Padina, Adanson, 1763

Pagana, Ilaria, Marroccia, Genny, Marletta, Giuliana & Alongi, Giuseppina, 2023, Re-examination of the distribution and species diversity of the genus Padina (Dictyotales, Phaeophyceae) in the Mediterranean Sea, Phytotaxa 619 (3), pp. 205-218 : 216

Key to the species of the genus Padina in the Mediterranean Sea.

1. Thallus with 2 layers of cells and/or a mixture of 2–3 layers of cells in the median portion ............................................................2

- Thallus with at least 3 layers of cells in the median portion ..............................................................................................................3

2. Thallus entirely composed of 2 layers of cells; hair lines are in concentric lines on the upper surface (ephemeral or absent in the lower surface); indusium absent................................................................................................................................ Padina boryana

- Thallus with a mixture of 2–3 cell layers in the median and basal portions; hair lines alternate on both surfaces; indusium present ......................................................................................................................................................................... Padina ditristromatica

3. Thallus with 3 layers of cells in the median portion and 3/ 4 in basal one; sporangial sori arranged in patches or in concentric lines; indusium present or absent .................................................................................................................................................................4

- Thallus with more than 3 layers of cells in the median and basal portion; sporangial sori arranged in continuous or discontinuous lines; indusium always absent ............................................................................................................................................................6

4. Thallus consisting entirely of 3 layers of cells except in the rolled apical portion; cells of the central layer and one cortical layer are more or less equal in height and usually smaller than the other cortical layer; sporangial sori arranged in concentric lines separated from each other by a sterile zone delimited by two consecutive hair lines on alternate surfaces of frond; indusium absent.............. .............................................................................................................................................................................. Padina boergesenii

- Thallus with 3 layers of cells in the median portion and 3/ 4 in basal ones; medullary layers (central layers) (1–2) consist of cells larger and less pigmented than the cells of the two peripheral layers; indusium present...................................................................5

5. Thallus with 3 layers of cells in the median portion and 3/ 4 in basal ones; sporangial sori arranged in continuous lines on both sides of the hair lines ......................................................................................................................................................... Padina pavonica

- Thallus with 3 layers of cells in both the median and basal portions; sporangial sori arranged in patches above hair line; indusium present................................................................................................................................................................. Padina pavonicoides

6. Thallus with 3–4 layers of cells in the median portion and 4 layers in the basal portion; sporangial sori present only on the lower surface arranged in continuous lines above the hair lines and discontinuous lines below the hair lines ....... Padina tetrastromatica

- Thallus with 3–4 layers of cells in the median portion and 6–8 layers of cells in the basal portion; sporangial sori present in both surfaces of the thallus arranged in more or less continuous lines ....................................................................... Padina gymnospora