XIV INTRODUCTION. 



to the naked eye a whitish, powdery substance, known by 

 the name of " mountain meal," 



In colour, the Alga^ exhibit three principal varieties, 

 with, of course, numerous intermediate shades, namely, 

 grass-green, olivaceous, red. The grass-green is character- 

 istic of those found in fresh water, or in very shallow parts 

 of the sea, along the shores, and generally above half-tide 

 level ; and is rarely seen in those which grow at any great 

 depth. But to this rule there are exceptions, sufficiently 

 numerous to forbid our assigning the prevalence of this co- 

 lour altogether to shallowness of water. Several of the 

 more perfect Confervece and Siphoneoi grow beyond the 

 reach of ordinary tides ; and others, as the beautiful Ana- 

 dyomene, are sometimes dredged from very considerable 

 depths. The great mass, however, of the green-coloured 

 species, are inconsiderably submerged. The olivaceous- 

 brown or olive-green is almost entirely confined to marine 

 species, and is, in the main, characteristic of those that 

 grow at half-tide level, Algae of this colour becoming less 

 frequent towards low-water mark ; but an olivaceous vege- 

 tation frequently occurs also at greater depths, in which 

 case it is very dark, and passes into brown or almost black. 

 The red also, is almost exclusively marine, and reaches its 

 maximum in deep water. When red sea-weeds grow 

 above half-tide level, they assume either purple, or orange, 

 or yellow tints, and sometimes even a cast of green, but in 

 these cases their colour is sometimes brightened by placing 

 the specimens, for a short time, in fresh water. The red is 

 rarely very pure much within the I'ange of extreme low- 

 water mark, higher than which many of the more delicate 

 species will not vegetate ; and those that do exist degene- 

 rate in form as well as in colour. How far below low- 

 water mark the red species extend has not been ascertained. 



