THE BROWN SEAWEEDS OF THE SALT MARSH. 375 
(c) Brown Alge as a covering Vegetation after Erosion. 
A bank of sand or mud, which is undergoing erosion, presents a vertical 
or concave face to the current, in contradistinction to an accumulating bank, 
which usually shows a broadly convex surface. This is of course due to the 
fact that the only effective mode of attack upon a vegetation-covered surface 
by the sea is the method of undercutting. The result of this, at the edges of 
marshes or streams, is the formation of cliff-faces, which are vertical or con- 
cave in contour. Оп these vertical banks the conditions are obviously quite 
different from those obtaining on the flat plains of the marsh proper, and 
they have, in consequence, a peculiar vegetation, a fact which was first 
recognised and described by Cotton (Clare Island Survey, p. 82). 
Mud cliffs of this type are frequent in the Blackwater marshes, and, 
wherever they occur, Fucus spiralis v. nana becomes the characteristic 
vegetation. On the banks which fringe the great tidal channels of the 
marshes the Fucus may occur in practically pure formation, covering 
the whole cliff-face, more or less sparsely ; but, so soon as the bank becomes 
sheltered from the swifter currents, by a bend in the cliff or the insertion of 
a small creek, a vegetation of filamentous alge occurs, which is distributed 
in fairly definite vertical bands, analogous to the associations described by 
Cotton (1. с. p. 83) from the Irish peat marshes. 
The commonest constituents of these were in the Blackwater marshes :— 
Uppermost Zone ............ Vaucheria Thuretii, 
Middle Zone .................. Rhizoclonium riparium 
with Enteromorpha (percursa ?), 
Lyngbya ferruginea, 
and occasionally Polysiphonia urceolata. 
Lowest Zone  .............. . ÆEnteromorpha (compressa ? and 
percursa ?). 
Where the filamentous alge predominated Fucus spiralis v. nana only 
appeared at the lower levels of the banks, especially with Rhizoclonium and 
Enteromorpha, and did not extend over the whole bank, as it did in more 
exposed places. 
It seems very doubtful whether the Fucus itself is of great value in 
preventing further erosion, as its habit is not well adapted to bind the soil, 
and the plants are usually rather distantly placed with their attachment- 
dises only just below the surface of the soil. 
However, the plant is very tolerant of upheaval, and continues to flourish 
on pieces of mud which have broken away from the upper parts of the 
cliff-face and settled down as hummocks at the foot of the cliff. It may 
be that the very tenacious attachment-bases of the Fucus prevent further 
2p2 
