70 PLANT-LIFE 



forms, which, however, are sheltered by the brown, 

 are also found. At and beyond low- water mark we find 

 a vigorous growth of the brown Kelps, or Tangles, 

 which also shelter red species. This belt is known as 

 the " Laminarian zone." Beyond this zone, in the 

 greater depths, unsheltered red forms occur. But sea- 

 weeds can flourish only within a limited depth; they 

 become rarer as the depth increases from 20 to 50 

 fathoms, and below the 50-fathom line they are ex- 

 ceptional. 



If brown and red Seaweeds are soaked in fresh water, 

 their brown and red pigments are dissolved, but the 

 green pigment, chlorophyll, remains. It is, then, 

 demonstrable that all Seaweeds contain chlorophyll, 

 which is used in the business of carbon assimilation, 

 the necessary carbon dioxide being held in solution by 

 the water in which the Algae live. The chloroplasts 

 are simply masked by the brown or red pigment, and 

 it becomes apparent that the pigments have a definite 

 relation to light; they are, indeed, light-filters, which 

 modify sunlight as it penetrates the water, and accommo- 

 date it to the requirements of the chloroplasts. It is 

 contended by some authorities that the pigments 

 increase the susceptibility of the chloroplasts to rays 

 of light that are most efficient in carbon assimilation. 

 If we pass a beam of white light through a prism, and 

 project it on a screen, we see that rainbow effect known 

 as a " spectrum." The colours of a rainbow appear 

 upon the screen. First, we have red, which shades off 

 into orange, yellow, green, blue, indigo, and violet; 

 and it is known that certain invisible rays occur at either 

 end of the spectrum. At the red end we have rays 



