Morphological analysis of shoots and roots in Thelethylax minutiflora and T. insolata (Podostemaceae Podostemoideae): taxonomic and evolutionary implications
published: Sep 4, 2008
The paper reports on the vegetative and floral morphology of Thelethylax minutiflora (Tul.) C. Cusset and T. insolata (Perr.) Jäger-Zürn stat. et comb. nov. (Podostemaceae — Podostemoideae). The two species endemic to Madagascar are characterized by dimorphic root-borne shoots: sessile floriferous shoots with few scale leaves and a terminal flower, and elongated shoots with foliage leaves. The latter remain vegetative in T. minutiflora, but may bear floriferous branches in T. insolata. Important characters are (1) axillary (not subfoliar) ramification of the elongated shoot, (2) absence of dithecous leaves otherwise largely common in subfam. Podostemoideae, (3) floriferous shoots arising not only from roots, but regularly also from the axils of foliage leaves of the elongated shoots in T. insolata, (4) transversally spreading pinnately divided leaves in T. minutiflora (in contrast to pinnately divided leaves spreading in the median plane in many neotropic species of subfam. Podostemoideae, highly elaborate “ensiform leaves”), (5) intercalary, (not marginally), growth of the entire blade in transverse direction by means of anticlinal divisions in T. insolata, and (6) position of carpels and stamens in continuation of the distichous leaf arrangements (not perpendicular to the leaf distichy as in other Podostemoideae). The position of the flower clusters along the foliated elongated shoots of T. insolata points to the inflorescence structure in the form of a thyrse or a panicle. The two species obviously bear ancestral characters and open new aspects of architecture and structural evolution of subfam. Podostemoideae.