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. Fu^us, 

 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- 



'f^cr 



Fig. 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, c. 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. 



