Comparative development of different shaped gametophytes of homosporous ferns, focusing on meristem behavior

Abstract

In homosporous ferns, gametophytes are cordate-thalloid (terrestrial) in many taxa but can also be strap-or ribbon-like (epiphytic), filamentous (epiphytic), and tuberous (subterranean) in some taxa. Recently developed long-term observational techniques of the same individual gametophyte during growth have led to a new classification of development types for planar gametophytes. We recognize five basic types (Lygodium-, Elaphoglossum-, Anemia-, Colysis-, and Vittaria-types). The five types reflect different combinations of the three meristems: apical cell-based, marginal and multicellular meristems. Cordate, spatulate, and asymmetrically cordate gametophyes are formed by the apical cell-based and subsequently formed multicellular meristems (Lygodium-, Elaphoglossum-, and Anemia-types). Irregularly branched, ribbon-like gametophytes are characterized by occurrence of the marginal meristem. During the marginal meristem phase, the gametophyte branches to form many lobes by cessation of the marginal meristem. In the Colysis-type, the marginal meristem phase occurs between the apical cell-based and the multicellular meristem phases, and in the Vittaria-type, the multicellular meristem phase is absent. The ability of many lobe formation leads to clone formation, resulting in slow growing and long-life of the gametophyte.