Flowering plants (angiosperms) form the backbone of NEET Biology Unit IV, contributing 2–3 questions per year. Mastery requires understanding structural modifications, seven diagnostic plant families, tissue types, and comparative anatomy of dicots and monocots.

Roots and Their Modifications

Roots anchor the plant and absorb water and minerals. Dicots develop a tap root system (primary root with lateral branches), while monocots develop a fibrous adventitious root system. Storage roots are among the most commonly tested modifications: conical (carrot, Daucus carota), fusiform (radish, Raphanus sativus), napiform (turnip), and tuberous adventitious roots (sweet potato, Ipomoea batatas — note: no buds/nodes, confirming root identity). Specialized aerial roots include prop roots (Banyan/Ficus benghalensis — from branches for additional support), stilt roots (maize, sugarcane — from lower nodes), and pneumatophores (Rhizophora mangroves — grow upward for gaseous exchange in waterlogged conditions).

Stems and Their Modifications

Stems bear leaves, branches, flowers, and fruits and are distinguished from roots by the presence of nodes, internodes, and axillary buds. Underground stem modifications include rhizomes (ginger — horizontal with nodes and scale leaves), tubers (potato/Solanum tuberosum — swollen tip with "eyes"/axillary buds, confirming stem identity), bulbs (onion/Allium cepa — reduced disc stem with fleshy scale leaves), and corms (Colocasia — solid, upright with nodes). Subaerial modifications include runners (Cynodon doob grass), stolons $\frac{Fragaria}{strawberry}$, offsets $\frac{Eichhornia}{water hyacinth}$, and suckers (Chrysanthemum). Aerial stem modifications include tendrils (Passiflora), thorns (Bougainvillea), and phylloclades (Opuntia — flattened photosynthetic stem with leaves reduced to spines).

Leaf Structure and Modifications

Dicot leaves have reticulate (net) venation; monocot leaves have parallel venation. Phyllotaxy describes leaf arrangement: alternate (one leaf per node), opposite (two per node), or whorled (three or more per node). Leaf modifications serve specialised functions: tendrils (pea — for climbing), spines (cacti — water conservation), pitchers (Nepenthes — insectivorous trap), bladders (Utricularia — aquatic insectivorous), and phyllodes (Acacia auriculiformis — flattened petiole taking over leaf function).

Inflorescence, Aestivation, and Placentation

Inflorescences are racemose (indefinite — flowers open base to apex: raceme, spike, spadix, umbel, capitulum) or cymose (definite — oldest flower at apex: monochasial, dichasial, polychasial). Aestivation describes petal/sepal arrangement in the bud: valvate (edges touching), twisted (one margin over the next), imbricate (irregular overlap), and vexillary (Fabaceae — large standard, two wings, two keel petals). Placentation types: marginal (pea — ovules along ventral suture), axile (tomato, lemon — central axis), parietal (mustard — inner wall of unilocular ovary), free central (Dianthus — central column, no septa), and basal (sunflower — single ovule at base).

Seven Plant Families

The seven families are defined by unique combinations of floral and fruit characters. Fabaceae: zygomorphic, vexillary corolla, diadelphous stamens (9+1), marginal placentation, legume fruit — examples: pea, gram. Solanaceae: actinomorphic, 5 epipetalous stamens, bicarpellary syncarpous ovary, axile placentation, berry/capsule — examples: potato, tobacco. Liliaceae: trimerous, 6 tepals, 6 stamens, axile placentation, capsule/berry — examples: onion, aloe. Malvaceae: epicalyx, monadelphous stamens, axile placentation — examples: Hibiscus, cotton. Brassicaceae: cruciform petals, tetradynamous stamens (4+2), parietal placentation, siliqua/silicula — examples: mustard, radish. Asteraceae: capitulum with ray + disc florets, syngenesious anthers, inferior ovary, cypsela with pappus — examples: sunflower, marigold. Poaceae: lodicules, 3 versatile-anther stamens, feathery stigma, caryopsis fruit — examples: rice, wheat, maize.

Tissues

Meristematic tissues (apical, lateral, intercalary) retain division capability. Simple permanent tissues include parenchyma (thin-walled, living, storage/photosynthesis), collenchyma (pectin-thickened corners, living, flexibility), and sclerenchyma (lignin-thickened, dead at maturity, mechanical support). Complex tissues: xylem (vessels and tracheids — dead, water transport; xylem parenchyma — only living component) and phloem (sieve tubes — living but enucleated; companion cells — living with nucleus; phloem fibres; phloem parenchyma).

Comparative Anatomy

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