Hypoglossum revolutum (Harv.) J.Agardh

Scientific Description

Habit and structure. Thallus medium red to greenish brown, forming spreading clumps of one to a few axes with only very short stipes, attached by discoid primary (and often secondary) holdfasts. Branching entirely sympodial with new blades (often in clumps of 2–4) arising from some distance below the apex of parent blades on their adaxial (convex) side, with the ends of all blades being typically revolute. Mature blades 3–5 mm broad, with a distinct corticated midrib 300–600 µm across, and wings convolute with margins irregularly coarsely spinous. Holdfast 0.5–1 mm across; epiphytic on various algae. Structure. All second-order cells producing third-order rows with the outermost 2 or 3 cells much smaller than inner blade cells, produced singly or occasionally in pairs and in some plants extending as short 2- or 3-celled spinous projections, at first comprising second-order rows extending beyond the anterior third-order rows but later composed mainly of extended adjacent posterior third-order rows which usually cut off fourth-order cells both anteriorly and posteriorly. Cortication of the midrib commences in young blades, and in mature blades extends almost to their apices; several layers of corticating cells form a broadly ovate to almost round midrib in section, but the lateral parts of the blades remain as wings throughout the thallus. Corticated midribs of older blades, either at their apices or below, often develop rhizoidal outgrowths which form secondary holdfasts. Mature cells multinucleate; rhodoplasts discoid.

Reproduction. Gametophytes dioecious. Procarps develop from the abaxial pericentral (supporting) cell, with a 4-celled carpogonial branch. Prior to fertilisation, the supporting cell cuts off an outer sterile cell which in turn produces a smaller sterile cell posteriorly. Several procarps may form in sequence, but usually on any blade only one procarp develops further. Carposporophyte with much branched gonimoblast filaments, with carposporangia developing terminally and sequentially but usually with chains of 2 or 3 maturing rapidly; subapical cells also produce carposporangia laterally. Carposporangia ovoid to pyriform, 25–45 µm in diameter. Cystocarps develop on the abaxial (concave) side of young blades, becoming subspherical to slightly urceolate, not or only slightly basally constricted, 0.5–1.5 mm in diameter, with a small ostiole. Pericarp moderately corticated, 4–6 cells thick. Spermatangia not observed. Tetrasporangia develop in broad sori at the revolute apices of young blades. No third-order cell rows form within the sorus although they are present outside the sorus. Tetrasporangia arise first from the lateral pericentral cells and each second-order cell within the sorus, and at an early stage from the first-formed corticating cells so that the sorus contains tetrasporangia of varying ages. Just prior to initiation of the sporangium, 1 or 2 corticating cells are cut off from the second-order cells and 2 or 3 outer corticating cells are usually produced from the first-formed ones; tetrasporangia 50–80 µm in diameter.

Distribution.Houtman Abrolhos, W. Aust., to Portland Bay, Vic.

Habitat. Recorded from 2–50 m deep.

[After Womersley, Mar. Benthic Fl. Southern Australia IIID: 43–45 (2003)]