Thallus development inDictyopteris membranacea(Phaeophyta, Dictyotales)

Abstract

The thallus ontogeny of Dictyopteris membranacea is the outcome of a highly co-ordinated series of cell divisions, which is completed at a long distance from the apex. Three types of “meristems” function: (a) the central apical initials, which give rise to the other meristems and to initial cells of the midrib; (b) the marginal apical initials, which contribute mainly to wing formation; and (c) a superficial meristem which is a true meristoderm and contributes to midrib formation. The central apical initials divide symmetrically in a longitudinal plane and asymmetrically in a transverse one. The initial cells of the midrib are derived by asymmetrical divisions of these central apical initials. The symmetrical divisions of the latter cells form new central apical initials and other apical ones of a determinate function. The meristodermal cells are separated by asymmetrical divisions of the initial cells of the midrib, which have been previously divided symmetrically on a periclinal plane. The meristoderm a...