Bijdr. (1825) 385
Sympodial epiphytes with very short to rather long rhizomes. Pseudobulbs present or not, consisting of one to several internodes, 1- to few-leaved; when pseudobulbs absent stem short, few- to many-leaved, with the leaves arranged in two rows. Leaves sheathing or not, glabrous, deciduous, duplicate, leathery. Inflorescence arising from the base of the pseudobulb, or lateral from the stem, a many-flowered raceme. Flowers very small, resupinate, often hardly opening, white or green, soon becoming yellowish. Sepals free or connate at the base. Petals free, usually about as long as the dorsal sepal but mostly dissimilar to it. Lip without spur, not mobile. Column-foot absent or very short. Pollinia 8, solid, caudicles present, stipe absent, viscidium present.
Southeast Asia, Indochina, Malaysia, Indonesia, the Philippines, Papua New Guinea, Australia, Solomon Islands. About 25 species; in New Guinea about 15 species.
Epiphytes in lowland and hill forest.
Thelasis is undoubtedly one of the least ornamental of all orchid genera. It is very closely related to Phreatia and rarely seen in cultivation, except in scientific collections. These are rather small epiphytes with green or whitish flowers that do not open widely. As in Phreatia, some species have distinct pseudobulbs while in others these are lacking. Delimitation against the related genus Phreatia is complicated by the existence of a group of intermediate-looking species, which Schlechter, after including them in Phreatia, decided to put in a genus of their own. This genus, called Rhynchophreatia, was taken up in the checklist on vol. 1 of this series, but after having studied living specimens we have to agree with J.J. Smith, who included this group of species as a section Hemithelasis in the present genus. These species differ from Thelasis in the strict sense by the possession of a short but distinct column-foot; on the other hand, the very fleshy flowers and the long rostellum (except in the especially problematic Thelasis sphaerocarpa) place them in Thelasis. The final word on this issue has yet to be spoken.
Key to the sections of Thelasis
1a Plant with distinct, one- or few-leaved-pseudobulbs ... section Thelasis
1b Plant without distinct pseudobulbs ... 2
2a Column-foot absent ... section Oxyanthera
2b Column-foot present ... section Hemithelasis