Inflorescence of Philadelphus

Abstract

Occasional branches of the common Philadelphus coronarius L. of our gardens illustrate a transition from the cymose (determinate or basipetal) to the racemose (indeterminate or acropetal) type of inflorescence. The flowers of this shrub are typically borne at and near the ends of leafy branches (Fig. 1). At the tip of such a branch is a dichasium of three flowers, the central blooming first. The remaining flowers are characteristically paired on opposite sides of the branch; the leaves which subtend them are reduced gradually upwards from foliage leaves to minute bracts. The uppermost pair of axillary flowers opens after the terminal flower but usually before the lateral flowers of the terminal dichasium; thenceforth flowering is basipetal down the branch. On the aberrant branches mentioned, which elongate more than the others, the flowers in the lower axils bloom first and flowering is acropetal; though on them also the terminal flower precedes the two immediately beneath it (Fig. 2). The character of the branch suggests that the change in order of flowering is brought about by physiological factors. One may suppose that in the rapidly maturing tip of the typical branch substances are formed which are concerned with flowering; in aberrant branches the greater degree of elongation delays development at the tip until after the lower axils have begun the production of flowers. Whatever the factors involved (and their identification would seem a fertile field for experiment), we see here the ease with which the cymose may pass to the racemose inflorescence; a transition which implies that the order in which flowers open is of little or no morpholcogical significance. This conclusion has been reached also in a consideration of the inflorescence of Crataegus (Rickett 1943). It may be objected that, since the apparently racemose branches of Philadelphus have terminal flowers, and since these are not the last to open, such branches are not true racemes. But, if this be granted, the same must be said of many inflorescences, commonly treated as racemes, which possess early-blooming terminal flowers (e.g. in Campanula, Rubus, Berberis). Which is the more primitive in Philadelphus, the cyme or the raceme? The presence on all flowering branches of the terminal dichasium suggests the former. On the other hand, textbooks often present inflorescences in general as having been derived from branches bearing solitary flowers in the axils of leaves. A comparison of several species of this genus is of interest here. The following notes were made from specimens preserved in the herbarium of the New York Botanical Garden.1