Morphology of Flowering Plants

The root is the descending, non-green, positively geotropic and hydrotropic vegetative axis of the plant body, lacking nodes, internodes, leaves, and buds. It provides mechanical anchorage, absorbs water and dissolved mineral nutrients, synthesizes plant growth regulators, and acts as a reservoir of reserve food materials.

• Tap Root System: In majority of dicotyledonous plants, direct elongation of the embryonic radicle leads to the formation of the persistent primary root, which grows deep into the soil. It bears lateral roots of several orders (secondary, tertiary roots). The primary root and its lateral branches constitute the taproot system (e.g., mustard plant Brassica campestris, gram, sunflower).
• Fibrous Root System: In monocotyledonous plants, the primary root is short-lived and soon ceases growth. It is replaced by a large cluster of thin, thread-like roots originating directly from the base of the stem. These roots spread shallowly in the topsoil (e.g., wheat Triticum aestivum, paddy, maize).
• Adventitious Root System: Roots that develop from any plant part other than the radicle of the embryo—such as stem nodes, internodes, branches, or leaves (e.g., grass, Monstera, banyan tree Ficus benghalensis).

A typical root apex exhibits four distinct morphological zones from apex to base:

1. Root Cap (Calyptra): A thimble-like or cup-shaped protective cellular covering over the tender root apex. It secretes mucilage to lubricate the passage of the root through coarse soil particles. (In aquatic plants like Pistia and Eichhornia, loose root pockets replace root caps).
2. Region of Meristematic Activity (Cell Division): Situated a few millimeters above the root cap. Cells are exceptionally small, thin-walled, dense with granular protoplasm, lacking large vacuoles, and undergo continuous rapid mitotic division.
3. Region of Elongation: Proximal to the meristematic zone. Cells undergo rapid longitudinal elongation and enlargement, responsible for the growth of the root in length.
4. Region of Maturation (Differentiation): Situated proximal to the elongation zone. Cells undergo structural and physiological differentiation into primary tissues (xylem, phloem, cortex, endodermis). The epidermal cells (epiblema) of this zone produce fine, delicate, unicellular tubular outgrowths called root hairs, which vastly increase the surface area for water and mineral absorption.

• Storage of Food Materials: Taproots become swollen and fleshy with accumulated reserve carbohydrates:
  • Conical root: Broad at base, tapering towards apex (e.g., carrot Daucus carota).
  • Fusiform root: Spindle-shaped, swollen in the middle, tapering at both ends (e.g., radish Raphanus sativus).
  • Napiform root: Spherical at base, abruptly tapering like a tail at apex (e.g., turnip Brassica rapa, beet).
  • Tuberous adventitious root: Swollen without definite shape (e.g., sweet potato Ipomoea batatas).
• Mechanical Support:
  • Prop Roots (Pillar Roots): Massive, pillar-like adventitious roots growing vertically downwards from heavy horizontal aerial branches into the soil, providing mechanical support against gravity (e.g., banyan tree Ficus benghalensis).
  • Stilt Roots: Stout, oblique adventitious roots arising from the basal nodes of the main stem, penetrating into soil to prevent lodging in tall, slender grasses (e.g., maize Zea mays, sugarcane Saccharum officinarum).
• Physiological Respiration (Pneumatophores): In halophytic mangrove plants growing in waterlogged, saline, oxygen-deficient swampy soils, numerous roots turn vertically upwards (negatively geotropic) and emerge out of water and mud. These specialized roots, called pneumatophores (respiratory roots), possess minute breathing pores called lenticels / pneumathodes through which oxygen diffuses directly into internal aerating aerenchyma tissues (e.g., Rhizophora, Avicennia, Sonneratia).

The stem is the ascending, positively phototropic and negatively geotropic axis of the plant body that develops from the plumule of the embryo. It bears foliage leaves, branches, flowers, and fruits, and is distinguished unambiguously from roots by the presence of nodes (where leaves arise) and internodes (portions between two successive nodes), as well as terminal and axillary buds.

Certain stems grow underground, becoming thick and fleshy with stored starch and reserve food, enabling perennation through unfavorable seasons:

• Stem Tuber: Swollen, succulent terminal portion of an underground branch lacking a definite shape. Possesses distinct depressions called 'eyes', which are nodes bearing minute scale leaves with axillary buds capable of vegetative sprouting (e.g., potato Solanum tuberosum).
• Rhizome: Prostrate, dorsiventrally flattened, thick horizontal underground stem growing parallel to soil surface. Distinctly marked with nodes and internodes, covered with dry brownish scale leaves, bearing terminal buds and adventitious roots on the ventral surface (e.g., ginger Zingiber officinale, turmeric Curcuma longa, banana).
• Corm: Stout, solid, unbranched, spherical or flattened vertically growing condensed underground stem with circular nodes bearing scale leaves and large apical buds (e.g., Amorphophallus [zaminkand / elephant foot yam], Colocasia, Crocus [saffron]).
• Bulb: Highly reduced, discoid, convex underground stem surrounded by concentric fleshy scale leaves storing water and soluble sugars (e.g., onion Allium cepa, garlic Allium sativum).

• Runner: Slender, prostrate branch that creeps horizontally along soil surface, rooting at nodes and producing new aerial shoots. When older parts die, new daughter plants become independent (e.g., doob grass Cynodon dactylon, strawberry Fragaria).
• Stolon: A slender lateral branch that arises from the base of the main axis, initially grows aerially upwards, and then arches downwards to touch the ground, producing adventitious roots and an aerial bud at its tip (e.g., mint Mentha, jasmine Jasminum).
• Offset: A short, stout, horizontal runner typically one-internode long found in aquatic rosette plants, producing a dense rosette of leaves at the water surface and a tuft of roots below (e.g., water hyacinth Eichhornia crassipes, water lettuce Pistia).
• Sucker: A lateral branch that originates from the underground portion of the main stem, grows horizontally beneath soil for some distance, and then turns obliquely upward to produce a leafy shoot (e.g., chrysanthemum, pineapple, banana).

• Stem Tendrils: Slender, spirally coiled, leafless thread-like structures modified from axillary buds, sensitive to contact (thigmotropic), coiling around supports to aid climbing in weak-stemmed plants (e.g., gourds: cucumber, pumpkin, watermelon; and grapevine Vitis).
• Thorns: Hard, woody, straight, pointed protective structures derived from modified axillary buds. They deter browsing herbivores and reduce transpiration (e.g., Citrus [lemon], Bougainvillea).
• Phylloclade (Cladode): In extreme xerophytes, leaves are reduced or modified into spines to minimize transpiration, and the stem becomes green, succulent, fleshy, flattened (e.g., Opuntia) or cylindrical (e.g., Euphorbia), taking over the entire photosynthetic role and storing copious mucilage and water.

The leaf is a lateral, generally flattened, vegetative outgrowth that develops exogenously at the nodes of the stem. It bears a bud in its axil (the axillary bud, which can develop into a branch) and is the primary photosynthetic and transpirational organ of the flowering plant.

A typical foliage leaf comprises three distinct morphological regions:

• Leaf Base (Hypopodium): The basal part by which the leaf attaches to the stem node. It may bear two lateral small leaf-like appendages called stipules. In monocots, the leaf base expands into a sheathing leaf base clasping the stem partially or wholly. In leguminous plants, the leaf base becomes swollen into a pulvinus, responsible for nyctinastic 'sleep' movements.
• Petiole (Mesopodium): The cylindrical stalk that holds the expanded lamina up to light and allows it to flutter in the wind, cooling the leaf surface and circulating fresh air. Leaves lacking a petiole are termed sessile.
• Lamina or Leaf Blade (Epipodium): The broad, green, expanded part of the leaf traversed by a network of vascular strands called veins and veinlets. A prominent central longitudinal vein is the midrib, which provides mechanical rigidity and conducts water, minerals, and carbohydrates.

The arrangement of veins and veinlets in the lamina is termed venation:

• Reticulate Venation: Veinlets branch repeatedly to form an irregular, complex network or meshwork across the lamina. Characteristic of almost all dicotyledonous leaves (e.g., mango Mangifera indica, peepal Ficus religiosa, hibiscus).
• Parallel Venation: Veins run parallel to each other along the length of the lamina without forming reticulate veinlets. Characteristic of almost all monocotyledonous leaves (e.g., grass, banana, maize, wheat).

• Simple Leaf: The lamina is either entire or, if incised, the incisions do not reach the midrib. An axillary bud is present in the axil of the petiole (e.g., mango, guava, peepal).
• Compound Leaf: The incisions of the lamina reach all the way down to the midrib, breaking the blade into a number of distinct, articulated segments called leaflets (pinnae). (An axillary bud is present in the axil of the compound leaf's petiole, but never in the axil of individual leaflets):
  • Pinnately Compound Leaf: Leaflets are arranged laterally along a common elongated axis called the rachis (representing the midrib) (e.g., neem Azadirachta indica, rose, tamarind).
  • Palmately Compound Leaf: Leaflets radiate from a single common point at the tip of the petiole, resembling the fingers of a palm (e.g., silk cotton Bombax ceiba).

The pattern of arrangement of leaves on the stem or branch to ensure maximum illumination without mutual shading:

• Alternate (Spiral): A single leaf arises at each node in an alternating, spiral sequence along the stem (e.g., China rose Hibiscus rosa-sinensis, mustard, sunflower).
• Opposite: A pair of leaves arises at each node on opposite sides of the stem:
  • Opposite decussate: Successive pairs stand at right angles to each other (e.g., Calotropis, holy basil Ocimum).
  • Opposite superposed: Successive pairs lie in the same vertical plane (e.g., guava Psidium guajava).
• Whorled (Verticillate): More than two leaves arise at a single node forming a circle or whorl (e.g., Alstonia scholaris, Nerium).

• Leaf Tendrils: The whole leaf or upper leaflets are modified into sensitive climbing coils (e.g., garden pea Pisum sativum, sweet pea Lathyrus).
• Leaf Spines: Leaves are reduced to sharp, stiff spines to curb transpiration and deter herbivores (e.g., cacti, Opuntia, Argemone).
• Fleshy Storage Leaves: Fleshy scale leaves store water and food (e.g., onion, garlic).
• Phyllode: In plants like Australian acacia (Acacia auriculiformis), the delicate bipinnate leaflets drop off early, and the petiole expands vertically into a green, flattened, sickle-shaped photosynthetic blade termed a phyllode.
• Insectivorous Modifications: Leaves modified into pitfall pitchers (Nepenthes), underwater bladders (Utricularia), or snap-traps (Dionaea) to capture insects and supplement nitrogen in nutrient-deficient soils.

The flower is a specialized, condensed, determinate reproductive shoot meant for sexual reproduction. The floral axis (pedicel and receptacle/thalamus) bears four successive whorls of modified leaves. The spatial arrangement of flowers on the floral axis is termed an inflorescence.

• Actinomorphic (Radial Symmetry - $\oplus$): A flower that can be divided into two equal radial halves in any vertical radial plane passing through the central axis (e.g., mustard Brassica, Datura, chilli Capsicum).
• Zygomorphic (Bilateral Symmetry - $\%$): A flower that can be divided into two identical halves only in one particular median vertical plane (e.g., pea Pisum, bean, gulmohar, Cassia).
• Asymmetric (Irregular): A flower that cannot be divided into two similar halves by any vertical plane passing through the center (e.g., Canna).

Based on the relative position of the calyx, corolla, and androecium with respect to the ovary on the thalamus, flowers are categorized into:

• Hypogynous Flower: The thalamus is convex or conical. The gynoecium occupies the highest position at the apex of the thalamus, while calyx, corolla, and androecium arise sequentially below it. The ovary is termed Superior ($\underline{\text{G}}$) (e.g., mustard, China rose, brinjal).
• Perigynous Flower: The thalamus forms a cup-shaped or saucer-shaped rim. The gynoecium sits in the center, and other floral whorls arise on the margin or rim of the thalamus at almost the same horizontal level. The ovary is termed Half-Inferior ($-\text{G}-$ or $\underline{\text{G}}$) (e.g., plum, rose, peach).
• Epigynous Flower: The margin of the hollow, cup-like thalamus grows upwards, completely enclosing the ovary and fusing irrevocably with the ovary wall. The calyx, corolla, and stamens arise on top of the ovary. The ovary is termed Inferior ($\bar{\text{G}}$) (e.g., guava Psidium, cucumber Cucumis, ray florets of sunflower).

Each complete flower consists of four concentric series of floral appendages: two non-essential accessory whorls (Calyx and Corolla, or Perianth) and two essential reproductive whorls (Androecium and Gynoecium).

The mode of arrangement of sepals or petals in the floral bud with respect to other members of the same whorl:

• Valvate: Margins of sepals or petals touch each other closely without any overlapping (e.g., Calotropis, mustard).
• Twisted (Contorted): One margin of the petal overlaps that of the adjacent petal, and its other margin is overlapped by the preceding one in a regular clockwise or anti-clockwise direction (e.g., China rose, lady's finger, cotton).
• Imbricate: Margins of petals overlap one another, but irregularly without any definite direction (e.g., Cassia, gulmohar).
• Vexillary (Papilionaceous): Unique to pea family (Fabaceae). The largest posterior petal is the Standard (Vexillum); it overlaps two lateral petals called Wings (Alae), which in turn overlap two anterior, smallest united petals forming a boat-shaped Keel (Carina) enclosing stamens and carpel ($C_{1+2+(2)}$) (e.g., pea, bean).

A stamen consists of a filament and an anther (usually bilobed and dithecous with 4 microsporangia). A sterile stamen is called a staminode.

• Cohesion (Union among stamens):
  • Monadelphous: Filaments are united into a single bundle or staminal tube around the style (e.g., China rose Hibiscus).
  • Diadelphous: Filaments are united into two bundles (e.g., pea Pisum, where 9 stamens are fused and 1 is free: $(9)+1$).
  • Polyadelphous: Filaments are united into more than two bundles (e.g., citrus / lemon).
  • Syngenesious: Anthers are fused into a cylinder while filaments remain free (e.g., sunflower family Asteraceae).
• Adhesion (Union with other whorls):
  • Epipetalous: Stamens are united to petals (e.g., brinjal, Datura, Solanaceae).
  • Epiphyllous (Epitepalous): Stamens are united to perianth tepals (e.g., lily, onion, Liliaceae).
• Variation in Stamen Length:
  • Didynamous: 4 stamens, 2 long and 2 short (e.g., Salvia, Ocimum [tulsi]).
  • Tetradynamous: 6 stamens, 4 long inner and 2 short outer (e.g., mustard, radish [Brassicaceae]).

Gynoecium consists of carpels (stigma, style, ovary). If carpels are free: Apocarpous (e.g., lotus, rose); if carpels are fused: Syncarpous (e.g., mustard, tomato). The distribution and arrangement of ovules on the ovarian placenta is termed placentation:

• Marginal Placentation: The placenta forms a longitudinal ridge along the ventral suture of a monocarpellary unilocular ovary, and ovules are borne in two alternating rows along the ridge (e.g., pea Pisum sativum, bean).
• Axile Placentation: In a multilocular syncarpous ovary with septa meeting at the center, ovules are attached to a central axial placenta (e.g., China rose, tomato, lemon, onion).
• Parietal Placentation: Ovules develop on the inner peripheral wall of a unilocular ovary. In Brassicaceae (mustard, Argemone), the ovary is initially one-chambered but becomes two-chambered due to the formation of a false septum called replum.
• Free Central Placentation: Ovules are borne on a prominent central column in an unilocular ovary, and septa are completely absent (e.g., Dianthus, Primula).
• Basal Placentation: A single ovule develops at the very base of a unilocular ovary (e.g., sunflower Helianthus, marigold).

The fruit is a characteristic feature of flowering plants—a mature, fertilized ovary. A fruit developed without fertilization is termed parthenocarpic (e.g., banana). The pericarp consists of epicarp (skin), mesocarp (pulp), and endocarp (inner wall). In simple fleshy fruits like the drupe (mango, coconut), the endocarp is stony hard.

• Vegetative Features: Trees, shrubs, or herbs; roots with nitrogen-fixing Rhizobium nodules; pinnately compound leaves with pulvinate leaf base and persistent stipules.
• Floral Features: Inflorescence racemose; flower bisexual, zygomorphic (%); Calyx: 5 sepals, gamosepalous $K_{(5)}$, valvate/imbricate; Corolla: 5 petals, polypetalous, vexillary aestivation $C_{1+2+(2)}$; Androecium: 10 stamens, diadelphous $A_{(9)+1}$, dithecous; Gynoecium: monocarpellary, superior ovary $\underline{G}_1$, unilocular with marginal placentation; Fruit: legume / pod; Seed: non-endospermic.
• Floral Formula: $\%\, \oint\, K_{(5)}\, C_{1+2+(2)}\, A_{(9)+1}\, \underline{G}_1$
• Economic Importance: Pulses (gram, arhar, sem, moong, soybean); edible oil (groundnut, soybean); dye (Indigofera); fibers (sunn hemp); fodder (Sesbania, Trifolium); ornamentals (lupin, sweet pea); medicine (mulethi Glycyrrhiza glabra).

• Vegetative Features: Mostly herbs or shrubs; stem herbaceous, branched, underground tubers in potato; leaves alternate, simple, exstipulate, reticulate venation.
• Floral Features: Inflorescence solitary, axillary, or cymose; flower bisexual, actinomorphic ($\oplus$); Calyx: 5 sepals, gamosepalous $K_{(5)}$, persistent (accrescent in brinjal, tomato), valvate; Corolla: 5 petals, gamopetalous $C_{(5)}$, valvate; Androecium: 5 stamens, epipetalous $A_5$; Gynoecium: bicarpellary syncarpous, superior ovary $\underline{G}_{(2)}$, bilocular, placenta swollen with many ovules on axile placentation, ovary oriented obliquely at 45° to mother axis; Fruit: berry or capsule.
• Floral Formula: $\oplus\, \oint\, K_{(5)}\, \overbrace{C_{(5)}\, A_5}\, \underline{G}_{(2)}$
• Economic Importance: Food (potato, tomato, brinjal); spice (chilli); medicine (Belladonna, Ashwagandha); fumigatory (tobacco Nicotiana tabacum); ornamental (Petunia).

• Vegetative Features: Perennial herbs with underground bulbs, corms, or rhizomes; leaves mostly basal, alternate, linear, exstipulate with parallel venation.
• Floral Features: Inflorescence solitary, cymose, or umbellate clusters; flower bisexual, actinomorphic ($\oplus$); Perianth: 6 tepals arranged in two whorls of three $P_{(3+3)}$, often united into a tube, valvate; Androecium: 6 stamens, epiphyllous $A_{3+3}$; Gynoecium: tricarpellary syncarpous, superior ovary $\underline{G}_{(3)}$, trilocular with many ovules on axile placentation; Fruit: capsule, rarely berry; Seed: endospermic.
• Floral Formula: $\oplus\, \oint\, \overbrace{P_{(3+3)}\, A_{3+3}}\, \underline{G}_{(3)}$
• Economic Importance: Ornamentals (tulip, Gloriosa); medicine (Aloe); vegetable (Asparagus); flavoring bulbs (onion Allium cepa, garlic Allium sativum); mitotic inhibitor (colchicine from Colchicum autumnale).