Reaction and Succession in Relation to the Plant Covering. 4 1 7 
Riimex Acetosella^ and Senecio sylvaticus, and several others, are frequent and 
characteristic species of some of the gravelly heaths inland. Rumex Aceto- 
sella in particular, and the abundant Cladonia on all the older banks, serve 
to illustrate the tendency on the older banks to develop an acid-tolerant 
type of vegetation. 
The Flanks. 
Occupying the slopes of the laterals on either side there are several 
zones, of which the most conspicuous is the Suaeda fruticosa zone clothing 
the lower edge, and more or less corresponding in position to the normal 
zone of drift deposition. Above the Suaeda zone is a zone of rather open 
vegetation with numerous bare pebbles , and characterized by the presence of 
Statice binervosa and Frankenia laevis. Locally, at the upper limit of this 
zone where it passes into the crest, sometimes at the upper limit of the 
Suaeda zone, but probably always where drift tends to be deposited by the 
highest tides and during storms, we find a zone in which Triticum pungens 
is a marked feature, sometimes accompanied by Atriplex littoralis. 
Within the upper portion of the Suaeda fruticosa zone, particularly on 
the basiscopic side of the banks where there is more shelter and the slope is 
gentler, Festuca rubra may form an interrupted zone, whilst on this side 
also Artemisia maritima frequently occupies the lower margin of the 
Suaeda zone. 
As might be expected, both the Suaeda and the Triticum zones show 
a high organic content, three localities in the former yielding a range from 
4-9 per cent, to 15-7 per cent. (cf. also marsh vegetation, p. 418), whilst a 
similar number for the Triticum zone showed a range of from 6-38 per cent, 
to 41-5 per cent. The corresponding hydrogen-ion estimations ranged from 
7‘2~7‘4 for the Suaeda zone and 7-0-7 -6 for the Triticum zone. From the 
marrams areg, only one carbonate determination was made for the Suaeda 
zone, viz. 2-55 per cent., though elsewhere on the area Suaeda fruticosa 
occurs in soil with as low a value as 0-26 per cent. In the Triticum zone 
the total carbonates range from 0-04 to 4-2 per cent. 
On the whole, then, the Suaeda and Triticum zones, that is, the zones of 
drift deposit, show a high organic content and an appreciable or often 
considerable proportion of carbonates. 
In contrast to these, the Binervosa zone has a low carbonate content, 
varying from o-oi to 0-07 per cent. (av. 0-028 per cent.), whilst the highest 
organic content observed was 4-91 per cent, and the lowest 0-47 per cent, 
(av. 2-63 per cent.). Appendix, Table XVI, shows also that the chloride 
content is subject to considerable variation in one and the same bank. 
These facts, combined with the slope of the surface, tend to render the habi- 
tat a decidedly xerophytic one, and it is significant that not only are the 
dominant species pronounced xerophytes, but that X. binervosa here 
