Azolla pinnata R.Br.

Plants triangular to trapezoidal; branching alternate, with basal branches sometimes similarly branched. Main stem with protostelic vascular system. Devoid of leaves except for those that subtend branches; trichomes present, although often sparse and aggregated near base of leaf lobes. Dorsal leaf lobes with variable degree of imbrication; hyaline margins generally of (2-)3(-4) cells diameter. Leaf trichomes consisting of a cell superficially positioned on an ‘epidermal’ cell. Red anthocyanin pigments evident under adverse environmental conditions. Ventral leaf lobes devoid of stomata and trichomes. Roots solitary, with obliquely arranged groups of (3-)5-6 root hairs; (9-)12 outer cortical cells and (13-)18(-19) epidermal cells in transverse section. Megaspore sporoderm surface of anastomosing baculae, often fused to form a surface of rounded nodules; baculae aggregated into prostrate or erect elongate or tuberculate excrescences, especially pronounced near distal pole. Excrescences of anastomosing baculae with interstices, or with smooth surface. Collar of megaspore apparatus comparatively small, without flange; both internal and external surfaces covered with filosum. Massulae with simple spiniform trichomes without glochidiate apex; 2-4(-12) trichomes on inner surface only, aseptate with occasional irregular internal cavities, sometimes with small irregular protuberances.

Growing in similar situations to, and often sympatric with, Azolla rubra.