Comparative Flower Development in the Cleistogamous Species Viola odorata. III. A Histological Study

Abstract

Histological study confirmed that fundamental differences exist between cleistogamous (CL) and chasmogamous (CH) shoot apices and developing floral primordia of Viola odorata. At maturity, CH flowers are four times larger than CL flowers and of a divergent form. These differences result from both smaller cell numbers and cell sizes in mature CL floral organs. Position-dependent variation in anther volumes occurs in every flower, with the smaller anthers in an anterior position in the CL flower and in a lateral position in the CH flower. In CL flowers, mature pollen germinates in the anther sac, sending pollen tubes through the wall at the apex of the anther when endothecial thickenings fail to develop. In CH flowers, a fully differentiated endothecium is produced throughout the anther. In both flower forms, pollen viability is low. Variation occurs in pollen size and morphology within a single anther. The viable pollen is large and ovoid in both CL and CH anthers.