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an order of brown algae (Phaeophyceae) comprising more than 100 species. They live in warm seas; in the USSR they live in the Black Sea and the Sea of Japan. There are four known genera. The thallus is eight to 25 cm high, dichotomously divided into two flat branches (Dictyota) or fan-shaped branches (Padina). It consists of one or several layers of large cells and a single-layer cortex of small cells: it grows from the apex. Reproduction is both sexual and asexual. The gametophytes and sporophytes have identical structure. The sexual process is oogamy. One immobile ovum grows in each oogonium. The male cells (antherozoids) have one flagellum and are formed in multilocular antheridia. The sporophytes usually produce tetraspores; meiosis occurs during this process. Often the sporangia, which do not produce spores, fall off and grow into new plants; sometimes the sporangia begin to grow on the mother plant. If the Dictyotales have tetraspores and single-flagellum antherozoids, they are sometimes placed in a separate class of brown algae—Aplanosporophy ceae.