WATER-PLANTS . 7 ee 
the water-surface, especially when it is disturbed, and 
that which enters is still further enfeebled by absorption. 
Beyond a certain depth no light penetrates, and vegeta- 
tion is impossible. The quality of the light is also altered. 
The red and yellow rays are gradually absorbed, and the 
light, as it descends, turns from white to green, then to 
blue, and, before it fades out altogether, to a pale ultra- 
marine. The deeper vegetation of the sea consists chiefly 
of coloured alge, and since the rays that are lost are just 
those which are most concerned in assimilation, it follows 
that these alge must be profoundly modified in order 
that adequate use may be made of the changed and 
weakened light which they receive. The difficulty is 
overcome by the adoption of colowr-adaptations. The 
pigments found in the brown and red seaweeds do not, 
however, replace chlorophyll; they merely mask it, acting 
as a screen to absorb the rays which chlorophyll alone 
cannot utilize. The marine alge are roughly zoned out 
in depth, according to their colour. Near the surface, 
where light is abundant, the green seaweeds flourish. 
At a lower depth their place is taken by the brown alge, 
and lowest of all grow the red seaweeds, in liquid regions 
where only a pale blue light reigns. The coloration of 
seaweeds is a striking illustration of adaptation to 
environment. 
All submerged aquatics suffer more or less from 
diminished light. If the surface is disturbed, only a 
fraction of the light that reaches the water passes into 
it ; the rest is scattered by the broken surface and lost. 
Beneath this liquid screen the aquatics live in partial 
shade, and show, in consequence, many of the characters 
of shade-loving plants. For example, they have long, 
thin, weak stems with distant nodes, and chlorophyll is 
present in the epidermal cells. 
Propagation in Aquaties. 
In this country water-plants experience, like land- 
plants, the vicissitudes of the seasons, but life in 
water is more uniform than on land, and winter to 
the aquatic is not the same thing as it is to the 
terrestrial plant. With the exception of some of the 
alge, summer is the season of vegetative activity, and 
