Fascicles and Filamentous Structures: Comparative Ontogeny of Morphological Novelties in Triuridaceae

Abstract

Floral ontogeny is described in five species—representing four (of 11) genera—of the mycoheterotrophic monocot family Triuridaceae in order to address hypotheses on the evolutionary origin of their reproductive units, in which the inflorescence–flower boundary is ambiguous. This study provides broader ontogenetic sampling across all three tribes of Triuridaceae (Kupeaeae, Triurideae, and Sciaphileae). The observations highlight some radical morphological shifts in floral units of Triuridaceae, marked by associated morphological novelties. These include the presence of carpel fascicles in Lacandonia, Triuris, and Peltophyllum, centrifugal carpel inception in Lacandonia and Triuris, paired carpel arrangement in some species, unusual filamentous structures in atypical positions in several species, and the remarkable inside‐out flowers of Lacandonia. The novel carpel fascicles are relevant to the inflorescence versus flower debate because fascicles are sometimes interpreted as axial structures, analogous to a compound leaf. A recent alternative hypothesis states that carpel multiplication in Triuridaceae occurred by fasciation, or folding of an enlarged floral apex. Carpels in fascicles of Peltophyllum show opposite orientation with respect to those of Lacandonia, which could support a fasciation hypothesis; the apparent inversion could have preadapted Lacandonia for central stamen formation. Two contrasting hypotheses for the origin of a single terminal stamen whorl in the Lacandonia flower are potentially supported by ontogenetic data, by either homeosis (heterotopy) or secondary carpel development, attributable to delayed determinacy (heterochrony).