6 SEAWEEDS 



has shown. It is precisely those rays that are most 

 efficient in the work of assimilation by plants that 

 are first intercepted, and only the blue and green 

 rays travel to greater depths. It may be taken, then, 

 that the red, brown, and yellow colouring matters, 

 added to the fundamental green, are adaptations 

 to the supply of sunlight. Whether they act in 

 the direction of heightening the susceptibility of 

 chlorophyll to a diminished supply of the useful 

 rays, or as a protection against a relative excess 

 of the blue rays, has not been settled experiment- 

 ally, but the balance of probability is in favour 

 of the latter theory, since it has been discovered 

 that certain pigments in other plants act as a shield 

 against illumination of this character. A microscopic 

 green Alga, Halosphcera viridis, has been obtained from 

 the great depths beyond the reach of sunlight, and 

 the speculation has been hazarded that the lumin- 

 osity of animals inhabiting those regions might in its 

 case be an efficient substitute for sunlight, but the 

 idea is wholly unsupported by experimental evidence. 

 The explanation that the plants in question were 

 swept there by currents of submerged waters is 

 much more in accordance with oceanographical facts. 

 The fact that colour, which affords a character of 

 notorious instability in determining claims merely 

 to specific rank among land plants, should be found 

 associated in the Algae with characters of more than 

 ordinal importance (though not constituting such 

 characters by itself) is not so puzzling when it is 

 remembered that it plays here a role of vital import- 

 ance in the matter of nutrition. 



