The Flower

The flower is the characteristic reproductive structure of angiosperms (flowering plants), typically borne at the tip of a stalk called the pedicel, which widens at its tip into the receptacle -- the platform on which the flower's parts are arranged, usually in four successive whorls.

The four floral whorls. The outermost whorl is the calyx, made up of sepals, which are usually green and leaf-like; the calyx's chief job is protecting the flower while it is still a bud. The next whorl inward is the corolla, made up of petals, which are often brightly coloured, and whose chief job is attracting pollinators. Because the calyx and corolla together serve mainly a protective and pollinator-attracting role rather than a directly reproductive one, they are jointly referred to as the perianth (a term used especially where the two whorls look alike and are not clearly distinguishable, as in many monocots). The two inner whorls carry out reproduction directly. The androecium is the male reproductive whorl, made up of stamens; each stamen typically consists of a slender filament bearing, at its tip, a two-lobed anther in which pollen grains are produced. At the very centre of the flower lies the gynoecium (or pistil), the female reproductive whorl, made up of one or more units called carpels; each carpel is differentiated into a swollen basal ovary containing one or more ovules, a slender style, and, at its tip, a stigma that receives pollen during pollination.

Symmetry. A flower is described as actinomorphic (or regular/radially symmetrical) when it can be divided into two exactly equal halves along more than one plane passing through its centre, as in mustard or Datura. A flower is described as zygomorphic (or irregular/bilaterally symmetrical) when only a single such plane of division produces two equal halves, as in the pea flower or Cassia.

Aestivation. Within a single whorl (usually the calyx or corolla), the mode in which the individual members are arranged relative to one another while still packed inside the unopened bud is called aestivation. Four patterns are recognised. Valvate aestivation is the simplest: the members merely touch one another at their margins, without any overlap, as in the calyx of China rose. Twisted (or contorted) aestivation has each member's margin overlapping the margin of the next member consistently in the same direction, like the sails of a pinwheel, as in China rose's corolla. Imbricate aestivation is more irregular: one member is completely external (outermost), one is completely internal (innermost), and the remaining members overlap each other only partly, as in Cassia and gulmohar. Vexillary (or papilionaceous) aestivation is a special, highly specific imbricate pattern found in the pea-type corolla of family Leguminosae: the single largest, posterior petal (the standard) is outermost and overlaps two lateral petals (the wings), which in turn overlap two smaller anterior petals fused along their lower edges into a boat-shaped structure called the keel.

Placentation. Within the ovary, the particular tissue to which the ovules are attached is called the placenta, and the pattern of this attachment is called placentation. In marginal placentation, a single carpel is folded lengthwise so that the ovules develop in two rows along its ventral suture (the line where the carpel's edges meet), as in pea. In axile placentation, several carpels are fused together with the placenta running along a central axis, and the ovary is divided into as many chambers (locules) as there are carpels, as in China rose and tomato. In parietal placentation, the ovules develop directly on the inner wall of the ovary, or on outgrowths of it, as in mustard and Argemone. In basal placentation, a single ovule is attached at the very base of the ovary, as in sunflower and marigold. In free-central placentation, the ovules are attached to a central column that rises from the base of the ovary but is not connected to the ovary wall by any partition, as in Dianthus and Primrose. …