Hydrodendron arboreum (Allman, 1888)

Hydrorhiza a tangle of thin, tubular, rugose stolons.

Stem and branches thick and woody, irregularly fascicled; ultimate branches monosiphonic; each new branch arising from side of a hydrophore; basal internode of some branches very long with several proximal constrictions, succeeding branch Internodes moderately long but variable, walls smooth, increasing slightly in diameter distally to hydrophore, nodes oblique to transverse, sloping away from hydrotheca, varying from a deep septum to a mere narrowing in perisarc, a tumescence above and often below node; longer internodes often with several weak proximal twists.

Hydrophores alternate, one on internode about two thirds distance up internode, length variable, hydrophore typically curving outwards at variable angles from internode, diameter about same internode; adnate to internode by a thick web of perisarc for three quarters of length then becoming free below hydrotheca; free part of hydrophore cylindrical, adcauline wall weakly convex, abcauline wall straight to weakly concave; hydrophores typically in linear series, each arising from diaphragm of preceding hydrotheca.

Hydrotheca shallow saucer-shaped, expanding from diaphragm to margin; margin circular, rim everted in a flange; diaphragm transverse, strong, a row of desmocytes above; occasional replications of margin.

Nematophores sparse, randomly scattered on internodes, small, goblet-shaped with outrolled rim.

Perisarc of branches thick, thinner and fragile on younger hydrophores and hydrotheca.

Colour (preserved material) of older stems honey brown, younger branches pale yellowish to colourless.