Comparative anatomy of the assimilatory organs of Nepenthes species

Abstract

There is a lack of data on comparative anatomy of the assimilatory organs of the enigmatic carnivorous Nepenthes species; the linkages between their leaf tissue anatomy and physico-chemical properties are also rarely considered. We examined the anatomy of the leaf (lamina) and its conjoint pitcher in five Nepenthes species (Nepenthes ampullaria, N. bicalcarata, N. gracilis, N. hemsleyana and N. rafflesiana). A Nepenthes leaf displays the usual cuticle–epidermis–hypodermis–palisade–spongy structure with ample stomata distribution for gas exchange. The conjoint pitcher has similar anatomy but lacks a palisade mesophyll layer, and its inner epidermal wall is endowed with digestive glands of three cell layers. A higher level of variation exists in the anatomy of the pitcher relative to the leaf. Both stomata and digestive glands, being similar in origin, display the usual negative log–log relationship between size and density. Across species, the mean size but not density of the glands varied across three readily identified zones of the digestive section of the pitcher. Leaf and pitcher thicknesses correlated (P < 0.05) with stomatal and digestive-gland sizes. Organ longevity, lignin content and construction cost negatively correlated with lower cuticle, epidermal and mesophyll dimensions, and positively so with stomatal and digestive-gland densities. In contrast, major nutrients of N, P, K, and total ash had minimal influence on anatomical size dimensions. It is likely that in Nepenthes leaf and its conjoint pitcher, both the protective and physiological tissues drive anatomical differences and organ functions. The observed bivariate relationships between the anatomical traits also fit into the worldwide leaf economy spectrum.