ORIGIN AND DEVELOPMENT OF THE RECEPTACLE OF CHRYSANTHEMUM MORIFOLIUM

Abstract

IN AN EARLIER paper Popham and Chan (1950) presented observation.s and a discussion of zonation in the vegetative shoot apex of Chrysanthemum morifolium Bailey. A commercially important variety, Bittersweet, was used as a basis for that study while occasional comparisons were made between this variety and the variety Orchid Queen. The plant material, culture methods, as well as microtechniques used, were described in detail. Temperatures in the ventilated greenhouses where the plants grew during the summer of 1947 were kept as low as practicable but ranged from 65? F. on some cool nights to as high as 1000 F. early on some afternoons. The present paper is a continuation of the same work; therefore, the same materials and methods were used and the observations are again based on the variety Bittersweet. The series of events occurring in the variety Bittersweet are nearly identical with those in Orchid Queen except for some variations (usually small) in the time factor. For this reason reference to the variety Orchid Queen will be made only in instances in which relatively great differences occur between the varieties. In the variety Bittersweet both the terminal bud of a branch and several subj,acent laterals develop into flower buds. The flower head or inflorescence of Chrysanthemum morifolium variety Bittersweet is of the daisy type, being composed of two rows of outer ray flowers (or florets) surrounding many tubular florets. In general, flower head development is characterized by three well-defined stages in a minutely transitory progression of events. The first is the vegetative condition of the stem tip (fig. 1) which has already been discussed (Popham and Chan, 1950). This stage may persist for long periods of time provided the plants are exposed to uninterrupted long photoperiods and proper temperatures. The second stage, the initiation and growth of the receptacle2 (fig. 2-7) and peduncle begins about the seventh day after exposure of plants to short (9 hr.) photoperiods and continues until the thirtieth day3 or later. It should be em-