Isoetaceae

Herbaceous, perennial, submerged aquatics or marsh plants, usually with annual grass-like leaves arising in a tuft from a lobed, flattened, corm-like stock. Stock divided into stem and rhizophore, 2—4-lobed, with black dichotomous roots arising from the furrows between two lobes. Roots monarch, with the stele attached to one side of a central cavity, vascular system protostelic, 2—4- lobed at base. Leaves distichous, crowded, with overlapping bases, terete or flattened above, with a broad spoon-like base. Blades with a simple trace and median, unbranched vein, accessory peripheral strands often present; mesophyll chambered with four longitudinal cavities divided by transverse diaphragms, Which give the leaf a muriform appearance when seen in transmitted light. Stomata present on one or both surfaces in some species and absent in others. Leaf-bases usually membranaceous and hyaline but in some species persistent as hard, brown, 2-lobed, horny structures. Ligule present near the base of the leaf above the sporangium, arising from a cavity called the ligular pit, cordate-triangular or subulate, 2—15 mm long, without chlorophyll or cuticle, secreting mucilage at least when young. All leaves potentially sporophyll with a sporangium seated in a pit (fovea) on the adaxial surface below the ligule. Megasporophylls normally arising below the microsporophylls; opening of fovea often wholly or partly covered by a membrane (velum) extending downwards from the apex. Sporangia large, 4—7 mm long, oblong, thin-walled (walls with 3—4 layers of cells), subdivided irregularly and incompletely by oblique sterile plates (trabeculae); of two kinds, megasporangia and microsporangia, sessile and broadly adnate. Sporangia with both megaspores and microspores have been reported and the megaspores often vary considerably in size. Megasporangium containing 50—300 trilete spores, 250—900 μ in diam., white, grey or black, smooth or with warts, spines, or ridges. Microspores monolete, elliptic, 20-45 μ long, smooth or papillose, 150.000— 1.000.000 in each sporangium. Annulus wanting, spores released by the decay of sporangial walls. Some species may be aposporous with young plants taking the place of the developing sporangia. Gametophytes dioecious. Female prothallus green, development starting within and the prothallus remaining attached to the wall of the megaspore. Archegonia one or more up to 30, deeply sunken. Rhizoids present, projecting beyond the spore wall. Male gametophyte arising within the microspore, consisting of only a single prothallial cell and an antheridium, with 4 peripheral cells and 4 central cells, each giving rise to a single antherozoid with 15 flagellae. Distribution. About 75 spp., in all parts of the world except the Pacific Islands (present in Tasmania and New Zealand), but mainly temperate, scarce in Asia, in Malaysia 3 spp., one in the hills and two