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 



