VARIETY OP COLOUR. 



105 



Callithamnion gracillimum, C. thuyoideum, C. byssoi- 

 deum and Dasya ocellata, and 

 other rarities are procured. 

 Mud-banks yield some of the 

 most beautiful Polysiphoniae, 

 as for instance, P. vaiiegata; 

 but most of this genus pre- 

 fer the purer water of rock- 

 pools. The exquisite P. pa- 

 rasitica is found only in 

 clear water at the verge of 

 low tide or on the banks of 

 Nullipores, which charac- 

 terise a still lower level. 



I have spoken of the Floridece, or Rhodosperms, as 

 the Red Sea-weeds ; but it must not be supposed that 

 they are all of a clear red colour, nor does colour sup- 

 ply us with more than an imperfect guide in determining 

 them. The red colour appears to depend in great de- 

 gree on the amount of direct light which reaches the 

 growing plant. The same species which exhibits a full 

 red colour when growing in the shade, assumes every 

 variety of paler tint till it ends in a clear yellow, as it 

 grows under the influence of sunshine, and in shallower 

 water. This is very apparent in the Chondrus crispus 

 or Carrigeen, well known for producing a peculiar gela- 

 tinous pi'inciple used in cookery and medicine. When 

 this plant grows in places shaded from the sun, its fronds 

 are of a very dark purple, reflecting prismatic colours 

 from the surface; but growing, as it frequently does, in 

 shallow pools exposed to full sunlight, it becomes green 



