§ 1 Position and Arrangement of Flowers, or Inflorescence
197. Flower-buds appear just where leaf-buds appear; that is, they are either terminal or axillary (47-49). Morphologically, flowers answer to shoots or branches, and their parts to leaves.
198. In the same species the flowers are usually from axillary buds only, or from terminal buds only; but in some they are both axillary and terminal.
199. Inflorescence, which is the name used by Linnæus to signify mode of flower-arrangement, is accordingly of three classes: namely, Indeterminate, when the flowers are in the axils of leaves, that is, are from axillary buds; Determinate, when they are from terminal buds, and so terminate a stem or branch; and Mixed, when these two are combined.
200. Indeterminate Inflorescence (likewise, and for the same reason, called indefinite inflorescence) is so named because, as the flowers all come from axillary buds, the terminal bud may keep on growing and prolong the stem indefinitely. This is so in Moneywort (Fig. 199).
201. When flowers thus arise singly from the axils of ordinary leaves, they are axillary and solitary, not collected into flower-clusters.
202. But when several or many flowers are produced near each other, the accompanying leaves are apt to be of smaller size, or of different shape or character: then they are called Bracts, and the flowers thus brought together form a cluster. The kinds of flower-clusters of the indeterminate class have received distinct names, according to their form and disposition. They are principally Raceme, Corymb, Umbel, Spike, Head, Spadix, Catkin, and Panicle.
203. In defining these it will be necessary to use some of the following terms of descriptive botany which relate to inflorescence. If a flower is stalkless, i. e. sits directly in the axil or other support, it is said to be sessile. If raised on a naked stalk of its own (as in Fig. 199) it is pedunculate, and the stalk is a Peduncle.
205. A Raceme (Fig. 200) is that form of flower-cluster in which the flowers, each on their own foot-stalk or pedicel, are arranged along the sides of a common stalk or axis of inflorescence; as in the Lily of the Valley, Currant, Barberry, one section of Cherry, etc. Each flower comes from the axil of a small leaf, or bract, which, however, is often so small that it might escape notice, and even sometimes (as in the Mustard Family) disappears altogether. The lowest blossoms of a raceme are of course the oldest, and therefore open first, and the order of blossoming is ascending from the bottom to the top. The summit, never being stopped by a terminal flower, may go on to grow, and often does so (as in the common Shepherd’s Purse), producing lateral flowers one after another for many weeks.
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).
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.
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.
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.
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.
215. The secondary or partial umbels of a compound umbel are Umbellets. When the umbellets are subtended by an involucre, this secondary involucre is called an Involucel.
216. A Compound raceme is a cluster of racemes racemosely arranged, as in Smilacina racemosa. A compound corymb is a corymb some branches of which branch again in the same way, as in Mountain Ash. A compound spike is a spicately disposed cluster of spikes.
217. A Panicle, such as that of Oats and many Grasses, is a compound flower-cluster of a more or less open sort which branches with apparent irregularity, neither into corymbs nor racemes. Fig. 209 represents the simplest panicle. It is, as it were, a raceme of which some of the pedicels have branched so as to bear a few flowers on pedicels of their own, while others remain simple. A compound panicle is one that branches in this way again and again.
218. Determinate Inflorescence is that in which the flowers are from terminal buds. The simplest case is that of a solitary terminal flower, as in Fig. 210. This stops the growth of the stem; for its terminal bud, becoming a blossom, can no more lengthen in the manner of a leaf-bud. Any further growth must be from axillary buds developing into branches. If such branches are leafy shoots, at length terminated by single blossoms, the inflorescence still consists of solitary flowers at the summit of stem and branches. But if the flowering branches bear only bracts in place of ordinary leaves, the result is the kind of flower-cluster called
220. Wherever there are bracts or leaves, buds may be produced from their axils and appear as flowers. Fig. 212 represents the case where the branches, b b, of Fig. 211, each with a pair of small leaves or bracts about their middle, have branched again, and produced the branchlets and flowers c c, on each side. It is the continued repetition of this which forms the full or compound cyme, such as that of the Laurestinus, Hobble-bush, Dogwood, and Hydrangea (Fig. 214).
221. A Fascicle (meaning a bundle), like that of the Sweet William and Lychnis of the gardens, is only a cyme with the flowers much crowded.
222. A Glomerule is a cyme still more compacted, so as to imitate a head. It may be known from a true head by the flowers not expanding centripetally, that is, not from the circumference towards the centre.
223. The illustrations of determinate or cymose inflorescence have been taken from plants with opposite leaves, which give rise to the most regular cymes. But the Rose, Cinquefoil, Buttercup, etc., with alternate leaves, furnish also good examples of cymose inflorescence.
224. A Cymule (or diminutive cyme) is either a reduced small cyme of few flowers, or a branch of a compound cyme, i. e. a partial cyme.
225. Scorpioid or Helicoid Cymes, of various sorts, are forms of determinate inflorescence (often puzzling to the student) in which one half of the ramification fails to appear. So that they may be called incomplete cymes. The commoner forms may be understood by comparing a complete cyme, like that of Fig. 215 with Fig. 216, the diagram of a cyme of an opposite-leaved plant, having a series of terminal flowers and the axis continued by the development of a branch in the axil of only one of the leaves at each node. The dotted lines on the left indicate the place of the wanting branches, which if present would convert this scorpioid cyme into the complete one of Fig. 215. Fig. 217 is a diagram of similar inflorescence with alternate leaves. Both are kinds of false racemes (219). When the bracts are also wanting in such cases, as in many Borragineous plants, the true nature of the inflorescence is very much disguised.
226. These distinctions between determinate and indeterminate inflorescence, between corymbs and cymes, and between the true and the false raceme and spike, were not recognized by botanists much more than half a century ago, and even now are not always attended to in descriptions. It is still usual and convenient to describe rounded or flat-topped and open ramification as corymbose, even when essentially cymose; also to call the reversed or false racemes or spikes by these (strictly incorrect) names.
227. Mixed Inflorescence is that in which the two plans are mixed or combined in compound clusters. A mixed panicle is one in which, while the primary ramification is of the indeterminate order, the secondary or ultimate is wholly or partly of the determinate order. A contracted or elongated inflorescence of this sort is called a Thyrsus. Lilac and Horse-chestnut afford common examples of mixed inflorescence of this sort. When loose and open such flower-clusters are called by the general name of Panicles. The heads of Compositæ are centripetal; but the branches or peduncles which bear the heads are usually of centrifugal order.
Notes
204 A peduncle on which a flower-cluster is raised is a Common peduncle. That which supports each separate flower of the cluster is a Partial peduncle, and is generally called a Pedicel. The portion of the general stalk along which flowers are disposed is called the Axis of inflorescence, or, when covered with sessile flowers, the Rhachis (back-bone), and sometimes the Receptacle. The leaves of a flower-cluster generally are termed Bracts. But when bracts of different orders are to be distinguished, those on the common peduncle or axis, and which have a flower in their axil, keep the name of bracts; and those on the pedicels or partial flower-stalks, if any, that of Bractlets or Bracteoles. The former is the preferable English name.
219 A Cyme. This is commonly a flat-topped or convex flower-cluster, like a corymb, only the blossoms are from terminal buds. Fig. 211 illustrates the simplest cyme in a plant with opposite leaves, namely, with three flowers. The middle flower, a, terminates the stem; the two others, b b, terminate branches, one from the axil of each of the uppermost leaves; and being later than the middle one, the flowering proceeds from the centre outwards, or is Centrifugal. This is the opposite of the indeterminate mode, or that where all the flower-buds are axillary. If flowering branches appear from the axils below, the lower ones are the later, so that the order of blossoming continues centrifugal or, which is the same thing, descending, as in Fig. 213, making a sort of reversed raceme or false raceme,—a kind of cluster which is to the true raceme just what the flat cyme is to the corymb.