Abstract: The seaweed Sargassum horneri is an marine benthic brown alga in the Northwest Pacific and discontinuous distributed along the coast of China. The morphological traits of S. horneri are brown and consist of the holdfast, stem, leaves, gas vesicles and receptacles. There are two ecological types of S. horneri, the floating type and the benthic type. Comparing the morphological traits of each ecological type, significant differences were found. However, when it came to the microscopic and submicroscopic structures, almost no variation was found between these two types. Compared with benthic S. horneri, the floating S. horneri had shorter burl distance and more densities of burls and gas vesicles， which might be influenced by the external environment and internal physiology in the process of floating. There were three kinds of cells that were observed in the algae, which were epidermis, cortex and medulla cells from the surface to the center. The size of the cells were gradually increased from epidermis, cortex to medulla cells; while the contents of chloroplast (C)and physode (Ph)decreased stepwise, especially, there existed abundant of plasmodesmata (Pla)in the intercellular of cortical cells.

Key words: Sargassum horneri, floating Sargassum horneri, benthic Sargassum horneri, morphology, structure