Thallus development inDictyopteris membranacea(Phaeophyta, Dictyotales)

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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 at first cuts off medullary cells and, later, cortical cells by asymmetrical periclinal divisions. The other apical initials undergo symmetrical and asymmetrical divisions, but this asymmetry, as well as their meristematic tendency, diminishes towards the margins. The central apical inititals are larger than the marginal ones and exhibit a zonation by the position of the nuclei on the same plane towards their base. The apical zone possesses many more organelles, vacuoles and inclusions than the basal zone. During asymmetrical divisions the latter zone is included in the initial cells of the midrib. At maturity the midrib is made up of three distinct cell types. (1) The epidermal cells, which are polarized and differentiated into a photosynthetic cell type; these cells have locally thickened anticlinal walls. (2) The cortical cells which are highly vacuolated and possess very thickened walls; among other functions they probably serve as supporting cells. (3) The medullary cells, which are slender cells of an enormous length, having thick longitudinal walls. Their transverse walls are perforated by numerous plasmodesmata. These cells resemble, in some aspects, the sieve elements of Laminariales.