DEVELOPMENT OF PLANTS 



207 



forking of the stem into two equal parts, a method of branching 

 called dichotomy in contradistinction to the axial branching 

 characteristic of the majority of our flowering plants. Fucus, 

 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- 



FlG. 122. Structural features of Fucus: A, cross-section of a portion of 

 the central stem-like part of the plant, showing an epidermal, e, cortical, 

 cr, and central region, <,. B, section of one of the cavities that appears to 

 the eye as a dot. See Fig. 121, B, g. This cavity contains only male game- 

 tangia. . C, section of a cavity from another plant contains only female game- 

 tangia. — ^After Oltmann. 



mentary epidermal, cortical and central region, the latter often 

 containing well-marked sieve tubes. These elongated cells of 

 the central region promote the rapid distribution of materials 

 and doubtless account in part for the size obtained by the kelps 

 and rockweeds. 



