Floral Morphogenesis in Lamium amplexicaule L. (Labiatae) with a Model for the Evolution of the Cleistogamous Flower

At inception and during early organogenesis, cleistogamous (CL, closed) and chasmogamous (CH, open) flowers are morphologically and histologically indistinguishable. Divergence does not occur until the late stages of anther differentiation, prior to meiosis. Subsequent to anther divergence, corolla shape divergence occurs in the two flowers; this is seen as a faster increase in the length/width growth in the CL flower. Precocious maturation occurs in CL flowers due to early anther dehiscence and stigma receptivity. A model is presented which describes the CL flower as a progenetic dwarf form of the CH flower.