🎯 Key Points
- Root tip zones (tip to base): root cap → meristematic → elongation → maturation (root hair) zone
- Stem modifications: rhizome (ginger), tuber (potato), bulb (onion), corm (colocasia) — all underground but only roots have root caps/root hairs, helping distinguish modified stems from roots
- Dicot stem: vascular bundles in a RING, cambium present (open, secondary growth possible); Monocot stem: bundles SCATTERED, no cambium (closed, no secondary growth)
- Dicot leaf: dorsiventral, palisade+spongy mesophyll, stomata mainly on lower surface; Monocot leaf: isobilateral, undifferentiated mesophyll, stomata on both surfaces
- Dicot root: few xylem bundles (2-6); Monocot root: many xylem bundles (polyarch, 6+) and a large well-developed pith
- Secondary growth: vascular cambium → secondary xylem/phloem; cork cambium (phellogen) → bark; annual rings used for dendrochronology (tree age estimation)
In a dicot stem, vascular bundles are arranged in a single ring (with cambium between xylem and phloem, enabling secondary growth); in a monocot stem, bundles are scattered throughout with no cambium, so monocots can't undergo secondary growth (no wood formation).
Root System
- Tap root: primary root with lateral branches (dicots); e.g., carrot, radish
- Fibrous root: many roots of similar size (monocots); e.g., grass, wheat
- Modifications: storage (carrot), breathing roots/pneumatophores (Avicennia), prop roots (banyan), stilt roots (maize)
- Root tip regions: root cap → meristematic zone → elongation zone → maturation zone (root hair zone)
Stem
- Conducts water and nutrients; supports leaves; site of photosynthesis (green stems)
- Modifications: rhizome (ginger), tuber (potato - swollen stem), bulb (onion), corm (colocasia), stem tendrils (grapevine), thorn (Bougainvillea), phylloclade (Opuntia)
Leaf
- Lamina (blade), petiole, leaf base; stipules may be present
- Venation: parallel (monocots), reticulate (dicots)
- Simple leaf: single undivided lamina; Compound leaf: divided into leaflets
- Phyllotaxy: arrangement of leaves on stem (alternate, opposite, whorled)
- Leaf modifications: tendrils (Pisum), spines (Opuntia/cactus), pitcher (Nepenthes/insectivorous)
Plant Tissues
- Meristematic tissue: dividing cells; apical (tip growth), lateral (girth), intercalary (between nodes)
- Permanent tissues: Simple (parenchyma, collenchyma, sclerenchyma) and Complex (xylem, phloem)
- Parenchyma: living, thin-walled, storage and packing
- Collenchyma: support in young growing plants
- Sclerenchyma: dead, thick walls, support (fibres and sclereids)
- Xylem: water transport (vessels, tracheids, fibres, parenchyma)
- Phloem: food transport (sieve tubes, companion cells, fibres, parenchyma)
Dicot vs Monocot Stem (anatomy)
- Dicot stem: vascular bundles in ring; open (cambium present); secondary growth possible
- Monocot stem: vascular bundles scattered; closed (no cambium); no secondary growth
Types of Inflorescence
- Racemose: the main floral axis continues to grow and produces flowers laterally in acropetal succession (older flowers toward the base, younger toward the tip); examples include raceme (mustard), spike (Adhatoda), and umbel (coriander)
- Cymose: the main axis terminates in a flower early, so growth and branching continue laterally below it; flowers are arranged in basipetal order (younger flowers toward the base); seen in cymes and verticillasters (Ocimum)
Anatomy: Dicot vs Monocot Leaf and Root
- Dicot leaf: dorsiventral (bifacial), with distinct upper and lower surfaces; mesophyll differentiated into palisade (upper, columnar, more chloroplasts) and spongy parenchyma (lower, irregular, air spaces); stomata mainly on the lower surface
- Monocot leaf: isobilateral, mesophyll not differentiated into palisade and spongy layers; stomata present on both surfaces; bulliform cells present in some grasses to help leaf rolling during water stress
- Dicot root: limited xylem bundles (usually 2 to 6, polyarch is rare), cambium develops later for secondary growth, pith small or absent
- Monocot root: many xylem bundles (polyarch, 6 or more), no cambium, large well-developed pith present
Secondary Growth
- Increase in girth of stems and roots due to activity of cambium (vascular cambium forms secondary xylem and phloem; cork cambium/phellogen forms the outer bark)
- Annual rings: alternating layers of spring wood (large vessels, lighter) and autumn wood (smaller vessels, darker) formed due to seasonal variation in cambium activity; used to estimate the age of a tree (dendrochronology)
- Heartwood (dark, non-conducting, resistant to decay) versus sapwood (lighter, peripheral, active in water conduction)
The Flower: Parts and Symmetry
- A flower is a modified shoot; the four whorls (outer to inner) are calyx (sepals, protective), corolla (petals, attract pollinators), androecium (stamens, male), and gynoecium (carpels/pistil, female)
- Symmetry: actinomorphic (radial — can be cut into equal halves along any plane, e.g. mustard, Datura); zygomorphic (bilateral — only one plane of symmetry, e.g. pea, Cassia)
- Number of parts: trimerous, tetramerous, or pentamerous (floral parts in multiples of 3, 4, or 5); monocots are typically trimerous, dicots tetra- or pentamerous
- Position of ovary vs other whorls: hypogynous (superior ovary, other parts below — mustard, china rose); perigynous (half-inferior ovary, parts at the same level — rose, plum); epigynous (inferior ovary, other parts above — guava, cucumber, sunflower)
- A complete flower has all four whorls; a bisexual flower bears both androecium and gynoecium, a unisexual flower only one
Aestivation and Placentation
- Aestivation (arrangement of sepals or petals in a floral bud): valvate (margins just touch, no overlap — Calotropis); twisted/contorted (one margin regularly overlaps the next — china rose, cotton); imbricate (overlapping without a definite direction — Cassia, gulmohar); vexillary/papilionaceous (a large standard covers two wings, which cover two keel petals — pea, bean)
- Placentation (arrangement of ovules within the ovary): marginal (ovules on one ridge — pea); axile (ovules on a central axis in a chambered ovary — china rose, tomato, lemon); parietal (ovules on the inner ovary wall — mustard, Argemone); free central (ovules on a central axis with no septa — Dianthus, Primrose); basal (a single ovule at the ovary base — sunflower, marigold)
Types of Fruits
- A fruit is the mature, ripened ovary formed after fertilisation; the ovary wall becomes the pericarp, which may be dry or fleshy
- True fruit develops only from the ovary (mango); false fruit has other floral parts contributing to it (apple — the fleshy edible part is the thalamus)
- Drupe: a fleshy fruit with a hard stony endocarp around the seed (mango, coconut, almond); in a berry the seeds are embedded in fleshy pulp (tomato, grape)
- Parthenocarpic fruit: develops from the ovary without fertilisation and is therefore seedless (banana)
Seed Structure: Dicot vs Monocot
| Feature | Dicot seed (bean/gram) | Monocot seed (maize/wheat) |
|---|---|---|
| Cotyledons | Two, often fleshy, store food | One (scutellum), thin |
| Endosperm | Usually non-endospermic (food in cotyledons) | Endospermic (food in endosperm) |
| Embryo axis | Plumule and radicle at the two ends | Plumule in coleoptile, radicle in coleorhiza |
| Seed coat | Testa and tegmen, distinct | Membranous, fused with the fruit wall (a grain) |
- The hilum is the scar where the seed was attached to the fruit; the micropyle is a small pore just above the hilum through which water is absorbed during germination
Floral Formula and Important Families
- Fabaceae (Papilionaceae, the pea family): zygomorphic bisexual flower; calyx 5 (fused), corolla 5 (papilionaceous — 1 standard, 2 wings, 2 keel), androecium 10 (diadelphous — 9 fused + 1 free), gynoecium 1 (superior, monocarpellary, marginal placentation); fruit a legume; examples pea, gram, groundnut; economically vital as pulses and for nitrogen fixation
- Solanaceae (the potato/nightshade family): actinomorphic bisexual; calyx 5 (fused, persistent), corolla 5 (fused), androecium 5 (epipetalous), gynoecium 2 (fused, superior, axile placentation, ovary obliquely placed); fruit a berry or capsule; examples potato, tomato, brinjal, tobacco, Datura
- Liliaceae (the lily family, a monocot): actinomorphic bisexual and trimerous; perianth of 6 tepals (3 + 3, often fused), androecium 6, gynoecium 3 (fused, superior, axile placentation); examples lily, onion, garlic, Aloe, tulip
🚀 NEET Advanced Edge
Distinguishing a modified stem from a root by clue words: If the structure has "nodes, internodes, scale leaves, or axillary buds" → it's a modified STEM (e.g. potato tuber has "eyes" which are axillary buds); if it lacks these and instead has a root cap → it's a true root (e.g. sweet potato, a root modification, NOT a stem tuber).
Why bulliform cells matter: In grasses (monocot leaves), bulliform cells on the upper epidermis lose turgor under water stress, causing the leaf to roll/fold inward — reducing the exposed surface area and transpiration rate, a structural drought-avoidance adaptation often tested alongside stomatal closure.
Reading a transverse section (T.S.) in an exam image: A T.S. showing scattered vascular bundles with no visible cambium ring → monocot stem. A T.S. showing bundles arranged in a ring WITH a continuous cambium layer → dicot stem. Polyarch xylem (6+ groups) arranged around a large pith → monocot root.