Abstract
The development and composition of compound inflorescences in poinsettia were observed from flower initiation to anthesis at 10-day intervals.
1. Under short day conditions, the primary cyathium differentiates at the tip of a shoot. Compound inflorescences made up of cyathia, bracts, and the inflorescence axis subsequently develop from axils of upper leaves. We termed each of them as a compound inflorescence unit. The terminal cyathium and upper three compound inflorescence units, which have been called a flower or an inflorescence, are considered a compound inflorescence in this paper.
2. The differentiation of cyathia and bracts in a compound inflorescence unit show a definite pattern; after the cyathium of the order n forms at the tip of the inflorescence axis of the order n, 2 cyathia and 4 bracts of the order n+1 differentiate from axils of the bracts of the order n.
3. In the average plant, one of the two differentiated primordia of cyathia and two of the four of bracts continue to abort from third and second order, respectively.
4. There was a close relation between the position of primordia and their abortion. As a result, a compound inflorescence unit develops as a scorpioid cyme at high order, while primordia differentiate as a dichasium.
5. When the source/sink ratio of a plant is high, compound inflorescences (flowers) with many-branched axes develop rapidly.
