Flower Structure -- A Recap

Sexual reproduction in flowering plants (angiosperms) takes place entirely within and around a

single, remarkable organ: the flower. Before the events of gamete formation, pollination and

fertilisation can be studied, it is worth recalling exactly how a flower is put together, because

every subsequent stage of this chapter unfolds inside one of its parts.

A typical flower is best understood as a highly modified, condensed shoot. In an ordinary

vegetative shoot, leaves are borne at nodes separated by internodes that keep elongating as the

shoot grows. In a flower, the internodes between successive whorls are extremely shortened, so

several distinct sets of appendages -- which are themselves modified leaves -- appear to arise

almost at the same level on a short axis called the thalamus, or receptacle. Once these whorls

have formed, growth of the floral axis is determinate: it does not continue to elongate

indefinitely the way a vegetative shoot apex does.

A complete, typical flower bears four such whorls, arranged concentrically on the thalamus from

outermost to innermost. The outermost whorl is the calyx, made up of individual units called

sepals. Sepals are usually green and leaf-like, and their principal role is protective -- shielding

the more delicate inner parts of the flower, especially while it is still a bud. Just within the

calyx lies the corolla, made up of petals. Petals are typically larger, thinner and often brightly

coloured (though sometimes white), and their chief function is to attract pollinating agents such

as insects and birds; many are additionally scented or produce nectar to reinforce this attraction.

Calyx and corolla are together described as the accessory, or sterile, whorls, because neither

directly produces the gametes needed for reproduction -- they support the process rather than

carry it out.

Within the corolla lies the androecium, the male reproductive whorl, composed of one or more

stamens. Each stamen typically consists of a slender filament bearing, at its tip, a fertile

anther. It is inside the anther that microspores are produced and eventually mature into pollen

grains, which carry the male gametes. At the very centre of the flower lies the gynoecium, the

female reproductive whorl, made up of one or more carpels, collectively forming the pistil. Each

carpel differentiates into an ovary (containing one or more ovules), a style, and a stigma at the

tip, which receives pollen. Androecium and gynoecium are together called the essential, or fertile,

whorls, because it is here that gametes are actually formed and fertilisation ultimately occurs.

The precise position of the ovary relative to the other whorls gives rise to a further useful

classification of flowers. In a hypogynous flower, the ovary occupies the topmost position on the

thalamus, with the calyx, corolla and androecium all attached at a lower level; the ovary is then

described as superior, since it sits above the other parts -- mustard and China rose are common

examples. In a perigynous flower, the thalamus grows up around the ovary so that the other three

whorls arise from roughly the same level as the ovary itself, which is then described as

half-inferior, as seen in plum and rose. In an epigynous flower, the thalamus grows completely

around and fuses with the ovary, so calyx, corolla and androecium all appear to arise from above

it; the ovary is then described as inferior, as in guava and the ray florets of sunflower.

This basic whorled architecture -- calyx, corolla, androecium, gynoecium, in that fixed order, on

a condensed and determinate floral axis -- is the structural stage on which every event described

in the rest of this chapter takes place: the differentiation of microsporangia within the anther,

the differentiation of the ovule and embryo sac within the ovary, the process of pollination that

connects anther to stigma, and the fertilisation and post-fertilisation events that ultimately

transform ovule into seed and ovary into fruit.