Cercis canadensis (Fabaceae, Caesalpinnoideae) has a simple, palmately veined leaf with a large upper pulvinus terminating in a cushion of pulvinus tissue that extends into the lamina. Controversy over how this leaf type, unique within the Fabaceae to the tribe Cercideae, was derived has long been debated. The derivation of this leaf is thought to be from either the fusion of leaflets of a palmately or pinnately compound leaf or the reduction of a compound leaf to a single terminal leaflet. Microscopic observations of the upper pulvinus in C. canadensis showed three cylinders of vascular tissue surrounded by pulvinus tissue. The central cylinder becomes the midrib and is formed from two adaxial and a central abaxial bundle at the proximal end of the upper pulvinus. Lateral vascular cylinders separate into three primary veins within the upper pulvinus. Partially fused secondary pulvini at the base of each primary vein form the pulvinus cushion. The pulvini and associated vascular tissue are homologous to those reported for leaflets. A mucro at the distal end of the midrib that abscises shortly after the unfolding of the lamina indicates that the leaflets originally formed a pinnately compound structure. This was expected as pinnately compound leaves are characteristic for the subfamily Caesalpinnoideae. Measurements of movement (over 12-hours) of the midrib with respect to the petiole and the lamina basal lobes with respect to the midrib were done on 24 leaves growing on six branches of two trees. All of these leaves were harvested for microscopic observation. Changes in the leaf movements studied can be attributed to the expansion and contractions of the partially fused secondary pulvini. Additional evidence that fusion of leaflets from a pinnately compound leaf gave rise to the simple leaf in Cercis is presented.

Key words: Cercis canadensis, pulvinus cushion, secondary pulvini