Key to species of Padina

1 Fronds heavily calcified on ventral "inner" surface, nearly obscuring hair and sporangial lines 2
1 Fronds lightly calcified, mostly as a very thin sheet of gray, hair and sporangial lines clearly observable 3
2 Calcification very chalky and continuous on ventral "inner" surface; bright golden-yellow to orange on dorsal "outer" surface P. melemele
2 Calcification thick but discontinuous, tan on upper part of ventral surface, and throughout dorsal surface P. japonica
3 Blades distromatic throughout 4
3 Blades distromatic except at base, or blades polystromatic 5
4 Fronds split through nearly to base; base thick, with rust-colored rhizoids forming a hump P. australis
4 Plants not split, but blades laterally adhering to each other, frequently to the base and difficult to separate without tearing; almost always in large clumps P. thivyae
5 Blades distromatic in upper parts, tristromatic below; sporangia grouped above every hair line on inner surface, hairs lacking on opposite surface P. tenuis
5 Blades three or more layers thick 6
6 Blades 4 cells thick in upper frond, up to 9 cells thick below; sporangia grouped above every hair line on inner surface, hairs lacking on opposite side P. crassa
6 Blades mostly 3-celled (rarely 2-celled) throughout, sporangia grouped above every hair line on inner surface, but both groups alternating with a hair line on opposite surface P.  boergesenii

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