CYTOLOGY OF THE APEX, THALLUS DEVELOPMENT AND REPRODUCTIVE STRUCTURES OF HORMOSIRA-BANKSII (FUCALES, PHAEOPHYTA)

Light, scanning and transmission electron microscopy were used to study the initiation of apical growth, the development of constrictions and branching in H. banksii (Phaeophyta). In the early stages of embryogenesis, a single apical cell arises form a cell lysing at the base of the apical hairs which grow out of the primary cryptostoma. Mature plants possess 4 apical cells. The constrictions between the monoiliform segments of Hormosira result from the intermittent activity of the apical cells. When the apical cells are inactive the surrounding meristematic region is absent and the terminal segment enlarges due to the development of the medulla. The initiation of new segments is associated with the renewed activity of the apical cells and the surrounding meristematic region. Cryptostomata are arranged in a radial pattern and they all mature into conceptacles. To describe branching in Hormosira as dichotomous is misleading, as 2 or more branches may arise simultaneously, and successive ''dichotomies'' do not lie in the same plane.