Inter-tidal Zonation — BEVERIDGE and CHAPMAN 
195 
for E. ( L ) .H.W.N.T. and E.(H).LW.N.T. 
are obtained by calculating the means of the 
extreme neap tides, i.e., those with the small- 
est range. 
A.H.B. datum 
(feet) 
E.H.W.S.T. . 
11.94 
M.H.W.S.T. . 
11.34 
M.H.W. . . 
10.30 
M.H.W.N. . 
9.26 
E.(L) .H.W.N. 
..... 8.79 
M.S.L. . . . 
6.22 
E,(H) .L.W.N. 
..... 3.82 
M.L.W.N. . . 
..... 3.24 
M.L.W. . . 
..... 2.14 
M.L.W.S. . . 
..... 1.03' 
E.L.W.S. . . 
..... 0.44 
ANALYSIS OF THE TIDAL FACTOR 
The tidal factor may be considered under 
three heads: 
1. Hours of submergence and air 
exposure. 
2. Periods of continuous air exposure 
or submergence. 
3. Number of submergences and air 
exposures. 
In most cases it is probable that it is a com- 
bination of factors that renders a certain level 
critical insofar as zonation is concerned. In 
respect to air exposure, the principal effect is 
the degree of desiccation (in its widest sense) 
to which the species are subjected. In the 
case of submergence the amount of incident 
light becomes important. Also to be consid- 
ered is the operation of these factors at dif- 
ferent periods in the life of the individual 
plants. 
Level and percentage of annual air exposure 
(Fig. 5) 
In most cases where tidal factors limit the 
distribution of a species, the limitation will 
be due not so much to a gradual change in 
conditions but to a more or less sudden varia- 
tion in some factor. In Fig. 5 such changes 
LEVEL IN FEET 
Fig. 5. Graph showing relationship between 
the level and percentage annual exposure. The 
major tide levels are also included. 
occur at the 1-, 2-, 4-, 10-, and 11-foot levels. 
A gradual change may, however, be equally 
important; thus the amount of exposure is 
trebled between -f- 2 and + 3 feet. The in- 
crease at this level may be far more impor- 
tant than a similar increase at + 9 to 10 feet. 
JAN. FEB. MAft. APR. MAY JUN. JUL AUS. SEP. OCT. NOV DEC. 
Fig. 6. The relationship between level and per- 
centage monthly exposure. 
