298
THE ESSEX NATURALIST
cover, and a few large plants may occupy the same total area as
many much smaller ones. In other words, the relationship
between frequency (which is dependant upon the size of the
square) and density, will differ between species.
To indicate, for each of the common species, the approximate
relationship between frequency and density, the number of occu-
pied squares was plotted against the average number of plants
in the squares of the same quadrat. This information was readily
transferred from the proformae and a line drawn through the
scattered points. The assumption was made that an average
of one plant in each square (1 plant/20 sq. ins.) was equivalent
to 7.2 plants/sq. ft.
(c) The profile
Initially referred to the zero point and with the vertical scale
exaggerated thirty times, a profile was constructed from the
levelling data and the relative level of high-water inserted.
Although the tide-tables predicted a height of 12.2 feet relative
to Chart Datum, the gauge at Harwich (a standard port) showed
that the level of high-water on the day was 12.5 feet. This value
was then used as a datum to which a scale of feet above Chart
Datum was referred, and it was a simple matter to indicate the
tidal levels as given in Admiralty Tide Tables, viz.,
EHWS MHWS MHWN MTL
13.7 12.8 10.4 6.9 feet above C. D.
This concluded the day's work on the survey.
For the final presentation of the data, all the information was
brought together on a single sheet and this is shown in Fig. 2.
The frequency with which each species occurred along the
profile is represented by a series of accurately located vertical
bars. The height of each bar indicates, for each quadrat, the
number of squares (0-25) within which the stated species were
found.
In the field it was not possible to ensure precise separation of
the two species of Salicornia present, and they have been included
under one heading. Forms intermediate between Limonium
vulgare and L. humile were shown, by means of microscopic
examination, to be hybrids. Although an attempt was made to
assign each plant to one or other of the two species, it was soon
decided to abandon such efforts and the records have been
combined.
For each of the common species, the approximate relationship
between the number of occupied squares and the number of
plants per sq. ft. is shown. Thus, within one quadrat, 25 squares
occupied by Spartina townsendii indicates a density of 20 plants/
sq. ft., but an equal number of occurrences by Salicornia spp.
implies a density of at least 60 plants/sq. ft.