INFLUENCE OF ENVIRONMENT 263 



the color of those which grow in deeper water. Most 

 of the red algae belong to this category, and the devel- 

 opment of a special red pigment allied to chlorophyll 

 seems to be a provision for increasing the absorp- 

 tion of certain of the light rays which pass through 

 the water, and is doubtless concerned in some way, 

 more or less directly, with the question of carbon 

 assimilation. The brown and yellow pigments of the 

 Phseophycese are probably purely protective, acting as 

 screens for the chlorophyll when the plants are exposed 

 at low tide. 



Living in a medium which is of approximately equal 

 density with the plant itself, most algse develop no 

 supporting or mechanical tissues, being buoyed up by 

 the water in which they float; such forms on being 

 removed from the water collapse completely. They 

 also have no protection against the loss of water by 

 evaporation, and this, when they are exposed to the air, 

 is very rapid and complete. Where, however, water 

 plants are exposed to the beating of the surf, as is the 

 case in many of the large kelps and some red algse, like 

 the common Irish moss ( Ohondrus erispus), the cell-walls 

 of the outer tissues become firm and cartilaginous in 

 consistence, so that the plant is very tough and flexible 

 and can endure the buffeting of the heavy surf without 

 injury, and the mucilaginous nature of their inner 

 tissues prevents too rapid loss of water when they are 

 exposed to the air. These surf plants develop root- 

 like holdfasts, which anchor them firmly, so that they 

 can be torn away from their moorings only by the exer- 

 cise of considerable force. In the largest of these kelps, 

 as we have seen, the leaves are provided with floats 



