THALLOPHYTES 



47 



cell. In such filamentous bodies as were met among the green algae, the filament 

 is elongated by the division of all the cells ; in other words, the power of cell-divi- 

 sion is distributed throughout the filament. In Ectocarpus this power of cell- 

 division to elongate the filament is more restricted, often being specially present 

 in a region behind the tip, where the divisions occur in unusually rapid succession. 

 In Sphacelaria this special power has become restricted to the apical cell, which 



FIG. 1 24. Macrocystis : leaves 

 between holdfast and floating part 

 omitted. After HARVEY. 



in this case is often extraordi- 

 narily large. It cuts off a suc- 

 cession of cells that adds to the 

 length of the filament, and 

 after each division it enlarges 

 again. This does not mean 

 that other cells do not have the 

 power of division, but that all 

 the cells are descendants of the 

 apical cell. An apical cell, there- 

 fore, is not merely the cell at the 

 apex, but a cell in that position 

 which has the power of giving 

 rise to the succession of cells 

 that organizes the plant body. 



Laminariaceae. These 

 are the kelps, the most 

 common and the largest of 

 the brown algae. They are 

 common on rocky coasts, 



FIG. 125. Nereocystis : showing the blades 

 arising from the bladder-like expansion of the tip 

 of the stipe. 



