The physodes or fucosan vesicles abundant in vegetative and reproductive cells of the brown seaweed Dictyota originate in the plastids. Precursor material expelled from the chloroplast matrix, may run together to produce the large physodes in the cell cytoplasm. Staining reactions indicate that the physodes are phenolic or tannin-like in nature. They may be products of photosynthesis. Physodes in the apical region of thalli are exceedingly resistant and stable compared with those in older parts of thalli; the latter are lost during glutaraldehyde fixation.