Comparative vegetative anatomy and systematics of Spiranthoideae (Orchidaceae)

Abstract

The anatomy of leaf, stem and root of plants in the orchid subfamily Spiranthoideae was studied and described from the viewpoint of systematics. Plants were available from most of the geographic range. Tribes Diceratosteleae and Tropidieae show sinuous anticlinal epidermal cell walls in leaves, glandular foliar hairs, tetracytic para-mesoperigenous stomata, unitary tracheary components in the foliar midrib, foliar and cauline stegmata and sclerenchyma, typical cruciate starch grains, thick-walled exodermal, endodermal, and pericyclic cells, and conductive strands of the root embedded in sclerenchyma. The tribe Cranichideae shows straight to curvilinear anticlinal epidermal cell walls in leaves, lack glandular foliar hairs, have variably patterned mesoperigenous stomata, lack sclerenchyma throughout the parts studied, have a binary tracheary component in the foliar midrib, store starch in specialized amyloplasts (spiranthosomes), lack stegmata, have thin-walled exodermal, endodermal, and pericyclic cells, show scalariform thickenings in exodermal cells, and have conductive strands of the root embedded in parenchyma. In Cryptostylis the tracheary component of the foliar midrib is unitary, stomata lack subsidiary cells, starch grains are of the typical cruciate configuration, and passage cells of the endodermis are apparently associated with tilosomes. Anatomical data, when analysed cladistically, support the hypothesis that Spiranthoideae, as currently delimited, are polyphyletic. Corymborkis, Tropidia, and Diceratostele are more closely related to Palmorchis, a likely representative of a basal clade within subfamily Epidendroideae, than to genera of Cranichideae. Likewise, members of Cranichideae are more closely related to Diuris , a representative of subfamily Orchidoideae-tribe Diurideae, than to Corymborkis, Tropidia and Diceratostele. The Corymborkis-Tropidia-Diceratostele -basal epidendroid ( Palmorchis) clade may be diagnosed by the foliar synapomorphies of sinuous anticlinal walls of epidermal cells and presence of glandular hairs. The Cranichideae-orchidoid ( Diuris) clade may be diagnosed by its variably patterned, mesoperigenous stomata, lack of vascular bundle sclerenchyma, absence of stegmata, unthickened endodermal cell walls in roots, and conductive cells of roots embedded in parenchyma. Relationships within this clade are quite unresolved, when only anatomical data are employed; however all studied genera of Cranichideae, except Cryptostylis, posess a binary tracheary component in the foliar midrib. Cranichideae, excluding Cryptostylis, may be considered monophyletic. All Cranichideae, except Hetaeria and Cryptostylis , possess spirathosomes. Hetaeria may be a basal member within Cranichideae. We consider the phylogenetic position of Crptostylis , in relation to Cranichideae vs. Diurideae, to be equivocal.