174 THE ESSEX NATURALIST
it occurs only in places which are wet for a large part
of the year, it obviously needs a damp habitat during
its development, but seems to be favoured by dry
summer conditions. It grows in creeks with chloride
content ranging from .093 grams per litre to 18.75
grams per litre, but is most abundant where the
chloride content is about 2.5 grams per litre. In such
places it seems to compete more easily with other
species. These preferences shown by Scirpus mari-
timus probably account for its absence from the
natural salt marsh.
(ii) Juncus Gerardi. This has a salinity tolerance similar
to that of Scirpus maritimus, although its range
probably extends into areas where there is no chloride
present. It does not. however, flourish in standing
water as does Scirpus, but prefers damp creek banks.
(iii) Hordeum marinum. The distribution of this seems
to be dependent on a high soil salinity. It is found
only in two places on the inned marshes, but the
salinity is high enough in many areas. It seems to
require fairly dry conditions, and presumably does not
compete well with other species.
(iv) Agropyron pungens. This is found along the steeper
creek banks and along the sea wall. Thus the slope
and good drainage of the ground are apparently the
determining factors in the distribution of the species.
Soil from about the roots of this plant contains no
detectable chloride.
(v) Puccinellia distans. This plant occurs in parts of the
inned marsh which are remarkably similar to the
central zone of the natural marsh. It also grows in
rather drier places where P. maritima is not found.
Probably these latter sites have allowed P. distans to
gain a footing and to spread to the inned marsh,
whereas in the natural marsh it cannot compete with
P. maritima.
(vi) Phragmites communis is found only in standing water
or in very damp places in the area, but the species