WATER-PLANTS 53 



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 algse, and since the rays that are lost are just 

 those which are most concerned in assimilation, it follows 

 that these algse 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 colour-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 utUize. The marine algse 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 algse, 

 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 Aquatics. 



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 

 algse, summer is the season of vegetative activity, and 



