FLORAL DEVELOPMENT IN SAURURUS CERNUUS (SAURURACEAE). 2. CARPEL INITIATION AND FLORAL VASCULATURE

Paired initiation of stamens and carpels on the apical meristem and bilateral symmetry of the flower are shown to prevail in Saururus cernuus. The carpels are initiated in two successive pairs of two members each, in a decussate arrangement. The floral apex shows plastochronic expansion in area between initiation of the last stamens and the first carpels, and also a less marked expansion between the two pairs of carpels. Vascularization appears to be whorled, with all carpels being supplied at once and at the same level. The vasculature of the flower is basically four‐stranded, such that each strand supplies a carpel and one, two, or three stamens. Carpels of a flower become unequal in size and level of attachment through post‐initiatory changes; these changes obscure the paired, decussate sequence of initiation. Phylogenetic conclusions about Saururus based on its supposedly spiral initiation of parts are in need of reexamination; Saururus exhibits a highly specialized pattern of paired organ initiation which is bilaterally organized, rather than radial and helical as in most primitive angiosperms. Vasculature of the flower involves divergence of appendage traces at approximately the same time for all stamens, and simultaneously for all carpels, contrary to previous reports that divergence is helical or spiral in succession. All carpel bundles diverge from sympodia which supply at least one stamen, also contrary to previous descriptions.