INFLUENCE OF ENVIRONMENT 263 
the color of those which grow in deeper water. Most 
of the red alge 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 
Pheophycee 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 alge 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 alge, like 
the common Irish moss ( Chondrus 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 
