14 PLANT-LIFE ON LAND [ch. 



below the sea-level as a rule all growth ceases, 

 though in some very transparent waters the limit 

 may be deeper. It is in fact the fading out of the 

 light necessary for the self-nutrition of these plants, 

 by its absorption in passage through the great depth 

 of water, that imposes a limit of extension upon the 

 seaweeds downwards. And so we may understand 

 that, exclusive of the floating Plankton, the character- 

 istic Flora of the sea is restricted to a comparatively 

 narrow zone lying between high -tide mark and some 

 150 feet below low water. 



The plants which constitute this Algal Flora, so 

 restricted in its spread, fall into three large groups, 

 which are roughly characterised by their colour as 

 the Green, Brown, and Red Seaweeds. Not that the 

 mere tint is absolutely distinctive, as the already 

 quoted case of Lemanea shows ; but it happens that 

 the colours mentioned run in a measure parallel with 

 other characteristics of form and propagative method 

 by which the large groups are more strictly defined. 

 It has been found that the colouring has its physio- 

 logical meaning and importance in relation to the light 

 so necessary for the self-nutrition of these plants. 

 In the Brown and Red Seaweeds the greatest activity 

 is carried on in the light of that part of the spec- 

 trum which is complementary to their own colour. 

 Ordinary green plants make special use of the rays 

 in the red end of the spectrum, but for Brown and 



