambium); no secondary growth typically.
Anatomy of leaf
A typical dicot leaf shows:
- Upper and lower epidermis with cuticle and stomata (more on lower surface).
- Mesophyll: palisade parenchyma (column of chloroplast-rich cells beneath upper epidermis) + spongy parenchyma (loose, with air spaces).
- Vascular bundles (veins) with xylem on adaxial side, phloem on abaxial.
Primary vs secondary growth
| Feature | Primary | Secondary |
|---|---|---|
| Meristem | Apical | Lateral (cambium) |
| Result | Length | Girth |
| Tissues added | Primary xylem/phloem, cortex, epidermis | Secondary xylem (wood), secondary phloem, periderm |
| Found in | All plants | Mainly dicots and gymnosperms |
Annual rings in temperate trees represent yearly secondary xylem growth — used in dendrochronology.
- Three plant tissue systems: dermal, ground, vascular.
- Casparian strip in root endodermis forces water through the symplast.
- Vessels are angiosperm-typical; gymnosperms rely on tracheids.
- Stomata are bordered by two guard cells that regulate gas exchange and transpiration.
- The cork cambium (phellogen) produces the protective bark.
Plant morphology — external form
A flowering plant has two systems:
- Root system — anchorage, absorption.
- Shoot system — stem, leaves, flowers.
Leaf
- Simple vs compound (pinnate, palmate).
- Venation: reticulate (dicots) or parallel (monocots).
- Phyllotaxy: alternate, opposite, whorled.
Flower
The reproductive organ; parts in concentric whorls:
- Calyx — sepals (protective).
- Corolla — petals (often showy).
- Androecium — stamens (filament + anther; produce pollen).
- Gynoecium — carpels/pistils (stigma + style + ovary).
Floral formula notation summarises symmetry, number and fusion.
Fruit and seed
- Fruit develops from the ovary; classified into simple, aggregate, multiple, and fleshy vs dry.
- Examples: drupe (mango), berry (tomato), pome (apple), capsule (cotton), legume (gram).
- Seed = matured ovule; contains embryo, endosperm, seed coat.
Specialised organs
- Tendrils (climbing) — modified stems (cucumber) or leaves (pea).
- Spines — modified leaves (cactus).
- Stolons / runners — vegetative reproduction.
- Bulbs, corms, rhizomes, tubers — underground storage organs.
For botany MCQs, remember the diagnostic dicot/monocot signals: in a dicot stem, bundles are in a ring and open; in a monocot stem, bundles are scattered. In a dicot root, xylem makes a star; in a monocot root, xylem is polyarch with central pith.
Studying plant anatomy
Tools: microtome sections, stains (safranin for lignified walls; fast green for cellulose), light and electron microscopy, GFP-tagged proteins for live imaging.
Major figures: Marcello Malpighi and Nehemiah Grew (17th c., founders), Katherine Esau (20th c., "Anatomy of Seed Plants").