72 SEAWEEDS 
gonal, but becoming somewhat ovate by unequal 
growth. Immediately within these are strands of 
hyphal filaments giving off smaller branches and 
traversing the peripheral portion of the mucilaginous 
mass which fills the interior of the elongated sac-like 
fronds, and running for the most part in the direction 
of the axis. The fronds are covered externally with 
a mucilaginous coating. The young cells of the 
outer epidermal layer give rise to septate hairs, the 
upper cells of which frequently become almost 
spherical or bulge unilaterally, imparting a moniliform 
appearance. These hairs bend towards the apical 
cell. Growth takes place as the result of the activity 
of a group of meristematic cells surrounding an 
apical cell unique in its character. This apical cell 
differs entirely from its neighbours in appearance, 
being larger and pear-shaped, with a thick mucila- 
ginous wall and a long filiform tail stretching towards 
the interior of the thallus. The apical cell and its 
surrounding meristem are sunk in a dimple of the 
tissue. The two outer layers of the meristem by 
radial division form the epidermal cells, which con- 
tinue to divide until the part of the thallus in which 
they lie attains maturity. The inner cells of the 
meristem form the cortical cells, which, when once 
formed, do not divide again, but increase in size, 
and by this means, as well as by separation, keep 
pace with the growing epidermal layers. The meri- 
stematic cells lying internally and immediately round 
the apical cell give rise to the filaments by tangential 
division. These branch and anastomose repeatedly, 
forming a plexus of filaments immediately beneath 
