22 SEAWEEDS 



The conditions of environment of seaweeds are, 

 as has been described, by no means so complex 

 as those of land plants, and their general adapta- 

 tion to their surroundings is expressed in a cor- 

 responding simplicity of structure. The aquatic 

 habit, fresh -water and marine, is accompanied in 

 flowering plants by a degradation of structure 

 in their vegetative organs, since the buoyancy 

 of water, aided by the air-spaces of the plants, 

 dispenses with the need of the mechanical aid 

 of vascular tissue, and partly of its conducting 

 function. This tissue is accordingly much reduced 

 in aquatic flowering plants, and there is a corre- 

 sponding reduction in the epidermal system, since 

 there is no need of a special cuticular or corky layer 

 to protect the plant from undue evaporation. A 

 favourite view of the evolution of plant forms 

 represents their ascent as a process of gradual 

 emancipation from an aquatic habit; and the 

 adoption of this habit by members of highly de- 

 veloped groups as of the nature of a relapse or 

 approximation to their primitive state. The student 

 of seaweeds is not concerned with the point farther 

 than it is founded on the fact that environ- 

 ment has made no demand on these organisms of a 

 kind that calls for much specialisation of their 

 tissues to enable them to adapt themselves to it, 

 and throughout the group there is a simplicity 

 of structure and a plasticity of form of express 

 character. In the most highly organised seaweeds 

 the vegetative tissues may be classified into a 

 cortical assimilative layer and a central conducting 



