INTERNAL STRUCTURE 217 



surface layer of cells (palisade or photo synthetic layer), 



(b) the larger isodiametric cells lying below (cortex), and 



(c) the central region of cells (medulla) the bodies of 

 which are separated from one another by more than 

 the thickness of an ordinary cell wall. 



(a) Palisade Layer. This consists of a single layer of 

 cells (Fig. 33, A, p) whose long axes are perpendicular to 

 the surface of the frond. Each cell of this layer 

 essentially resembles a mesophyll cell (especially a 

 palisade cell), of the leaf of a higher plant (Figs. 10, u). 

 It contains a central vacuole and a peripheral layer 

 of cytoplasm containing the nucleus and packed with 

 phaeoplasts. In this layer (as in the palisade layer of 

 the mesophyll of a typical leaf) the greater part of 

 the work of photosynthesis is carried on. These cells 

 contain the greatest mass of phaeoplasts, and the raw 

 materials of the process (water, dissolved carbon 

 dioxide and mineral salts) have direct access to them 

 when the plant is covered by the sea at high tide. 



(b) Cortex. These cells (Fig. 33, A, c) are larger than 

 those of the palisade layer, having larger vacuoles and 

 fewer phaeoplasts per unit bulk. The nucleus is often 

 suspended in the vacuole by cytoplasmic bridles. 



(c) Medulla. The central region of the frond is 

 occupied by cells (Fig. 33, A, me.) most of which are 

 apparently isolated from one another. They are not, 

 however, separated by air spaces like many of the 

 cells of the tissues of a higher plant, but by a mucila- 

 ginous substance (mu.}> which is really formed by the 

 swelling of the middle layer of the joint wall between 

 two adjacent cells. When these cells are first formed 

 in development, the cell bodies are separated by thin 

 walls, but the walls gradually increase in thickness 

 and the middle layer becomes mucilaginous, takes up 



