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Alsidium corallinum C. Agardh from the Mar Grande of Taranto (Ionian Sea, Mediterranean Sea) reproduced primarily by propagules. These were starch-filled branchlets subtended by little-pigmented axes, which reached the maximum length of 6 mm with a diameter of about 300 µm. Propagules could both be branched up to three orders and form other propagules in their turn. On mature plants propagules originated by the transformation of the ultimate order branch whereas on over-wintering plants they derived from the transformation of lower order branches. On some individuals, some tetrasporangial branchlets grew as propagules after having formed a variable number of tetrasporangia. Some propagules also formed tetrasporangia. Generally, propagules detached above the little-pigmented axis which successively regenerated a new propagule. In the field, propagules settled on various substrata and propagule-derived plantlets were often observed. As the other species of Alsidium do not form propagules, propagule production by A. corallinum may be considered as a specific character. [ABSTRACT FROM AUTHOR]