Roots of Sobralia Macrantha (Orchidaceae): Structure and Function of the Velamen‐exodermis Complex

Roots of Sobralia macrantha, a largely terrestrial orchid, bear a typical multilayered velamen underlaid by a cortical exodermis. The latter consists of densely cytoplasmic passage cells regularly interspersed among much larger, elongate, vacuolate cells. A structure which may be peculiar to Sobralia, or at least unusually well developed there—the fibrous body—consists of numerous intermeshed and divided wall outgrowths arising from two or three velamen cells positioned directly over the passage cell. This body is well designed to retard root transpiration. Should the fibrillar components be appropriately hygroscopic, it may act as a valve, promoting movements of water into the cortex while the velamen is engorged. The general mode of moisture and mineral procurement proposed for the Sobralia macrantha root probably applies to many other soil‐ and canopy‐dwelling orchids with a velamen‐exodermis complex.