Floral Parts and their Structure

All floral parts arise from the thalamus, one whorl per node; the thalamus is also called the torus or receptacle, and because it is green it too can carry out photosynthesis.

1. Calyx (K). The outermost floral whorl, whose individual members are called sepals — usually green and capable of photosynthesis. If all the sepals are fused, the condition is gamosepalous (e.g. China rose); if they remain free, it is polysepalous (e.g. Brassica/mustard). The main function of the calyx is to protect the inner floral parts while the flower is still in bud. In some plants the sepals become brightly coloured (petaloid sepals) and help attract insects for pollination, e.g. Mussaenda. In others the calyx modifies into stiff, hairy structures called a pappus; such a persistent calyx helps disperse the fruit by wind, e.g. Tridax, Sonchus.

2. Corolla (C). The second whorl from the outside, made up of variously coloured petals, which may be sweet-tasting or carry scent/aroma. If the petals are free, the condition is polypetalous (e.g. Rose); if fused, it is gamopetalous (e.g. Datura). The main function of the corolla is to attract pollinating agents.

Perianth (P). In many flowers, the calyx and corolla are not clearly differentiated from one another; each member is then called a tepal, and the whole whorl of tepals is the perianth. If all the tepals are free, this is polyphyllous; if fused, gamophyllous. A sepaloid perianth is green, while a petaloid perianth is brightly coloured, e.g. Lily, Amaranthus, Celosia. The perianth protects the other floral whorls; a petaloid perianth also helps pollination, while a sepaloid one can photosynthesise.

Aestivation. The way sepals, petals or tepals are arranged relative to each other, within one whorl, inside the flower bud is called aestivation. The main types (see Table 9.43) are:

• Valvate — the margins of adjacent sepals/petals stay in contact or lie close to one another without overlapping, e.g. calyx of Datura, Calotropis.
• Twisted — one margin of each sepal/petal is directed inward and overlapped, while its other margin is directed outward and overlaps the margin of the next member, e.g. corolla of China rose, Cotton.
• Imbricate — one member is entirely internal and gets overlapped on both its margins, one member is entirely external (completely outside the rest), and the remaining members both overlap and get overlapped, e.g. calyx of Cassia, Bauhinia.
• Vexillary — a butterfly-shaped corolla of five petals: the outermost and largest petal is the standard or vexillum, two lateral petals that partly overlap it are the wings, and the two smallest inner petals fuse to form a boat-shaped structure called the keel, e.g. corolla of Pisum sativum.

Epicalyx. An extra whorl of sepal-like structures, formed by bracteoles, that occurs just outside the calyx; there are usually 5–8 of them, and they are protective in function. This is a characteristic feature of family Malvaceae, e.g. Ladies finger.

3. Androecium (A). The third floral whorl from the outside — the male reproductive part of the flower — whose individual member is called a stamen. If all the stamens remain free, the condition is polyandrous. (Note: cohesion is fusion between members of the same whorl, while adhesion is fusion between members of different whorls.)

A typical stamen has three parts: the anther, terminal in position, which produces pollen grains inside its locules — it is usually bilobed, and since each lobe usually has two locules the anther is tetralocular/tetrasporangiate (e.g. Datura), though in some plants a single anther lobe holds two locules, and in others fusion of the two locules leaves only one locule per lobe, a condition called monothecous (e.g. Hibiscus); the filament, the stalk of the stamen that bears the anther at its tip and raises it to a suitable height for pollen dispersal; and the connective, a continuation of the filament that, much like a midrib, joins the two anther lobes together and connects the anther to the filament.

Cohesion and adhesion in the androecium: when stamens are united by their filaments while their anthers remain free, the condition is called adelphy, and depending on whether this forms one, two or several bundles it is monoadelphous, diadelphous or polyadelphous. When stamens are instead fused to the petals or tepals (i.e. adhesion to another whorl), they are called epipetalous (e.g. Datura) or epiphyllous (e.g. Lily). When the anthers alone are united while the filaments stay free, this is called syngeny/syngenesious (e.g. Sunflower); when both the anthers and filaments are fused, the stamens are said to be synandrous (e.g. Cucurbita).

4. Gynoecium (G). The female reproductive part of the flower, innermost in position, also called the pistil; its individual member is the carpel. The number of carpels may range from one to many. If all the carpels are fused, the gynoecium is syncarpous; if they remain free, it is apocarpous. A polycarpellary (many-carpelled) gynoecium may be described as bicarpellary (two carpels, e.g. Datura), tricarpellary (three carpels, e.g. Cucurbita), pentacarpellary (five carpels, e.g. Hibiscus) and so on. …