An Anatomical Basis for the Divergent Floral Forms in the Cleistogamous Species, Lamium Amplexicaule L. (Labiatae)

The pattern of variation in mature corolla form has been documented throughout the main shoot inflorescence of the cleistogamous species Lamium amplexicaule L. A heteroblastic development occurs within the cymose inflorescence at a single node as well as between different nodes. Variation involves an increase in cell number and expansion in the upper corolla and also an increase in cell number in the anther sacs, expressed as pollen count increases, in progressively produced flowers in the inflorescence. The corolla base region in all flowers shows an increased cell expansion from the first produced cleistogamous (CL) flowers to the later produced chasmogamous (CH) flowers which expand at anthesis. Cell numbers are equivalent in the corolla base region of all flowers. It appears that in successively produced flowers in the inflorescence, prolonged cell division and expansion occurs in both the upper corolla region and the anthers until a threshold is reached, at which point flowers may undergo anthesis and become CH. The CL corolla form in Lamium amplexicaule is not caused by a lack of cell expansion alone; rather, both cell division and expansion are arrested in the upper corolla region.