Comparative vegetative anatomy and systematics of the angraecoids (Vandeae, Orchidaceae) with an emphasis on the leafless habit

Abstract

The vegetative anatomy and morphology of 142 species of the angraecoid orchids (Angraecinae + Aerangidinae) and 18 species of Aeridinae were examined using light and scanning electron microscopy. Leafless members of Vandeae were of particular interest because of their unique growth habit. Leafy and leafless members of Angraecinae and Aerangidinae were examined and compared with specimens of Aeridinae. Vandeae were homogeneous in both leaf and root anatomy. A foliar hypodermis and fibre bundles were generally absent. Stegmata with spherical silica bodies were found associated with sclerenchyma and restricted to leaves in almost all specimens examined. Distinct inner tangential wall thickenings of the endovelamen occurred in several vandaceous genera. Exodermal proliferations and aeration units commonly occurred in both leafy and leafless Vandeae. Cladistic analyses of Angraecinae and Aerangidinae with members of Aeridinae and Polystachyinae as outgroups using 26 structural characters resulted in 20 000+ equally parsimonious trees. Vandeae formed the only well-supported clade in bootstrap analyses and were characterized by having a monopodial growth habit, spherical stegmata, loss of mucilage, and loss of tilosomes.