Comparative vegetative anatomy and mycorrhizal morphology of three South Indian Luisia species (Orchidaceae) with the note on their epiphytic adaptations

Abstract

The anatomical characteristics of the vegetative structures and mycorrhizal morphology of Luisia pulniana Vatsala, Luisia tenuifolia Blume and Luisia tristis Hook.f. belonging to the subfamily Epidendroideae (Orchidaceae) were investigated. Luisia species have amphistomatic leaves covered by a thick cuticle, paracytic stomata, and homogenous mesophyll containing raphides, silica bodies, water-storage cells and collateral vascular bundles. The leaf sheath has an upper epidermis of conical papillate shaped cells, homogenous mesophyll, water storage cells, raphides and sclerenchymatous fibers. The process of stem thickening through cortical proliferation was observed for the first time in Luisia species. The uniseriate epidermis is dimorphic in L. pulniana stem. The stem cortex is parenchymatous comprising of fibrous idioblasts. The ground tissue is parenchymatous (L. pulniana) or sclerenchymatous (L. tenuifolia, L. tristis) containing scattered collateral vascular bundles. Xylem and phloem capped by sclerenchymatous patches. The root hairs are restricted to root regions attached to the substratum. Velamen 2–4 layered, exodermal cells with greatly thickened outer tangential walls, cover cells present. Endodermis O-thickened, vascular cylinders 15–23 arched. Mycorrhizal fungi entered roots through the root hairs and formed pelotons in the root cortex. The anatomical characteristics varied significantly among the Luisia species. Some of these anatomical features relate Luisia with epiphytic and xerophytic habitats.