Stem structure and its bearing on the systematics of Pleurothallidinae (Orchidaceae)

Abstract

Pleurothallids comprise a subtribe of numerous orchids consisting of mostly diminutive New World epiphytes. The cauline system involves a branching rhizome bearing unifoliate ramicauls. The inflorescence usually originates from an adaxial invagination toward the apex of the ramicaul. Two or more cycles of vascular bundles traverse the ramicaul; typically, there is an outer, branching series of smaller strands and an inner, non-branching series of larger strands. In the ultimate internode subtending the leaf the smaller, outer bundles migrate between the larger, inner bundles and become aggregated in the medullary region. They bend adaxially to vascularize the inflorescence. The leaf is articulated at an abscission layer to the prolongated apex of the ramicaul more or less distal to the cauline invagination. In some pleurothallids, a cauline annulus appears at the insertion of the inflorescence and cither below or at the attachment of the leaf to the apex of the ramicaul. Anatomical studies show that the annulus is the external manifestation of an intercalary meristem. Except for Pleurothallis, genera are consistent in having or lacking an annulus. Genera with two pollinia either possess or lack the annulus; those with four to eight pollinia always lack the annulus. It is suggested that the intercalary meristem is a derived characteristic providing survival value and a selective advantage to those pleurothallids in which it occurs. Presence of the intercalary meristem is correlated with other structural features considered to represent specializations.