Ultrastructural studies on thallus development inDictyota dichotoma(Phaeophyta, Dictyotales)

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Abstract
Three successive asymmetrical divisions, one occurring in the apical cell of Dictyota dichotoma and two in its first discoid segment, lead to the formation of two different cell types, the epidermal and medullary cells. The change in cell length with distance from the thallus apex is the same for the four series of nascent epidermal cells as for the medullary cells. Thereafter, and for a distance of 2 mm from the apex, the elongation of the epidermal cells differs and reflects a different pattern of cell growth. The derivatives of the initial epidermal cells exhibit greater meristematic activity than those of the initial medullary cell and grow mainly by the synthesis of protoplasm. During differentiation they retain a highly secretory ribosomal cytoplasm and develop well-differentiated chloroplasts and microbodies. Besides, many mitochondria underlie the longitudinal anticlinal walls of the differentiated epidermal cells, forming an intimate relationship with the plasmalemma, and the anticlinal walls become locally thickened. Finally, the epidermal cells lose their secretory properties and become very active photosynthetically. The medullary cells grow mainly by vacuolation, lack well-formed plastids and their cytoplasm loses its structural complexity. The plastids and the other organelles are perinuclear. These cells finally become photosynthetically less active and parenchymatous, probably with a storage function. A well-developed symplastic communication characterizes the thallus cells, and in mature cells is finally restricted in primary pit fields. A description of the apical cell as well as of its first segments is included.