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, Halosphwra 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 Algz with characters of more than 
ordinal importance (though not constituting such 
characters byitself) 1s not so puzzling when it is 
remembered that it plays here a réle of vital import- 
ance in the matter of nutrition. 
