PIGMENTS OF SEAWEEDS 71 



wHch are capable of raising the temperature of a body : 

 these are dark heat-rays. At the violet end we have the 

 ultra-violet rays, which produce a distinct chemical 

 effect upon a photographic plate. Now, it is known 

 that the red and yellow rays are absorbed by chlorophyll, 

 and we naturally conclude that they have all-important 

 effects in carbon assimilation. It is also known that an 

 excess of blue rays is hostile to green plants. The water 

 in which the Seaweeds grow also acts as a light-filter; it 

 not only reduces the quantity of the sunlight in propor- 

 tion to its depths, but it also affects its quality. The 

 very rays that are most efficient in carbon assimilation 

 are the first to be intercepted by the water, and the 

 green and harmful blue rays penetrate to greater depths. 

 It would seem, then, that the use of the brown and red 

 pigments of Seaweeds is either to heighten the suscepti- 

 bility of the chloroplasts to red and yellow rays, or to 

 screen them from the excess of blue rays; the weight of 

 probability is in favour of the latter contention. In 

 Nature " nothing walks with aimless feet." The pig- 

 ments of Seaweeds are not the products of creative 

 caprice, but perform a definite use. The peculiar 

 pigment of the brown Algae accommodates them to the 

 light conditions of the zone they occupy, while the red 

 pigment of the red Algse accommodates them to the 

 light conditions of deeper water. It is significant that 

 when red species occur in the brown zone, they are 

 usually sheltered by brown species. 



The ubiquity of Algse on the world's coasts is remark- 

 able. Seaweeds occur from the Arctic regions to the 

 Tropics, and from the Tropics to the Antarctic; but 

 there is diversity of species in relation to climate and 



