Leafless and shootless epiphytic orchids rely essentially on CAM photosynthesis in roots for carbon gain. However, it is believed that a proportion of carbon is obtained by endomycorrhizal associations. In this study, we show that Dendrophylax lindenii possesses a dimorphic exodermis with smaller, thin-walled passage cells that are depleted in flavonoids. No hyphae succeeded in penetrating into the cortex from a non-passage cell, but 20% of the hyphae in contact with a passage cell managed to penetrate into the cortex. The passage cells represent 40% of the amount of cells in the centre of the side that touches the substrate, but no passage cells are observed in the upper side of the root. This distribution and density of exodermal passage cells define a strategy for controlling the extent and location of fungal invasion in the orchid root. This strategy provides a mechanism for restricting fungal growth to the lower cortex and thus maximising carbon gain from photosynthesis while enabling further trophic exchanges from mycorrhizal associations
