I 



I 



Fig. Ill Fucus. A, adult plant of F. serratus ( x 0-30). B, a marsh 

 form of F. vesiculosus ( x 0-30). C-E, seedling stages of F. vesicu- 

 losus showing origin of rhizoids and apical tiift of hairs. F, diagram 

 to show method of segmentation of apical cell, A. bs = basal seg- 

 ment, 55 = side segments. G, apical cell of young thallus. H, apical 

 cell of old thallus. (A, B, after Taylor; C-H, after Oltmanns.) 



or alae are developed, these latter being of varying width with 

 either entire or serrate margins. Branching, although apparently 

 dichotomous, is in fact monopodial, the degree of branching in- 

 creasing as the rate of growth decreases. In many species the 

 branches bear swollen vesicles or pneumatocysts. In at least one 

 species, F. vesiculosus, these are known to be seasonal in formation, 

 but the nature of the habitat is also involved because more are pro- 

 duced in quiet than in rough waters. Their production appears to 

 be linked with high photosynthetic activity in the spring and 



197 



