16 



CHAPTEE II. 



ON THE COLOUR, DISTRIBUTION, ETC., OP SEA-WEEDS. 



Among the higher orders of plants, the diflPerent parts of 

 the same individual vary in colour ; but the same part 

 in different individuals is always approximately of the 

 same hue. Thus the trunk, or stem, is generally of 

 some shade of brown, the leaf green, and the flower of a 

 brighter colour. 



Among Sea- weeds, on the other hand, the colour of all 

 the parts of an individual is the same, while that of in- 

 dividuals — of different families, of course — is different. 



This variation has been used as a basis of classifica- 

 tion, and all Sea-weeds are divided into Olive-green 

 weeds, or MelanospermecB ; Red weeds, or Rhodosperme(B; 

 and Grass-green weeds, or Chlorospermea. Several purple 

 weeds are included in the last class; and beside this 

 there are others which are not in all circumstances con- 

 stant to the colour of the class to which they belong. 



The Olive-green, weeds are for the most part of large 

 size, and very abundant. They grow chiefly between, 

 and a little beyond, the tide-marks, and furnish the 

 great bulk of our shore vegetation. Some exotic species 

 grow at a great depth, and are of enormous size. 



