Isoetaceae

Abstract

Terrestrial or aquatic plants, either evergreen, or in seasonally wet habitats, losing their leaves in the dry periods. Stem covered by an irregular cortex layer which sloughs off periodically as new layers are formed from a secondary meristem below, often producing bulbils at the edges of epidermal lesions, usually erect, elongate or short and corm-like, bibbed or trilobed in transection, or more rarely short-creeping and rhizome-like; apices naked, usually sunken. Roots arising from a basipetal growing point beneath the “corm”, or from one or more median grooves running longitudinally up the stem from that growing point. Leaves spirally arranged, 2–100 cm long, up to 10 mm wide, occasionally filiform, either terete, often winged below, with a subulate or tapered apex, few to many, arising in an apical tuft or open rosette, or more or less flat, linear, with a spathulate or rounded, apiculate apex, forming an imbricate rosette at the stem apex; base frequently spathulate and much broader than the blade, often colourless with broad membranous margins or with blackish-brown sclerotic patches that harden and remain as usually tricuspate scales which protect the stem growing point during drought periods; occasionally viviparous on leaf margin; vascular strands running centrally throughout the length of the leaf, surrounded by four longitudinal air canals which are separated from each other by parenchyma and divided by transverse septa; stomata, when present, in longitudinal rows above the air canals; ligule narrowly triangular to ovate-deltate, arising from a sac-like depression on the adaxial face of the leaf base, above the sporangium when this is present; blade sometimes vestigial and leaf replaced by phyllopodia formed by persistent scale-like base (Fig. 6).