Chapter 5: Morphology of Flowering Plants (Structural Organisation)

Phyllotaxy is the pattern of arrangement of leaves on a stem or branch. (i) Alternate: a single leaf at each node, alternately, e.g. china rose, mustard, sunflower. (ii) Opposite: a pair of leaves at each node, opposite each other, e.g. Calotropis, guava. (iii) Whorled: more than two leaves at a node forming a whorl, e.g. Alstonia.

(a) Aestivation: the mode of arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl (valvate, twisted, imbricate, vexillary). (b) Placentation: the arrangement of ovules within the ovary (marginal, axile, parietal, free central, basal). (c) Actinomorphic: a flower that can be divided into two equal radial halves by any radial plane through the centre, e.g. mustard, datura, chilli. (d) Zygomorphic: a flower divisible into two similar halves by only one vertical plane, e.g. pea, gulmohur, bean, Cassia. (e) Superior ovary: the ovary is at the highest position on the thalamus with the other floral parts below it (hypogynous flower: mustard, brinjal). (f) Perigynous flower: the gynoecium is in the centre and the other parts are on the rim of the thalamus at almost the same level; ovary half-inferior (plum, rose, peach). (g) Epipetalous stamen: a stamen attached to the petals, e.g. brinjal.

(a)

(b)

Labelled diagrams: a gram seed (two cotyledons, no endosperm) and V.S. of a maize grain (endosperm, aleurone layer, scutellum, coleoptile, coleorhiza)

Floral diagram and floral formula of Solanaceae (e.g. Solanum nigrum)

(i) Marginal: the placenta forms a ridge along the ventral suture and ovules are in two rows (pea). (ii) Axile: placenta axial in a multilocular ovary; ovules on it (china rose, tomato, lemon). (iii) Parietal: ovules on the inner wall (periphery); the one-chambered ovary becomes two-chambered by a false septum (mustard, Argemone). (iv) Free central: ovules on a central axis without septa (Dianthus, primrose). (v) Basal: the placenta is at the base of the ovary with a single ovule (sunflower, marigold).

A flower is a modified shoot meant for sexual reproduction, in which the shoot apex becomes a floral meristem and the condensed axis bears floral appendages. Its four whorls sit on the thalamus at the end of the pedicel: (i) calyx: green sepals that protect the bud (gamo- or polysepalous); (ii) corolla: coloured petals that attract insects (gamo- or polypetalous); (iii) androecium: stamens, each with a filament and a bilobed anther making pollen; (iv) gynoecium: carpels, each with stigma, style and ovary containing ovules on placentae. Calyx and corolla are accessory whorls; androecium and gynoecium are reproductive. A flower may also have bracts.

Inflorescence is the arrangement of flowers on the floral axis. The basis is whether the apex of the main axis develops into a flower or continues to grow. If the main axis continues to grow and flowers are borne laterally in acropetal succession, it is racemose. If the main axis ends in a flower, so that its growth is limited and flowers are borne in basipetal order, it is cymose.

(i) Hypogynous: the gynoecium occupies the highest position and calyx, corolla and androecium are below it; ovary superior (mustard, china rose, brinjal). (ii) Perigynous: the gynoecium is central and the other parts are on the rim of a cup-shaped thalamus at about the same level as the ovary; ovary half-inferior (plum, rose, peach). (iii) Epigynous: the margin of the thalamus grows up, encloses and fuses with the ovary, and the other parts arise above the ovary; ovary inferior (guava, cucumber, ray florets of sunflower). Quick-reference sheet

Case study (practice): a kitchen-garden walk. A student examines (1) a pea pod, (2) a mustard flower, (3) a guava flower, (4) a coconut. (i) Name the aestivation and placentation of (1). (ii) Write the floral formula of (2). (iii) Where is the ovary of (3) placed? (iv) Name the fruit type of (4) and its fibrous layer.