LIFE-HISTORY, REPRODUCTIVE MORPHOLOGY AND DEVELOPMENT OF THE ANTARCTIC BROWN ALGA DESMARESTIA-MENZIESII J-AGARDH

The life history, reproduction and development of Desmarestia menziesi i J. Agardh from Antarctica is described. Unilocular sporangia occur s ingly or in small groups in the outermost cortical layer of the sporop hyte. They are formed by periclinal division of cortex cells into a st alk cell and the sporangium initial. Meiospores germinate into dioecio us microscopic filamentous gametophytes. As in other perennial Antarct ic species of the Desmarestiales, gametangia are formed in culture und er short-day conditions or in darkness. In nature, juvenile sporophyte s should therefore be formed in winter. They develop only attached to the oogonium. At first they are uniseriate and elongate by means of an intercalary meristem located in their middle part. Laterals are forme d predominantly in this region, and they subsequently give rise to sec ondary laterals. The branching pattern is opposite to alternate in bot h young and adult plants. Cortication of the main axis is initiated by filaments growing out from the lowermost cells of the primary lateral s. Tn sporophytes of this developmental stage the meristem of the main axis is confined to a small region where cortication starts and above . Lateral branches elongate and become corticated in the same way as t he main axis. In mature plants, cells of the inner cortex can become m eristematic again and form a meristoderm which contributes to axis thi ckness by periclinal and anticlinal divisions. The observations are di scussed in relation to possible evolutionary relationships in the genu s Desmarestia and in the order Desmarestiales.