WHAT IS THE DITHECOUS LEAF? AN INVESTIGATION OF THE NEOTROPICAL PODOSTEMUM RUTIFOLIUM SUBSP. RICCIIFORME (PODOSTEMACEAE – PODOSTEMOIDEAE)

The so-called dithecous leaves with two sheaths (adaxial and abaxial) which occur at branching in species of subfamily Podostemoideae of the aquatic flowering plant family Podostemaceae are enigmatic structures. The fact that in many species the shoot axes and leaves are fused has previously prevented an understanding of the real nature of dithecous leaves. This study reports on the position of the dithecous leaf in Podostemum rutifolium subsp. ricciiforme, the simple shoot structure of which has revealed that the dithecous leaf is the first leaf of the branch and does not arise from the main shoot as previously believed. Accordingly, the two sheaths of the dithecous leaf are interpreted anew: the sheath towards the branch, and its flower bud, is the true (ventral, adaxial) sheath of the dithecous leaf (formerly viewed as an additional new structure), and the other (dorsal, abaxial) sheath towards the flower bud of the main shoot of the branch is a novel structure. Consequently, the branch does not develop from the reverse side of the dithecous leaf in a subfoliar position, but rather is axillary above its subtending leaf. A similar structure was described in a previous paper on Apinagia riedelii, and in both species, and thus in subfamily Podostemoideae, the ramification pattern conforms to the pattern in other angiosperms.