THE THALLUS OF THE HIGHER ALGM. 37 



The compact small cells forming the surface are nutritive 

 and probably in part protective ; the bladders serve to in- 

 crease the buoyancy of the plants when the tide is in ; while 

 the abundant mucilage, formed in the interior from the cell- 

 walls, serves to retain the moisture when the plants are ex- 



FiG. 47. — A plant of Sargassum, showing differentiation of thallus. Natural size. — 

 After Bennett & Murray. 



posed by the ebbing tide j the hair-pits are functionless, so 

 far as known ; and the strong, elastic cells of the disk and 

 stalk above hold the plants in place as they sway constantly 

 back and forth in every wave of the rising or falling tide. 



45. Color. — The coloring matter in the chloroplasts of 

 Fucus and other brown seaweeds is chlorophyll (green) and 



