II] THE BEACH AND ROCKS 15 



Red Seaweeds rdys further along the spectrum are 

 specially effective, and it is such rays, in the direction 

 of the blue end of the spectrum, which penetrate 

 furthest into the depths of the sea-water. Thus 

 the colourings have a probable relation to the 

 nutrition of the Seaweeds which show them : and 

 in particular, the possession of a brown or red tint 

 makes self-nutrition at great depths possible, while 

 it does not appear injuriously to aifect life under 

 less rigorous conditions. 



From the circumstance that the Red and Brown 

 Seaweeds are by their colouring specially able to 

 use in their nutrition such light as penetrates far 

 into the depths of sea-water, it might be expected 

 that they Avould find their place exclusively at these 

 low levels. But it is not so. There is no exact scale 

 of zonation of the three types of seaweeds according 

 to depth of water. Representatives of all may be 

 found near to the high- tide mark. It is, on the other 

 hand, an interesting fact that such full-green Algae 

 as Struvea, which show no colour specialisation 

 different from land-plants, may live at so great a 

 depth as 150 feet. But speaking generally the Red 

 Seaweeds are certainly more prevalent at the lower 

 levels, where the green are present in less numbers. 

 The Brown extend from the highest levels downwards, 

 but stop short of the greater depths. As regards 

 genera and species, however, there is more definite 



