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Summary

From a study of young plants of Ascophyllum nodosum raised from fertilized eggs it has been shown that an apical cell differentiates from the basal cell of one of a group of apical hairs. This apical cell builds up an active meristem around itself. This meristem and its subsequent products function throughout the life of the thallus. Generally the apical meristem dichotomizes once a year. Behind it a succession of lateral nodes develop during spring and summer. These originate from the meristoderm in a similar way to the conceptacles. The branches, whether vegetative or fertile, which are produced from the lateral nodes, are superficial in origin.

The apical cell of a meristem exerts apical dominance and inhibits the development of branches from nearby lateral nodes. Meristems from lateral nodes will inhibit the development of branches from a wound callus. Lateral branches, gametangia in conceptacles and branches from a wound callus all require some interruption of the dominating effect of the apical cell before they can complete their normal development.