Atypical fruit of Talinum triangulare (Jacq.) Willd., the type species of the genus Talinum (Talinaceae, former Portulacaceae)

Talinum triangulare sharply contrasts to the vast majority of Talinum species in explosive dehiscence of its fruits. Distinctively, Talinum species have a 3-valvate seed vessel that splits tangentially into outer and inner semi-valves. Such a seed vessel is exemplified by T. paniculatum. Its coriaceous outer semi-valves decay while its scarious inner semi-valves separate from the receptacle but remain hanging on mostly fibrous dorsal bundles. The outer semi-valves are traditionally considered epicarp (= exocarp). We have concluded, however, that the outer semi-valves really consist of lignified 1-layered exocarp (= outer epidermis) and unlignified parenchymatous mesocarp. The inner semi-valves consist of heterogeneous obliterated endocarp. Differently shrinking meso- and endocarp interact with a resistant exocarp and dorsal bundles to generate stresses in the seed vessel and cause loculicidal and basal circumscissile dehiscing of the capsule as well as tangential fissuring of the seed vessel. The lignified exocarp of T triangulare is an augmentative homologue of T. paniculatum's one as it consists of an outer epidermis and 1 to 3-layered hypodermis. The unlignified parenchymatous mesocarp of T triangulare is thicker than in T paniculatum, though it corresponds only to the inner part of the latter one (defective homologue). The heterogeneous endocarp of T triangulare is also thicker than in T paniculatum. Besides, it partly obliterates. Thicker endo- and mesocarp coherently shrink to interact with the more resistant thicker exocarp in order to generate higher stresses in the seed vessel. The dorsal and basal circumscissile faults of the seed vessel are flanked by reinforcing lignified tissues to prevent from precocious fissuring of the seed vessel and thus to promote accumulating stresses. The stresses accumulated cause hasty fissuring of the seed vessel and longitudinal bending of each of the 3 resultant valves. It is this hasty bending that induces centrifugal force in the receptacle that pushes the valves away.