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 
FlG. 124. Macrocystis: show- 
Ing the long axis bearing numer- 
ous blades. 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. 
