18 PELOPHILOUS FORMATION OF LEFT BANK OF SEVERN ESTUARY 
A curious feature of the vegetation of this zone has been the oc- 
casional appearance of a number of plants of Salicornia europcea, L., 
in the early autumn or late summer, apparently after some high 
tide. A number of these have been found growing amongst the 
close turf at this time, but before they have grown more than a 
few inches they die down again. 
(IV.) Above this zone there is occasionally to be seen traces of 
a fourth zone, which closely resembles the associations to be found 
on the reclaimed levels within the dune, and which therefore seems 
to owe its existence largely to human activity in improving 
drainage, &c, and is an example, therefore, of what Dr. W. G. 
Smith 1 terms a " substituted " association. This zone is so 
typically not halophytic (see p. 21 for soil salt content) that I 
have paid but little attention to it, but it closely agrees so far as I 
have observed with the account given by Moss. 2 
These four zones are not always all present. IV. in particular 
is only occasionally seen ; it occurs between St. George's wharf and 
Portishead, and in several places above Aust. 
III. occurs nearly all the way up from Portishead to Oldbury- 
on-Severn, except where sand and shingle break the sequence. 
It is also missing just above New Passage. 
II. occurs at Portishead, is disappearing at St. George's wharf, 
is occasionally missing above Avonmouth, and disappears just 
below Aust, only to reappear very occasionally. 
I. is a fringe narrowing from Portishead upwards, ceasing 
below Aust. 
CAUSES OF ZONATION. 
With zones so sharply delimited by features in their vegeta- 
tion, it was natural to seek to establish a relation between the 
zones and some edaphic factors. 
The sharp fall between the zones results in efficient drainage 
at the margin of each zone, and this in its turn probably reacts 
upon the salt content. This may be made clearer by reference to 
a diagram (Fig. I). 
Text Figure I. 
1 Smith, W. G., loc. cit. 
2 Moss, loc. cit., p. 20. 
