HORIZONTAL STRUCTURES ON PNEUMATOPHORES OF AVICENNIA-MARINA (FORSK) VIERH - A NEW SITE OF OXYGEN CONDUCTANCE

Conductance of pneumatophores of the grey mangrove, Avicennia marina, to oxygen depended on lenticel number up to about 25 functional lenticels per pneumatophore, and probably on cross-sectional area of the gas space in pneumatophores with over 25. The dependence on lenticel number was far from perfect, because of a substantial non-lenticellular component of conductance. This non-lenticellular conductance was attributed to 'horizontal structures' on the most recently formed part of the pneumatophore. These were thought to result from persistent portions of the root cap which remained attached to the surface of the pneumatophore, and around which breaks in the surface of the periderm developed. Following flooding, the lenticels dried and resumed full conductance within about three minutes. Non-lenticellular conductance imputed to the horizontal structures took about ten times as long to recover after flooding. Nevertheless, these horizontal structures are likely to represent a significant pathway for admission of oxygen, especially in rapidly growing roots where the tip region lacks lenticels. A name, 'subrisules', for these horizontal structures is proposed.