[207.] An Umbel (Fig. [203]) as in the Milkweed, a sort of flower-cluster where the pedicels all spring apparently from the same point, from the top of the peduncle, so as to resemble, when spreading, the rays of an umbrella; whence the name. Here the pedicels are sometimes called the Rays of the umbel. And the bracts, when brought in this way into a cluster or circle, form what is called an Involucre.
208. The corymb and the umbel being more or less level-topped, bringing the flowers into a horizontal plane or a convex form, the ascending order of development appears as Centripetal. That is, the flowering proceeds from the margin or circumference regularly towards the centre; the lower flowers of the former answering to the outer ones of the latter.
209. In these three kinds of flower-clusters, the flowers are raised on conspicuous pedicels ([204]) or stalks of their own. The shortening of these pedicels, so as to render the flowers sessile or nearly so, converts a raceme into a Spike, and a corymb or an umbel into a Head.
[210.] A Spike is a flower cluster with a more or less lengthened axis, along which the flowers are sessile or nearly so; as in the Plantain (Fig. [204]).
Fig. 204. Spike of the common Plantain or Ribwort.
[211.] A Head (Capitulum) is a round or roundish cluster of flowers, which are sessile on a very short axis or receptacle, as in the Button-ball, Button-bush (Fig. [205]), and Red Clover. It is just what a spike would become if its axis were shortened; or an umbel, if its pedicels were all shortened until the flowers became sessile. The head of the Button-bush is naked; but that of the Thistle, of the Dandelion, and the like, is surrounded by empty bracts, which form an Involucre. Two particular forms of the spike and the head have received particular names, namely, the Spadix and the Catkin.
