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 
