ULTRASTRUCTURAL DIFFERENTIATION OF THE TANNINIFEROUS CELLS IN THE MARINE PHANEROGAM POSIDONIA-OCEANICA (L) DELILE

The mediterranean seagrass Posidonia oceanica (L.) Delile differs from the Australian species of Posidonia in the occurrence of special tanniniferous cells which are located in the first superficial cell layers of mesophyll and sometimes among the sieve elements of the vascular system. Ultrastructural observations of the basal leaf growing region have made it possible to study the development pattern of these secretory cells. Different processes must be involved in their differentiation, some of them related to the cell and its vacuolar system and others more especially to the secretion of tannins. Important lytic processes occur in small grouped and undifferentiated cells and induce the formation of highly vacuolated cells dispersed among less differentiated cells. There is no formation of intercellular spaces at the cell group centre by which secretion products might be eliminated. Tannin compounds are enclosed within these newly formed vacuoles. It is suggested that several sites exist for their synthesis. These observations are compared to the processes described in various secretory structures present in higher land plants.