Piaranthus is a succulent plant genus in the tribe Stapeliae, milkweed subfamily Asclepiadoideae, in the family Apocynaceae (dogbane).
The large flowering plant genus Grewia is today placed by most authors in the mallow family Malvaceae, in the expanded sense as proposed by in the APG.
 Formerly, it was placed in either the linden family (Tiliaceae) or the Sparrmanniaceae.
 However, these were both not monophyletic with respect to other Malvales - as already indicated by the uncertainties surrounding placement of "Grewia" and similar genera - and have thus been merged into the Malvaceae.
 Together with the bulk of the former Sparrmanniaceae, "Grewia" is in the subfamily Grewioideae and therein the tribe Grewieae, of which it is the type genus.
The large flowering plant genus Grewia is today placed by most authors in the mallow family Malvaceae, in the expanded sense as proposed by in the APG.
 Formerly, it was placed in either the linden family (Tiliaceae) or the Sparrmanniaceae.
 However, these were both not monophyletic with respect to other Malvales - as already indicated by the uncertainties surrounding placement of "Grewia" and similar genera - and have thus been merged into the Malvaceae.
 Together with the bulk of the former Sparrmanniaceae, "Grewia" is in the subfamily Grewioideae and therein the tribe Grewieae, of which it is the type genus.
Duvalia is a succulent plant genus in the tribe Stapeliae, milkweed subfamily Asclepiadoideae, in the family Apocynaceae (dogbane).

Question: Which plant is in the genus Stapeliae, Duvalia or Grewia?
Answer: Duvalia