The leading characteristic of the order is that denoted by its name; for the flowers, which are generally very small and white, are arranged in umbels. In a few instances these umbels are simple; but in most they are compound--that is, the stalks which radiate from the same point on the main peduncle, and thus form the _primary umbel_, give rise to the lesser stalks of the _secondary umbels_, which are similarly arranged and bear the flowers. There are often bracts at the base of the primary umbel, in which case they are termed the _primary bracts_; and there are frequently _secondary bracts_ or _involucels_ at the bases of the secondary umbels.
The flowers have a superior calyx; with five teeth; but this is often so inconspicuous that it appears like a mere rim round the top of the ovary. There are also five petals, which generally have their points turned inwards; and five stamens. The inferior ovary consists of two united carpels, surmounted by a fleshy disc that supports the petals and the stamens, and bears two styles.
Special attention must be given to the structure of the fruits of umbellifers, for a close examination of these is often necessary for purposes of identification. The two carpels are close together, with their adjacent surfaces flattened, and are fixed to a central axis called the _carpophore_. As the fruit ripens, the carpophore often divides, from above downwards, becoming Y-shaped; and the carpels, thus separated, are for a time suspended on its two arms. Each carpel is marked by vertical ridges, generally nine in number, five of them (_primary ridges_) being more prominent than the four intermediate or _secondary ridges_. The ridge on each side of the carpel, nearest to the fissure that divides the fruit into two parts, is often extended so as to form wings by means of which wind-distribution is greatly facilitated; and between the various ridges are the _furrows_ of the fruit. In addition to these features, there are often narrow, light-coloured streaks running parallel with the ridges, in the walls of the fruit. There are usually six of these in each carpel, sometimes more than one in the same furrow, and they mark the positions of narrow oil-sacs or _vittæ_. Each carpel contains only one seed.