24 PELOPHILOUS FORMATION OF LEFT BANK OF SEVERN ESTUARY. 
tidal submersion, while at the same time its retentivity for soil 
water raises the water content of the soil above that of the 
neighbouring- halophytic zones. 
While, then, the halophytic formations of the shore seem to be 
directly connected with the varying" salt content, I am unable to 
suggest in what manner this salt content directly affects the plant. 
Its effect is probably in part a direct physiological action on the 
plant and at the same time an indirect action due to its action 
on the structure of the soil. 
The presence of salt in the water causes this clay to more 
rapidly settle out of a state of colloidal suspension. Thus, in a 
typical experiment, if 5 grams of the dry soil be shaken up in 
one case with 250 c.cs. of distilled water, in the other with 250 c.cs. 
of 1 % sodium chloride, at the end of 15 minutes more of the 
soil will be deposited as sediment from the salt solution. In an 
actual experiment 472 grams settled from the salt solution as 
compared with 4-63 grams from the distilled w T ater. Sodium 
chloride then behaves as a normal electrolyte and precipi- 
tates these colloidal suspensions, and probably does to some 
extent obstruct the drainage of the heavy clay. At the same time, 
so long as tidal scour prevents the accumulation of humus, the 
drainage of the formation cannot much improve, and probably 
that is the main factor in keeping the salt in the soil and the 
vegetation typically halophytic. 
SUMMARY. 
The vegetation of the pelophilous formation of the Severn shore 
provides an excellent example of zonation of halophytic associations. 
These zones occur at different levels owing to the gradual subsi- 
dence of the land outside the seaward dunes, as a consequence 
of tidal scour. 
Below the reclaimed pasture forming an occasional upper layer 
outside the dune, the land steps down in three main zones, which 
may be termed the Fesiiica, Sclerochloa, and Salicornia zones 
respectively. 
The vegetation of these zones is described in some detail, and 
the fact brought out that they tend to pass into one another as 
the land subsides. The series may be diagrammatically ex- 
pressed thus — 
Reclaimed pasture 
\ f 
Fesluca zone 
1 
Sclerochloa zone 
\ 
Salicornia zone 
1 
Bare mud swept by tides. 
