THE BROWN SEAWEEDS OF THE SALT MARSH. 373 
Vertical Zonation of the Marsh Fucoids. 
The Fucoids occupy a definite vertical range in the salt marsh, in the 
same way as they have a definite position upon the seashore. The lower 
limit of this range seems to be determined solely by the mobility of the 
ground ; for on the sheltered banks of deep creeks the limicolous Fucus 
vesiculosus may extend several feet below the level of the marsh. The upper 
limit is, however, well defined, and differences of an inch or two in level 
may determine the persistence of a species, with great precision. In no case 
have the alge been found to extend above the more or less mobile part of 
the salt marsh, unless it be as occasional relicts (cf. Oliver, in Journ. of 
Ecology, 1913, p. 12). 
The nature of the phanerogamic overgrowth seems to be immaterial: thus 
Е. vesiculosus megecad limicola occurs with Spartina in the Hurst Castle, 
French, and American marshes; with Aster and Salicornias at Blakeney 
and Mersea Island, and under the small ecad ecspitosus with a close Glyceria- 
Armeria turf at Mochras, Carnarvon, and Clare Island. It is possible that 
some of the variations in form noticeable in specimens of this species from 
different localities may be partly due to differences in illumination, ete., 
referable to the different aspect of the associated phanerogams. The Fucoids 
disappear as a rule in well-established saltings, such as the Marams in the 
Blakeney area, or where they penetrate, as they do in the Blackwater 
marshes, they are rigidly confined to the banks of creeks and channels. 
The vertical range of the mobile flats of the salt marsh is only about 
12 inches over all, with an 18-ft. tide; but there may be a definite zonation 
of the algal undergrowth, within this range. Thus, at Blakeney there is a 
distinct zonation between the free-growing Pelvetia, which occupies the upper 
parts of the Salicornia- Aster marshes, and the embedded Fucus vesiculosus of 
the lower levels. Again, in the Blackwater marshes Ascophyllum ecad scor- 
pioides occurs only in a narrow vertical range, commencing slightly below 
the upper limit of Fucus vesiculosus and extending a short distance downwards, 
but not nearly to the lower limit of that species. 
It is noteworthy that the order of zoning is the same in both cases as that 
of the parent species on a sheltered rocky shore. (The relative vertical dis- 
tribution of Fucus vesiculosus and Ascophyllum varies with the exposure (see 
Borgesen, 1908, p. 743)). This is shown in the diagram (fig. 14, p. 363), 
concocted from the data collected in various localities. On the salt marsh, 
however, the factor determining the vertical range of any species appears 
to be, not the time of immersion per tide, but the number of tides reaching 
any level. The duration of the long exposure over the neap tides, which may 
be curtailed by a whole day with the access of an extra tide, caused by a 
change in level of a few inches, is in this case the most important factor in 
determining the zonation. 
LINN. JOURN.—BOTANY, VOL. XLIII. 2p 
