204 Plant Life and Evolution 



Siphoneae and Characeae, have many small chromato- 

 phores like those of the higher plants. 



The prevalence of the small chromatophores in 

 the more highly specialized red and brown algae, as 

 well as in the higher land plants, would indicate that 

 the numerous small chromatophores are probably 

 more efficient than the single large chromatophore 

 of the lower types. Aside from the modifications of 

 the individual chromatophores, the character and ar- 

 rangement of the cells containing them may safely 

 be assumed to be related to light exposure. In the 

 larger and more massive seaweeds, the chromato- 

 phores are mainly developed in the superficial cells, 

 where they are best exposed to the light, and in 

 the more delicate algae the assimilative cells may be 

 spread out in thin leaf-like plates, exposing a large 

 area to the light, and sometimes the same result is 

 obtained by the development of dense tufts of fine 

 branches composed of single rows of small cells, 

 which are thus exposed on all sides. 



Among the red and brown seaweeds, secondary 

 pigments are developed, but there has been much dis- 

 cussion as to their composition and as to the role 

 they play in the plants' economy. The brown pig- 

 ments of the kelps are pretty generally recog- 

 nized as protective, screening the chlorophyll from 

 too strong illumination. The brown algae very com- 

 monly grow close to the surface of the water or 

 actually floating upon it, or they may be completely 

 exposed by the ebbing tide. Some of them grow 



