THE ANATOMY AND ULTRASTRUCTURE OF CONDUCTING CHANNELS IN ASCOSEIRA-MIRABILIS (ASCOSEIRALES, PHAEOPHYCEAE)

The conducting channels' of Ascoseira mirabilis are unique, aseptate structures running the length of the blade in the central medulla. Their function is presently unknown, and this study of their anatomy and ultrastructure was carried out to obtain evidence of their possible functional role. Conducting channels contain a continuous mass of cytoplasm and have frequent connections with cells of ensheathing medullary filaments via thin sieve-like cell walls perforated by many pits. The cytoplasm is multinucleate, and contains chloroplasts, mitochondria, Golgi bodies and endoplasmic reticulum. Mature conducting channels are densely packed with physodes containing phlorotannins. The anatomical relationships of conducting channels suggest that they could have a role in translocation of solutes, but it is concluded that the high frequency of physodes in mature channels makes this unlikely. It is possible that young conducting channels in newly developing tissue may have a transport role, and the accumulation of phlorotannins in mature tissue may have a protective function. The significance of similarities between conducting channels in Ascoseira, sieve tubes in the Laminariales, and trumpet hyphae in the Desmarestiales is discussed. © 1990, IEEE. All rights reserved.