THE BROWN SEAWEEDS OF THE SALT MARSH. 371 
Sargasso weed can be explained in the same terms as those of the limicolous 
Fuci, and this affords a strong confirmation of Borgesen’s contention, that 
the two floating species, Sargassum natans and fluitans, are direct vegetative 
descendants of the saxicolous Sargassums, which continue to propagate 
themselves indefinitely, by vegetative means, under the new conditions. 
PART 3. 
RELATIVE DISTRIBUTION OF THE Marsa FUcorps. 
The various species of Brown Algee which occur in salt marshes are by no 
means an inconspicuous component of the marsh vegetation. They may 
function in three ways in the economy of the marsh as :— 
(a) Pioneer vegetation ; 
(b) Undergrowth ; 
or (c) Vegetation of eroded surfaces. 
(a) Brown Alyw us Pioneers. 
There is only one species which seems at all important in this connexion, 
the ubiquitous Fucus vesiculosus megecad limicola. Occasionally small 
bunches of Ascophyllum nodosum ecad scorpioides may be found in similar 
situations, but never in sufficient quantity to have any marked effect on the 
topography. 
When, either by the accumulation of silt or the protection of the locality, 
an area of mud ог sand becomes sufficiently stable, it is at once colonized by 
sand-binding alge. The most important of these, in the earliest stages, 
seem to be various species of Enteromorpha, Ithizoclonium, Vaucheria, and 
Microcoleus. 
At Mersea Island, as well as at Blakeney, Enteromorpha comes in first, 
and, after the ground has reached a certain level, either Rhizoclonium, Micro- 
coleus, or Vaucheria may follow it and build up the mud-bank very rapidly. 
When a certain level has been reached the ground becomes infected with 
Fucus, which forms a dense tangled vegetation, and no doubt assists materially 
in the accretion of mud and the stabilization of the bank, besides affording 
humus by the decay of its subterranean parts. When buried by the deposited 
silt, the Fucus has the property of pushing up fresh shoots to the surface 
from its underground parts, and this happens pretty rapidly (the process is 
illustrated in Во. 18). The new marsh at Blakeney, called the Samphire 
Marsh (Oliver, in Journ. of Ecology. 1918, р. 13), which is being rapidly 
built пр from a sterile mud-bank, shows the function of this Fucus in a 
particularly striking way. Every bare area in this marsh is being overgrown 
by vigorous Fucus plants, and, after these are established, seedling Salicornias 
inevitably follow closely. An accreting bank under Fucus is shown in РІ. 28. 
