Fruit structure and pericarp histogenesis of Cyclospatheae and Ceroxyleae (Arecaceae: Ceroxyloideae)

Abstract The Ceroxyloideae subfamily of Arecaceae, encompassing the tribes Cyclospatheae, Ceroxyleae, and Phytelepheae, displays diverse fruit morphology, yet their anatomical structure remains underexplored. The present study illustrates the pericarp structure and fruit development from Cyclospatheae (Pseudophoenix) and Ceroxyleae (Ceroxylon, Juania, Oraniopsis, and Ravenea) and aims to define their morphogenetic types. Fruits at various developmental stages were analysed using histological techniques, including microtome sectioning, phloroglucinol–HCl reaction, and light microscopy. This revealed that all studied taxa produce syncarpous, indehiscent drupaceous fruits characterized by a one-layered exocarp, a multilayered mesocarp (often differentiated into peripheral and inner zones), and a one-layered endocarp of palisade or cuboid sclereids, classifying the fruits as pyrenariums of the Ilex type. The distinct features of members of the Cyclospatheae and Ceroxyleae include a dentate-shaped exocarp in Ceroxylon utile, a belt of brachysclereids in Ceroxylon and Juania, and tannin-containing cells in Juania. It was noted that more elongated sclereids of the endocarp develop in Caribbean and northwestern South American taxa and shorter cuboid sclereids in Madagascar, Comoros, and Australian species. As in Cyclospatheae and Ceroxyleae, other Arecaceae taxa show the development of a one-layered sclerenchymatous endocarp in Phytelepheae (Ceroxyloideae), Caryoteae (Coryphoideae), and many Arecoideae.