The simplest form of the definite type of the inflorescence is seen in Anemone nemorosa and in gentianella (Gentiana acaulis), where the axis terminates in a single flower, no other flowers being produced upon the plant. This is a solitary terminal inflorescence. If other flowers were produced, they would arise as lateral shoots from the bracts below the first-formed flower. The general name of cyme is applied to the arrangement of a group of flowers in a definite inflorescence. A cymose inflorescence is an inflorescence where the primary floral axis before terminating in a flower gives off one or more lateral unifloral axes which repeat the process—the development being only limited by the vigour of the plant. The floral axes are thus centrifugally developed. The cyme, according to its development, has been characterized as biparous or uniparous. In fig. 16 the biparous cyme is represented in the flowering branch of Cerastium. Here the primary axis t ends in a flower, which has passed into the state of fruit. At its base two leaves are produced, in each of which arise secondary axes t′ t′, ending in single flowers, and at the base of these axes a pair of opposite leaves is produced, giving rise to tertiary axes t″ t″, ending in single flowers, and so on. The term dichasium has also been applied to this form of cyme.
In the natural order Carophyllaceae (pink family) the dichasial form of inflorescence is very general. In some members of the order, as Dianthus barbatus, D. carthusianorum, &c., in which the peduncles are short, and the flowers closely approximated, with a centrifugal expansion, the inflorescence has the form of a contracted dichasium, and receives the name of fascicle. When the axes become very much shortened, the arrangement is more complicated in appearance, and the nature of the inflorescence can only be recognized by the order of opening of the flowers. In Labiate plants, as the dead-nettle (Lamium), the flowers are produced in the axil of each of the foliage leaves of the plant, and they appear as if arranged in a simple whorl of flowers. But on examination it is found that there is a central flower expanding first, and from its axis two secondary axes spring bearing solitary flowers; the expansion is thus centrifugal. The inflorescence is therefore a contracted dichasium, the flowers being sessile, or nearly so, and the clusters are called verticillasters (fig. 17). Sometimes, especially towards the summit of a dichasium, owing to the exhaustion of the growing power of the plant, only one of the bracts gives origin to a new axis, the other remaining empty; thus the inflorescence becomes unilateral, and further development is arrested. In addition to the dichasial form there are others where more than two lateral axes are produced from the primary floral axis, each of which in turn produces numerous axes. To this form the terms trichasial and polychasial cyme have been applied; but these are now usually designated cymose umbels. They are well seen in some species of Euphorbia. Another term, anthela, has been used to distinguish such forms as occur in several species of Luzula and Juncus, where numerous lateral axes arising from the primary axis grow very strongly and develop in an irregular manner.
| (From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.) |
| Fig. 16.—Cymose inflorescence (dichasium) of Cerastium collinum; t-t″", successive axes. (After Duchartre.) |
| Fig. 17.—Flowering stalk of the White Dead-nettle (Lamium album). The bracts are like the ordinary leaves of the plant, and produce clusters of flowers in their axil. The clusters are called verticillasters, and consist of flowers which are produced in a centrifugal manner. |
In the uniparous cyme a number of floral axes are successively developed one from the other, but the axis of each successive generation, instead of producing a pair of bracts, produces only one. The basal portion of the consecutive axes may become much thickened and arranged more or less in a straight line, and thus collectively form an apparent or false axis or sympodium, and the inflorescence thus simulates a raceme. In the true raceme, however, we find only a single axis, producing in succession a series of bracts, from which the floral peduncles arise as lateral shoots, and thus each flower is on the same side of the floral axis as the bract in the axil of which it is developed; but in the uniparous cyme the flower of each of these axes, the basal portions of which unite to form the false axis, is situated on the opposite side of the axis to the bract from which it apparently arises (fig. 18). The bract is not, however, the one from which the axis terminating in the flower arises, but is a bract produced upon it, and gives origin in its axil to a new axis, the basal portion of which, constituting the next part of the false axis, occupies the angle between this bract and its parent axis—the bract from which the axis really does arise being situated lower down upon the same side of the axis with itself. The uniparous cyme presents two forms, the scorpioid or cicinal and the helicoid or bostrychoid.
| Fig. 18.—Helicoid cyme of a species of Alstroemeria. a1, a2, a3, a4, &c., separate axes successively developed in the axils of the corresponding bracts b2, b3, b4, &c., and ending in a flower f2, f3, f4, &c. The whole appears to form a simple raceme of which the axes form the internodes. Fig. 19.—Scorpioidal or cicinal cyme of Forget-me-not (Myosotis palustris). Fig. 20.—Diagram of definite floral axes a, b, c, d, e, &c. Fig. 21.—Flowering stalk of Ragwort (Senecio). The flowers are in heads (capitula), and open from the circumference inwards in an indefinite centripetal manner. The heads of flowers, on the other hand, taken collectively, expand centrifugally—the central one a first. |
In the scorpioid cyme the flowers are arranged alternately in a double row along one side of the false axis (fig. 19), the bracts when developed forming a second double row on the opposite side; the whole inflorescence usually curves on itself like a scorpion’s tail, hence its name. In fig. 20 is shown a diagrammatic sketch of this arrangement. The false axis, a b c d, is formed by successive generations of unifloral axes, the flowers being arranged along one side alternately and in a double row; had the bracts been developed they would have formed a similar double row on the opposite side of the false axis; the whole inflorescence is represented as curved on itself. The inflorescence in the family Boraginaceae are usually regarded as true scorpioid cymes.
In the helicoid cyme there is also a false axis formed by the basal portion of the separate axes, but the flowers are not placed in a double row, but in a single row, and form a spiral or helix round the false axis. In Alstroemeria, as represented in fig. 18, the axis a1 ends in a flower (cut off in the figure) and bears a leaf. From the axil of this leaf, that is, between it and the primary axis a1 arises a secondary axis a2, ending in a flower f2, and producing a leaf about the middle. From the axil of this leaf a tertiary floral axis a3, ending in a flower f3, takes origin. In this case the axes are not arranged in two rows along one side of the false axis, but are placed at regular intervals, so as to form an elongated spiral round it.
Compound definite inflorescences are by no means common, but in Streptocarpus polyanthus and in several calceolarias we probably have examples. Here there are scorpioid cymes of pairs of flowers, each pair consisting of an older and a younger flower.
Forms of inflorescence occur, in which both the definite and indefinite types are represented—mixed inflorescences. Thus in Composite plants, such as hawk weeds (Hieracia) and ragworts (Senecio, fig. 21), the heads of flowers, Mixed inflorescence. taken as a whole, are developed centrifugally, the terminal head first, while the florets, or small flowers on the receptacle, open centripetally, those at the circumference first. So also in Labiatae, such as dead-nettle (Lamium), the different whorls of inflorescence are developed centripetally, while the florets of the verticillaster are centrifugal. This mixed character presents difficulties in such cases as Labiatae, where the leaves, in place of retaining their ordinary form, become bracts, and thus might lead to the supposition of the whole series of flowers being one inflorescence. In such cases the cymes are described as spiked, racemose, or panicled, according to circumstances. In Saxifraga umbrosa (London-pride) and in the horse-chestnut we meet with a raceme of scorpioid cymes; in sea-pink, a capitulum of contracted scorpioid cymes (often called a glomerulus); in laurustinus, a compound umbel of dichasial cymes; a scorpioid cyme of capitula in Vernonia scorpioides. The so-called catkins of the birch are, in reality, spikes of contracted dichasial cymes. In the bell-flower (Campanula) there is a racemose uniparous cyme. In the privet (Ligustrum vulgare) there are numerous racemes of dichasia arranged in a racemose manner along an axis; the whole inflorescence thus has an appearance not unlike a bunch of grapes, and has been called a thyrsus.