B. FLOWER CLUSTERS WITH DETERMINATE
INFLORESCENCE.
829. The simplest mode of determinate inflorescence is a plant with a solitary terminal flower, as in the hepatica, the tulip, etc. The leaves in these two plants are clustered in the form of a rosette, and the aerial shoot is naked and bears the single flower at its summit. Such a flower-shoot is a scape. As in the case of the indeterminate inflorescence, so here the larger number of flower-shoots are more complex and specialized, resulting in the evolution of flower clusters or masses. Accompanying the association of flowers into clusters there has been a reduction in leaf surface on the flower-shoot so that the flowers predominate in mass and are more conspicuous. Among the recognized modes of determinate inflorescence, the following are the chief ones:
830. The cyme.—In the cyme the terminal flower on the main axis opens first and the remaining flowers are borne on lateral shoots, which arise from the axils of leaves or bracts, below. These lateral shoots usually branch and elongate so that the terminal flowers on all the branches reach nearly the same height as the terminal flower on the main shoot, forming a somewhat flattened or convex top of the flower cluster. This is illustrated in the basswood flower. The anthesis of the cyme is centrifugal, i.e., from the inside outward to the margin. But it is often more or less mixed, since the lateral shoots if they bear more than one flower are diminutive cymes and the terminal flower opens before the lateral ones. Where the flower cluster is quite large and the branching quite extensive, compound cymes are formed, as in the dogwood, hydrangea, etc.
Fig. 453.
Diagrams of cymose inflorescence. A, dichasium; B, scorpioid cyme; C, helicoid cyme. (After Strasburger.)
831. The helicoid cyme.—Where successive lateral branching takes place, and always continues on the same side a curved flower cluster is formed, as in the forget-me-not and most members of the borage family. This is known as a helicoid cyme ([fig. 453, C]). Each new branch becomes in turn the “false” axis bearing a new branch on the same side.
832. The scorpioid cyme.—A scorpioid cyme ([fig. 453, B]) is formed where each new branch arises on alternate sides of the “false” axis.
833. The forking cyme is where each “false” axis produces two branches opposite, so that it represents a false dichotomy (example, the flower cluster of chickweed).
834. Some of these flower clusters are peculiar and it is difficult to see how the helicoid, or scorpioid, cymes are of any advantage to the plant over a true cyme. The inflorescence of the plant being determinate, if the flowering is to be extended over a considerable period a peculiar form would necessarily result. In the helicoid cyme continued branching takes place on one side, and the result in the forget-me-not is a continued inflorescence in its effect like that of a continued raceme (compare shepherd’s-purse). But we should not expect that all of the complex and specialized structures from simple and generalized ones are beneficial to the plant. In many plants we recognize evolution in the direction of advantageous structures. But since the plant cannot consciously evolve these structures, we must also recognize that there may be phases of retrogression in which the structures evolved are not so beneficial to the plant as the more simple and generalized ones of its ancestors. Variation and change do not result in advancing the plant or plant structures merely along the lines which will be beneficial. The tendency is in all directions. The result in general may be diagramed by a tree with divergent and wide-reaching branches. Some die out; others remain subordinate or dormant; while still others droop downward, showing a retrogression. But in this backward evolution they do not return to the condition of their ancestors, nor is the same course retraced. A new downward course is followed just as the downward-growing branch follows a course of its own, and does not return in the trunk.