DEVELOPMENT OF PLANTS 



195 



called dichotomy in contradistinction to the axial branching 

 characteristic of the majority of our flowering plants. Fticus, 

 like many of the gross brown algae, contains air cavities or 

 bladders which buoy it up in the water; this feature accounts 

 for its popular name of bladder wrack. A cross section of the 

 stem shows that the tissues of these plants have attained a con- 

 siderable differentiation (Fig. 122, A) as is attested by a rudi- 

 mentary epidermal, cortical and central region, the latter often 

 containing well marked sieve tubes. 



Fig. 121. Two common forms of the Fucaceae: A, Sargassum, the stem- 

 like axis bearing air sacs, s, and leaf-like organs; g, reproductive branches. B, 

 Fucus — s, air sacs; g, reproductive branch. C, young plant. 



The most characteristic feature of these plants, however, and 

 the one separating them sharply from the Laminarias appears 

 in their method of reproduction. None of the Fucaceae develop 

 zoospores, although an asexual reproduction may be effected by 

 the detachment of small branches from the plants. Sexual re- 

 production is a step in advance of any of the preceding types 

 of the brown algae, in that the female gamete becomes still 



