Chapter overview: flowering plant morphology
Flowering plants show great external diversity, but are characterised by roots, stems, leaves, flowers and fruits. The underground portion forms the root system and the aerial portion forms the shoot system. Standard morphological terms make description, identification and classification possible.
5.1 The root
Roots absorb water and minerals, anchor the plant, store reserve food and synthesise plant growth regulators. In dicots, the radicle elongates to form a primary root with lateral roots: the tap root system (mustard). In monocots, the short-lived primary root is replaced by roots from the stem base: the fibrous root system (wheat). Roots arising from any part other than radicle are adventitious roots, as in grass, Monstera and banyan.
Regions of the root tip
The root cap protects the tender apex. Above it is the meristematic region of actively dividing cells, followed by the region of elongation that increases root length. The region of maturation bears root hairs that absorb water and minerals.
5.2 The stem
The stem develops from the plumule and is the ascending axis bearing branches, leaves, flowers and fruits. It has nodes (where leaves arise), internodes (between nodes), terminal buds and axillary buds. Young stems are usually green and later may become woody.
| Stem modification | Function / examples |
|---|---|
| Underground | Storage, perennation and vegetative propagation: potato (tuber), ginger (rhizome), colocasia (corm), onion (bulb) |
| Subaerial | Vegetative propagation: runner (grass), stolon (strawberry), sucker (mint), offset (Pistia) |
| Aerial | Support, protection or photosynthesis: tendrils (grape), thorns (citrus), phylloclade (Opuntia) and cladode (Asparagus) |
5.3 The leaf
A typical leaf has leaf base, petiole and lamina. The leaf base may bear stipules; in monocots it commonly forms a sheath, while legumes may have a swollen pulvinus. The lamina has a midrib, veins and veinlets that give rigidity and conduct materials.
Venation, leaf type and phyllotaxy
Reticulate venation forms a network and is typical of dicots; parallel venation is typical of monocots. A leaf is simple when incisions do not reach the midrib or petiole; it is compound when they do. Compound leaves are pinnately compound (neem) or palmately compound (silk cotton).
Phyllotaxy may be alternate (mustard, china rose), opposite (guava, Calotropis) or whorled (Alstonia).
5.4 Inflorescence and 5.5 the flower
Inflorescence is the arrangement of flowers on the floral axis. In racemose inflorescence the main axis continues to grow and flowers show acropetal succession. In cymose inflorescence the main axis ends in a flower and the succession is basipetal.
A flower is a modified shoot and the reproductive unit of angiosperms. It has four whorls on the thalamus: calyx and corolla (accessory) plus androecium and gynoecium (reproductive). A flower with both reproductive whorls is bisexual; one with only one is unisexual.
| Position of ovary | Flower type / examples |
|---|---|
| Superior | Hypogynous: thalamus and other whorls below gynoecium; mustard, china rose, brinjal |
| Half-inferior | Perigynous: other whorls arise on thalamus rim; plum, rose, peach |
| Inferior | Epigynous: thalamus encloses and fuses with ovary; guava, cucumber, sunflower ray florets |
Floral parts and important terms
Calyx consists of sepals and may be polysepalous (free) or gamosepalous (united). Corolla consists of petals and may be polypetalous or gamopetalous. When calyx and corolla are not distinct, the whorl is called perianth.
Aestivation is the arrangement of sepals/petals in a bud: valvate (Calotropis), twisted (china rose), imbricate (Cassia) or vexillary/papilionaceous (pea). The androecium consists of stamens with filament and bilobed anther. Stamens can be epipetalous (brinjal), epiphyllous (lily), polyandrous, monadelphous, diadelphous or polyadelphous.
The gynoecium has one or more carpels, each with stigma, style and ovary. Carpels are apocarpous when free (lotus, rose) and syncarpous when fused (mustard, tomato). Placenta arrangements include marginal (pea), axile (tomato/lemon), parietal (mustard), basal and free-central.
5.6 Fruit and 5.7 seed
After fertilisation, the ovary develops into fruit and ovules develop into seeds. The fruit wall is pericarp and may differentiate into epicarp, mesocarp and endocarp. A fruit developing without fertilisation is parthenocarpic. Mango and coconut are drupes; mango has fleshy mesocarp while coconut has fibrous mesocarp.
Dicot and monocot seeds
A dicot seed has seed coat (testa and tegmen), hilum, micropyle, embryonal axis, radicle, plumule and two cotyledons. Castor is endospermic; bean, gram and pea are non-endospermic at maturity. A monocot seed such as maize is generally endospermic. Its single cotyledon is the scutellum; coleoptile encloses the plumule and coleorhiza encloses the radicle.
NCERT revision prompts
- Differentiate tap, fibrous and adventitious roots with examples.
- State the regions of the root tip and their functions.
- Compare stem modifications for storage, propagation, support and protection.
- Differentiate simple and compound leaves, and the types of phyllotaxy.
- Compare racemose and cymose inflorescence.
- Explain hypogynous, perigynous and epigynous flowers.
- Compare a dicot seed with a monocot seed.