What is morphology? Parts of a flowering plant
Morphology means the study of the outer shape and parts of a living thing. A flowering plant (angiosperm) has two big parts.
- Root system: everything below the soil. It holds the plant and takes in water and minerals.
- Shoot system: everything above the soil. It has the stem, leaves, flowers and fruits.
Each part has a normal form. Many parts also change their form to do a special job. This change is called a modification.
The root
Types of root system
- Tap root system: the first root (primary root) grows long and gives side roots (lateral roots). Seen in most dicots, like mustard and gram.
- Fibrous root system: the primary root dies soon; many thin roots come from the base of the stem. Seen in monocots, like wheat and rice.
- Adventitious roots: roots that grow from any part other than the radicle, like the stem or leaf. Seen in grass, banyan and Monstera.
Regions of a root tip
From the tip upwards: root cap (a cap that protects the soft tip), region of meristematic activity (cells divide fast), region of elongation (cells grow long) and region of maturation (cells become mature and form root hairs that absorb water).
Modifications of root
- Food storage: carrot and turnip (tap roots), sweet potato (adventitious roots).
- Support: prop roots of banyan hang from branches; stilt roots of maize and sugarcane come from lower nodes.
- Breathing: in marshy soil, Rhizophora has roots that grow up out of the mud (pneumatophores) to get oxygen.
The stem
The stem grows from the plumule of the embryo. It has nodes (where leaves grow) and internodes (gaps between nodes). It carries buds, which can be at the tip (terminal) or in the angle between leaf and stem (axillary). The stem holds leaves, flowers and fruits up to the light and carries water, minerals and food.
Modifications of stem
- Food storage (underground): potato (tuber), ginger (rhizome), Colocasia (corm), onion (bulb).
- Tendrils for climbing: gourds, cucumber, watermelon, grapevine. They come from axillary buds.
- Thorns for protection: Citrus, Bougainvillea. They are also from axillary buds.
- Flat green stems that make food in dry places: Opuntia (phylloclade), Euphorbia (fleshy, cylinder-like).
- Spreading stems for new plants: runner in grass, stolon in mint and jasmine, offset in Pistia and Eichhornia, sucker in banana and pineapple.
The leaf
A leaf grows at a node and has a bud in its axil. It has three parts: leaf base (may have small side leaves called stipules; in legumes the base swells into a pulvinus), petiole (the stalk) and lamina (the flat green blade with veins).
Venation (pattern of veins)
- Reticulate: veins form a net. Most dicots.
- Parallel: veins run side by side. Most monocots.
Simple and compound leaves
A simple leaf has one blade, even if it is cut in. A compound leaf is cut all the way to the midrib, forming many leaflets. Pinnately compound: leaflets on a common axis called the rachis (neem). Palmately compound: leaflets from one point at the tip of the petiole (silk cotton). A bud is found in the axil of the whole leaf, never in the axil of a leaflet.
Phyllotaxy (arrangement of leaves)
- Alternate: one leaf at each node (china rose, mustard, sunflower).
- Opposite: two leaves at each node, facing each other (Calotropis, guava).
- Whorled: more than two leaves at a node, in a ring (Alstonia).
Modifications of leaf
Tendrils (pea), spines (cactus), fleshy food-storing leaves (onion, garlic), phyllode — a flat green petiole that does the leaf's work (Australian Acacia), and insect-catching pitchers or bladders (pitcher plant, Venus fly trap).
The inflorescence
An inflorescence is the way flowers are arranged on the floral axis.
- Racemose: the main axis keeps growing. Flowers are born on the sides in acropetal order: older flowers at the base, younger at the top.
- Cymose: the main axis ends in a flower, so its growth stops. Flowers form in basipetal order: the oldest at the top.
The flower
The flower is the reproductive shoot. It sits on a stalk (pedicel) whose top swells into the thalamus. It has four whorls: calyx (sepals), corolla (petals), androecium (stamens, the male part) and gynoecium (carpels, the female part). If calyx and corolla look alike, they are called perianth (lily).
Symmetry and sex
Actinomorphic (⊕): can be cut into two equal halves through any line through the centre (mustard, chilli). Zygomorphic (% or ↑): only one such line (pea, gulmohur). Bisexual (⚥) flowers have both stamens and carpels; unisexual ones have only one.
Position of the ovary
- Hypogynous: ovary is highest, others below it = superior ovary (mustard, brinjal).
- Perigynous: ovary in the middle, others at the rim of a cup at the same height (plum, rose, peach) = half-inferior.
- Epigynous: thalamus grows around and joins the ovary, others sit above it = inferior ovary (guava, cucumber, sunflower ray florets).
Aestivation (how petals or sepals sit in the bud)
Valvate: edges just touch (Calotropis). Twisted: each overlaps the next one in one direction (china rose, cotton). Imbricate: overlapping without a fixed direction (Cassia, gulmohur). Vexillary (papilionaceous): one big standard petal, two wings, two fused keel petals (pea, bean).
Androecium and gynoecium
Stamens joined to petals are epipetalous (brinjal); joined to perianth are epiphyllous (lily). Stamens joined in one bundle = monoadelphous (china rose), two = diadelphous (pea), many = polyadelphous (citrus). Carpels free = apocarpous (lotus, rose); fused = syncarpous (mustard, tomato).
Placentation (how ovules sit inside the ovary)
Marginal: along one edge (pea). Axile: on a central axis in a many-chambered ovary (china rose, tomato, lemon). Parietal: on the inner wall; chamber split by a false wall (mustard, Argemone). Free central: on a central axis with no walls (Dianthus, Primrose). Basal: one ovule at the base (sunflower, marigold).
The fruit and the seed
A fruit is a ripe ovary formed after fertilisation. A fruit formed without fertilisation is parthenocarpic (seedless). The fruit wall is the pericarp. In fleshy fruits it has three layers: epicarp (skin), mesocarp (middle) and endocarp (inner). Mango and coconut are drupes: one-seeded fruits from a single-carpel superior ovary. In mango the mesocarp is the juicy pulp and the endocarp is the hard stone; in coconut the mesocarp is fibrous.
Dicot seed (gram, bean, pea)
Outer seed coat has two layers: testa (outer) and tegmen (inner). The hilum is the scar where the seed was attached to the fruit; a small hole above it is the micropyle. Inside is the embryo: an embryonal axis with two cotyledons (often fleshy with stored food), a radicle (future root) and a plumule (future shoot). Seeds with food stored in a separate tissue called endosperm are endospermic (castor); others are non-endospermic (bean, gram, pea).
Monocot seed (maize)
Usually endospermic (orchids are an exception). The seed coat is fused with the fruit wall. The big endosperm stores food and is covered by a protein-rich aleurone layer. The embryo is small, with one shield-shaped cotyledon called the scutellum, and an axis with the plumule covered by a coleoptile and the radicle covered by a coleorhiza.
Describing a flowering plant: floral formula and diagram
Botanists write a flower in short form using a floral formula. Symbols: Br bracteate, K calyx, C corolla, P perianth, A androecium, G gynoecium. A number in brackets means the parts are joined (fused). A line above G (G̅) means an inferior ovary; a line below G (G̲) means a superior ovary. A curved line joining two whorls means they are fused with each other (like epipetalous stamens). A floral diagram is a top view that shows the number of parts, their arrangement and their relation to the mother axis (a dot at the top of the drawing).
Family Solanaceae (the potato family)
A large family found in the tropics, sub-tropics and temperate zones. It is also called the potato family.
- Vegetative characters: mostly herbs and shrubs, sometimes small trees. Stem is herbaceous, rarely woody, often hairy or prickly; in potato it forms an underground tuber. Leaves are alternate and simple (rarely pinnately compound), without stipules, with reticulate venation.
- Inflorescence: solitary, axillary or cymose (like a cyme in Solanum).
- Flower: bisexual and actinomorphic.
- Calyx: 5 sepals, united (gamosepalous), stays on the fruit (persistent), valvate.
- Corolla: 5 petals, united (gamopetalous), valvate.
- Androecium: 5 stamens, epipetalous (stuck to the petals).
- Gynoecium: bicarpellary (2 carpels), syncarpous; ovary superior, two-chambered, placenta swollen with many ovules, axile placentation; the ovary sits obliquely.
- Fruit: berry (tomato, brinjal) or capsule (Datura). Seeds: many, endospermic.
Floral formula: ⊕ ⚥ K(5) C(5) A5 G(2) with a line under G for the superior ovary and C joined to A (epipetalous).
Economic importance
- Food: tomato, brinjal, potato.
- Spice: chilli.
- Medicine: belladonna, ashwagandha.
- Fumigatory: tobacco.
- Ornamental: petunia.
Key formulas and definitions
- Solanaceae: ⊕ ⚥ K(5) C(5) A5 G(2) — superior ovary, epipetalous stamens
- K = calyx, C = corolla, P = perianth, A = androecium, G = gynoecium, Br = bracteate
- ( ) = parts fused; G̲ (line below) = superior ovary; G̅ (line above) = inferior ovary
- ⊕ = actinomorphic; % = zygomorphic; ⚥ = bisexual; ♂ = male; ♀ = female
- Racemose = acropetal (oldest at base); Cymose = basipetal (oldest at top)
- Pericarp = epicarp + mesocarp + endocarp
Worked examples
1. A flower has 5 fused sepals, 5 fused petals, 5 free stamens stuck to the petals and 2 fused carpels in a superior ovary. It is actinomorphic and bisexual. Write its floral formula.
Step 1: symmetry and sex → ⊕ ⚥. Step 2: calyx 5 fused → K(5). Step 3: corolla 5 fused → C(5). Step 4: 5 stamens free from each other → A5, joined to C by a curved line (epipetalous). Step 5: 2 fused carpels, superior → G(2) with a line below. Answer: ⊕ ⚥ K(5) C(5) A5 G(2) — this is a Solanaceae flower.
2. Read this formula: % ⚥ K(5) C1+2+(2) A(9)+1 G1. Say the symmetry, number of petals and how the stamens are grouped.
% = zygomorphic. C1+2+(2) = 1 standard + 2 wings + 2 fused keel petals = 5 petals (vexillary). A(9)+1 = 9 stamens fused in one group + 1 free = diadelphous. G1 = one carpel. This is the pea family pattern.
3. In a plant, flowers on the axis are numbered from the bottom: flower 1 (bottom) is open, flower 5 (top) is a small bud. What type of inflorescence is it and why?
The oldest flower is at the base and the youngest at the top, so flowers form in acropetal order. The axis keeps growing. It is racemose.
4. Why is a potato called a stem and not a root, while a sweet potato is a root?
A potato has 'eyes', which are nodes with buds; buds and nodes are found only on stems, so it is a stem tuber. A sweet potato has no nodes or buds; it is a swollen adventitious root that stores food.
5. A tomato cut across shows two chambers with seeds attached to a thick central column. Name the placentation and the fruit type.
Ovules are on a central axis in a many-chambered (here two-chambered) ovary, so the placentation is axile. The fleshy many-seeded fruit is a berry. Both match family Solanaceae.
6. Compare the seeds of gram and maize in 4 points.
(1) Cotyledons: gram 2, maize 1 (scutellum). (2) Endosperm: gram non-endospermic, maize has a big endosperm. (3) Seed coat: gram's is separate; maize's is fused with the fruit wall. (4) Covers: maize plumule has a coleoptile and radicle has a coleorhiza; gram has no such sheaths.
Common mistakes
- Calling every underground part a root. Potato, ginger, onion and Colocasia are underground stems or leaves; check for nodes and buds.
- Mixing up racemose and cymose. Racemose: axis keeps growing, oldest flower at the bottom. Cymose: axis ends in a flower, oldest at the top.
- Writing the superior-ovary line on the wrong side. Line below G = superior (hypogynous); line above G = inferior (epigynous).
- Thinking a leaflet of a compound leaf has an axillary bud. Only the whole leaf has a bud in its axil.