The Structure and Development of the Thallus in the British Species of Gelidium and Pterocladia

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Abstract
The apical structure and the development of the thalli of all the British species of Gelidium and Pterocladia have been investigated; the development of G. pulchellum is described in detail. Each axis is terminated by one or more apical cells, which by their segmentation form the tissues of the thallus. An axial filament is distinct for a short distance behind each apical cell, but secondary pit-connexions develop rapidly so that in section the mature axis has the appearance of a multi-axial structure. Lateral branches of the frond develop by the segmentation of the lateral branch apical cells, which are formed by the transformation of superficial cortical cells, either in the meristematic or mature parts of the axes. The extreme variability of external appearance is due principally to the indeterminate origin of all lateral branches. The thallus in the British species of Gelidium and Pterocladia consists of erect fronds borne on creeping axes. The relative proportions of the frond and creeping axes in various species and their survival through adverse conditions are discussed.

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