ration: Fig. 201. A raceme. 202. A corymb. 203. An umbel.]

206. =A Corymb= (Fig. 202) is the same as a raceme, except that it is
flat and broad, either convex, or level-topped. That is, a raceme
becomes a corymb by lengthening the lower pedicels while the uppermost
remain shorter. The axis of a corymb is short in proportion to the lower
pedicels. By extreme shortening of the axis the corymb may be converted
into

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 i