TISSUE STRUCTURE 



221 



jr IGi 23. A, longitudinal section through the apex of frond of Fucus 

 (the cells are in outline, the intercellular mucilage dotted); a, apical 

 cell surrounded by the cells produced from its previous divisions ; 

 p, photosynthetic layer; c, cortex; me., medullary cells (mu., 

 mucilage formed from their walls). Further from the apex the 

 cortical cells increase in size and the medullary cells in length. B, 

 young plant of Fucus in the cylindrical stage. Apical groove just 

 developing. Medullary cells beginning to lengthen and to separate ; 

 rh., first rhizoids which develop into the " holdfast." C, longitu- 

 dinal section of part of the bristle-like thread of Stictyosiphon 

 (6 cells thick) showing dense protoplasmic contents with phaeoplasts 

 of surface cells (photosynthetic layer), which are half the 

 length of the next layer (cortex). These, again, are half the 

 length of the central cells (medulla), which are slightly separated 

 longitudinally. This arrangment is due to the more numerous 

 divisions of the surface cells. Compare B. 



