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). [Illustration: 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_. [Illustration: Fig. 205. Head of the Button-bush (Cephalanthus).] 212. =A Spadix= is a fleshy spike or head, with small and often imperfect flowers, as in the Calla, Indian Turnip, (Fig. 206), Sweet Flag, etc. It is commonly surrounded or embraced by a peculiar enveloping leaf, called a SPATHE. [Illustration: Fig. 206. Spadix and spathe of the Indian Turnip; the latter cut through below.] 213. =A Catkin, or Ament=, is the name given to the scaly sort of spike of the Birch (Fig. 207) and Alder, the Willow and Poplar, and one sort of flower-clusters of the Oak, Hickory, and the like,--the so-called _Amentaceous_ trees. [Illustration: Fig. 207. Catkin, or Ament, of Birch.] 214. _Compound_ flower-clusters of these kinds are not uncommon. When the stalks which in the simple umbel are the pedicels of single flowers themselves branch into an umbel, a _Compound Umbel_ is formed. This is the inflorescence of Caraway (Fig. 208), Parsnip, and almost all of the great family of Umbelliferous (umbel-bearing) plants. [Illustration: Fig. 208. Compound Umbel of Caraway.]