10 PELOPHILOUS FORMATION OF LEFT BANK OF SEVERN ESTUARY. 
Finally, my apology for the incomplete state of this work, in 
spite of the fact that it has now been in progress for several 
years, must be that it has of necessity been restricted to occasional 
holiday afternoons and week-ends, and subsequent work in the 
laboratory at incidental hours. The pressure of a large amount 
of teaching work, coupled with a considerable amount of more 
directly physiological research, have prevented me giving more 
time to the investigation. 
AREA OF INVESTIGATION. 
The detailed investigation I desired to make, limited the area 
of ground which could be covered with advantage. 
The investigation, therefore, has been confined to the narrow 
strip of land outside the seaward dune, and submerged at the 
highest tides, which runs from Portishead up the shore of the 
Severn to Oldbury-on-Severn. 
For the greater part of the distance this narrow strip consists 
of alluvial mud, covered at the higher levels with a close carpet 
of vegetation, but at one point below New Passage for a distance 
of about one mile, the vagaries of tidal currents are replacing the 
mud by shingle and sand with as a consequence changes in the 
vegetation. 
From this shingle beach up to New Passage the whole frontage 
has been artificially changed, either by soil thrown out during 
excavations in connection with the Severn railway tunnel or by the 
construction of a strong stone barrier against the further incursion 
of the tides. This part of the shore owing to its partially artificial 
nature has received less attention from me, and is not described 
here, though the problem of the colonization of the shingle is an 
exceedingly interesting one. My attention has been chiefly directed 
to the vegetation formation that is characteristic of the alluvial 
mud within occasional or frequent reach of the tide, and it is 
this alone that is discussed in this paper. 
THE STRAND FLORA. 
The vegetation under observation then, has been what is often 
termed a strand flora. One of the characteristics of such a flora 
is that it is usually regarded as determined by edaphic rather than 
climatic factors ; that is to say the flora of the shore seems to be 
chiefly determined by some factor connected with the nature of 
the soil of the shore itself. Thus the same main type of vegetation 
is found existing in many different climates, for instance from 
the Baltic, the Bay of Fundy, the Netherlands, and parts of the 
Tropics, the same general type of strand vegetation is described. 
The main edaphic factor establishing this similarity of type of 
vegetation is almost undoubtedly the salt, for it may be found 
away from the sea shore, in the neighbourhood of inland salt 
marshes. 
