Why don't the gas spaces of submerged organs of wetland plants flood extensively when damaged? In addressing this intriguing question, Soukup et al. (pp. 71–75 in this issue) report on the role of rhizome diaphragms as barriers to flooding in Phragmites australis. This should prompt some reappraisal of the ways in which flooding resistance can be realized, even perhaps in undamaged organs.

Most emergent wetland macrophytes have an abundance of interconnected internal gas space, much of it in the form of large voids transversely partitioned at intervals by perforated cellular plates termed diaphragms. Functionally, it provides a low-resistance pathway for internal oxygen transport to support the respiratory needs of submerged and buried organs (Armstrong, 1979; Armstrong et al., 1988; Crawford, 1992) and facilitates carbon dioxide removal. However, it does more than this, since it enables oxygen to be released from the root to where it can support aerobic microbial activity in otherwise anaerobic sediments, and phytotoxin immobilization or destruction (Armstrong et al., 1992; Begg et al., 1994; Gilbert & Frenzel, 1998). This oxygen release is regarded by some as a valuable aid to effluent purification by constructed wetlands. Perhaps a less desirable property of this gas-space provision is its recently discovered role in enhancing the emissions of greenhouse gases such as methane from wetlands (Brix et al., 1992; Chanton & Whiting, 1996; Crutzen, 1991; Dacey & Klug, 1979).