The use of medullary unit patterns of intergenicula and genicula in the taxonomy of Amphiroa (Corallinaceae, Rhodophyta)

Abstract

Branch structure and growth in Amphiroa generated by primarily dividing apical medullary cells was documented by light and scanning electron microscopy. Specific numbers of long- and short-celled tiers are formed when intergenicular and genicular medullary cells periodically divide, leading to the unit pattern concept. Zonations on the intergenicular surface, grooves on the decalcified intergenicular surface and spaces in longitudinal section of intergenicula underlie units. Three unit pattern types are recognized: angled medullary tiers arranged in a straight horizontal row with lateral initials cut off abruptly from peripheral medullary cells in A. annulata and A. anastomosans (type A), medullary tiers whose length gradually reduces as they curve into the cortex in A. fragilissima var. debilis, A. rigida var. antillana, A. spina and A. verruculosa (type B), and rounded medullary tiers arranged in slightly irregular horizontal rows with lateral initials or cortical cells surrounding circularly peripheral medullary cells that are cut off from the medulla in A. cuspidata (type C). These patterns were statistically analysed in four Bermudan species: A. annulata, A. cuspidata, A. fragilissima var. debilis and A. rigida var. antillana. Unit patterns can usefully distinguish species. The association of intergenicula and genicula at the branch apex during the initial stage of branch formation is explained for the following four species: A. annobonensis, A. crustiformis, A. fragilissima var. debilis, and A. itonoi.