4 



STRUCTURE OF SEA-WEEDS. 



least, certain : — they appear to be as much influenced as 

 other plants by the soil in which they grow ; for different 

 species of rock afford different kinds in greater perfection 

 than others, and a large number of those that are parasi- 

 tical confine themselves to particular species ; a fact 

 that should not fail to be recollected by the young algo- 

 logist. The selection of habitat, it has been argued, 

 would seem to prove that the root is not so sluggish an 

 organ as it has been supposed to be. It does not, how- 

 ever, present much modification, and rarely attains a 

 large size, differing from that of most land-plants, in 

 which, as they grow in size and height, the root also 

 extends, and increases in proportion. 



The cellular tissue of Algae presents some varieties. 

 The most common form of the cellule is cylindrical, 

 often of very small diameter in proportion to its length ; 

 and in such cases the cellules always cohere, by the 

 ends, into threads, or filaments, bundles of which, either 

 branched or simple, form the frond,* by lateral cohesion. 



Algae do not derive any nourishment whatever from the 

 roots, and that these organs serve merely as hold-fasts. I 

 have frequently gathered as fine specimens growing on the 

 mooring buoys in Plymouth Harbour, covered thickly with 

 pitch as well as copper, as in other places ; neither of which 

 substances would, I presume, be likely to afford nutritive 

 matter for promoting vegetation. 



* The term "frond," when applied to a sea-weed, 



