Salvinia leaves. I. Origin and early differentiation of floating and submerged leaves

Abstract

A study of floating and submerged Salvinia leaves using light and scanning electron microscopy shows unique features in the arrangement of leaves and their growth. Leaves are produced in phyllotactic units of six; within each phyllotactic unit are two sets or groups of three leaves each. The genetic spiral of leaf initiation is not unidirectional but alternates from clockwise to counterclockwise with the production of each group of three leaves. Within each group of leaves, the sequence of primordial expansion is the reverse of their inception. Observations of floating leaf apical cells show that during development they undergo configurational changes from rectangular to hemisperical to lenticular to tetrahedral. Floating and submerged leaves diverge structurally when they are 70–90μm in length. The general course of leaf development appears to differ from previously described ferns and angiosperms in that each floating leaf blade panel is generated from the abaxial primordial surface.