Morphology of Flowering Plants

Morphology is the study of the external form and structure of a plant. A typical flowering plant has an underground root system and an aerial shoot system (stem, leaves, flowers, fruits).

The Root. Roots are usually non-green, positively geotropic and grow underground. There are two main types: the tap root system (one main root with branches; in dicots, e.g. mustard) and the fibrous root system (a cluster of thin roots from the stem base; in monocots, e.g. wheat). Some plants have adventitious roots arising from parts other than the radicle (e.g. prop roots of banyan). A young root has four regions from tip upward: the root cap, the region of meristematic activity, the region of elongation, and the region of maturation (bearing root hairs). Roots are modified for special functions: storage (carrot, sweet potato — tuberous), support (prop roots of banyan, stilt roots of maize), and respiration (pneumatophores in mangroves like Rhizophora).

The Stem. The stem develops from the plumule, bears nodes (where leaves arise) and internodes, and is usually positively phototropic. It supports leaves, flowers and fruits, conducts water and food, and may store food or photosynthesise. Stems too are modified: underground stems for storage (potato tuber, ginger rhizome, onion bulb); sub-aerial stems for vegetative propagation (runner of grass, stolon, offset, sucker); and aerial modifications such as stem tendrils (for climbing, e.g. grapevine) and stem thorns (for defence, e.g. Citrus, Bougainvillea).

The Leaf is a flattened, green, lateral outgrowth of the stem that arises at a node. Its main function is photosynthesis. A typical leaf has three parts: the leaf base, the petiole (stalk) and the lamina (blade). Leaves may be simple (one undivided lamina) or compound (the lamina is divided into leaflets — pinnately compound like neem, or palmately compound like silk cotton).

Two features describe leaf arrangement and pattern:

• Venation — the pattern of veins: reticulate (net-like, in dicots) or parallel (in monocots).
• Phyllotaxy — the arrangement of leaves on the stem: alternate (one leaf per node, e.g. China rose), opposite (two per node) or whorled (more than two per node).

Leaves are also modified for special jobs: tendrils for climbing (pea), spines to reduce water loss/defence (cactus), storage (onion fleshy leaves), and even traps in insectivorous plants (pitcher plant).

The Inflorescence is the arrangement of flowers on the floral axis. There are two main types: racemose (the main axis keeps growing and flowers are borne laterally in an acropetal manner — older flowers at the bottom, e.g. mustard) and cymose (the main axis ends in a flower, so growth is limited; flowers open in a basipetal order — older flower in the centre/top).

The Flower is the reproductive unit of angiosperms, made of four whorls on the thalamus: calyx (sepals), corolla (petals), androecium (stamens — male) and gynoecium (carpels/pistil — female). Useful terms: aestivation (the arrangement of sepals/petals in the bud — valvate, twisted, imbricate, vexillary) and placentation (the arrangement of ovules within the ovary — marginal, axile, parietal, basal, free-central).

After fertilisation, the ovary becomes the fruit and the ovule becomes the seed. A true fruit has a wall (pericarp) and seeds. A seed consists of a seed coat, an embryo (with radicle and plumule) and cotyledons; dicot seeds have two cotyledons, monocot seeds one (with endosperm).

A flower is summarised by a floral formula (using symbols for the whorls, their number and fusion) and a floral diagram (a cross-sectional sketch). Three important families:

• Fabaceae (pea family) — flowers bisexual, zygomorphic; corolla papilionaceous (vexillary aestivation); 10 stamens (diadelphous); fruit a legume. Examples: pea, gram, groundnut. Economically very important (pulses, oil, nitrogen fixation).
• Solanaceae (potato family) — flowers bisexual, actinomorphic; 5 fused sepals and petals; 5 stamens; bicarpellary, axile placentation; fruit a berry/capsule. Examples: potato, tomato, brinjal, tobacco.
• Liliaceae (lily family) — typically monocots; flowers trimerous, actinomorphic; perianth of 6 tepals; 6 stamens; tricarpellary, axile placentation. Examples: onion, garlic, tulip, Aloe.