Morphology & Anatomy of Flowering Plants

Flowering plants (angiosperms) display an extraordinary range of morphological adaptations in their roots, stems, and leaves, each subject to modifications for storage, support, climbing, and defense. Understanding these modifications alongside plant families, tissue systems, and internal anatomy is essential for NEET.

Roots anchor the plant and absorb water and minerals. Dicots develop a tap root system with a primary root and lateral branches, while monocots develop a fibrous (adventitious) root system. Root modifications include storage roots (conical — carrot; fusiform — radish; napiform — turnip; tuberous adventitious — sweet potato), pneumatophores (breathing roots in Rhizophora/mangroves that grow upward for gaseous exchange), prop roots (adventitious aerial roots from branches providing additional support — banyan/Ficus benghalensis), and stilt roots (adventitious roots from lower nodes — maize, sugarcane).

Stems bear leaves, branches, flowers, and fruits. Stem modifications include underground (rhizome — ginger/Zingiber; tuber — potato/Solanum tuberosum with "eyes" or buds confirming stem nature; bulb — onion with fleshy scale leaves around a reduced stem; corm — Colocasia with a solid stem base), subaerial (runner — Cynodon/doob grass; stolon — Fragaria/strawberry; offset — Eichhornia/water hyacinth; sucker — Chrysanthemum), and aerial (tendrils — Passiflora; thorns — Bougainvillea; phylloclade — Opuntia, a flattened photosynthetic stem).

Leaves show reticulate venation in dicots and parallel venation in monocots, with phyllotaxy patterns of alternate, opposite, or whorled. Leaf modifications include tendrils (pea), spines (cacti — for water conservation), pitcher (Nepenthes — insectivorous), bladder (Utricularia — aquatic trap), and phyllode (Acacia auriculiformis — flattened petiole resembling a leaf).

Inflorescence is classified as racemose (indefinite growth — flowers open from base to apex: raceme, spike, spadix, umbel, capitulum/head) or cymose (definite growth — oldest flower at apex: monochasial, dichasial, polychasial). Aestivation types include valvate (sepals/petals touching without overlap), twisted (one margin overlapping the next), imbricate (irregular overlap), and vexillary (characteristic of Fabaceae — one large standard petal, two wings, two fused keel petals). Placentation includes marginal (ovules along ventral suture — pea), axile (central axis in partitioned ovary — tomato), parietal (ovules on inner wall of unilocular ovary — mustard), free central (ovules on central column, no septa — Dianthus), and basal (single ovule at base — sunflower).

Fruits are classified as simple (from one ovary of one flower: drupe — mango; berry — tomato; capsule — cotton), aggregate (from multiple free carpels of one flower: etaerio — Polyalthia), or composite/multiple (from entire inflorescence: sorosis — pineapple, jackfruit; syconus — fig/Ficus).

Seeds differ between dicots and monocots. Dicot seeds (gram/chickpea) have two cotyledons, a seed coat of testa and tegmen, and an embryo axis with radicle, plumule, hypocotyl, and epicotyl. Monocot seeds (maize) have one cotyledon (scutellum), a coleoptile covering the plumule, a coleorhiza covering the radicle, and a prominent endosperm.

Seven important plant families are distinguished by diagnostic features. Fabaceae (Papilionaceae): zygomorphic flowers, vexillary aestivation (papilionaceous corolla — standard, wings, keel), diadelphous stamens (9 fused + 1 free), marginal placentation, legume/pod fruit; examples — pea, gram, soybean. Solanaceae: actinomorphic, 5 fused petals, 5 epipetalous stamens, bicarpellary syncarpous ovary with axile placentation, berry or capsule fruit; examples — Solanum (potato, brinjal), Nicotiana (tobacco), Datura, Petunia, Withania. Liliaceae: actinomorphic, trimerous, 6 tepals (3+3), 6 stamens, superior ovary with axile placentation; examples — Allium (onion, garlic), Colchicum, Asparagus, Aloe, Gloriosa. Malvaceae: actinomorphic, epicalyx present (bracteoles outside calyx), monadelphous stamens (all filaments fused into a staminal tube), axile placentation; examples — Hibiscus, Gossypium (cotton), Abelmoschus (okra). Brassicaceae (Cruciferae): actinomorphic, 4 sepals + 4 cruciform petals, tetradynamous stamens (4 long + 2 short), parietal placentation, siliqua or silicula fruit; examples — Brassica (mustard), Raphanus (radish), Iberis, Capsella. Asteraceae (Compositae): capitulum (head) inflorescence with ray florets (ligulate, zygomorphic) and disc florets (tubular, actinomorphic), inferior ovary, syngenesious stamens (anthers fused, filaments free), cypsela fruit with pappus; examples — Helianthus (sunflower), Tagetes (marigold), Tridax, Lactuca (lettuce). Poaceae (Gramineae): lodicules (2 reduced perianth scales), 3 stamens with versatile anthers, feathery stigma (wind pollination), caryopsis fruit (pericarp fused with seed coat), floret enclosed by lemma and palea; examples — Oryza (rice), Triticum (wheat), Zea mays (maize), Saccharum (sugarcane), Bambusa (bamboo).

Plant tissues include meristematic (apical — tips; lateral — cambium; intercalary — base of internodes) and permanent. Simple permanent tissues are parenchyma (thin-walled, living, storage and photosynthesis), collenchyma (pectin-thickened corners, living, provides flexibility), and sclerenchyma (lignin-thickened, dead at maturity, provides mechanical support — includes fibres and sclereids/stone cells). Complex tissues include xylem (vessels — dead, cylindrical, efficient water transport; tracheids — dead, tapering; xylem fibres; xylem parenchyma — only living component) and phloem (sieve tubes — living but enucleated at maturity; companion cells — living, nucleated, assist sieve tubes; phloem fibres; phloem parenchyma).

Anatomy comparisons: Dicot roots have 2-4 xylem bundles with cambium; monocot roots have polyarch xylem with no cambium and a large pith. Dicot stems have vascular bundles arranged in a ring (conjoint, open with cambium allowing secondary growth); monocot stems have scattered closed vascular bundles (no cambium, no secondary growth). Dicot leaves are dorsiventral (differentiated palisade and spongy mesophyll); monocot leaves are isobilateral (undifferentiated mesophyll) with bulliform cells for leaf rolling during water stress.

The key testable concept is the diagnostic features of the seven plant families (especially Fabaceae, Brassicaceae, Asteraceae, and Poaceae), stem vs root modifications (potato = stem; sweet potato = root), and the distinction between living and dead cells in xylem and phloem.