The Causes of the Zoning of Brown Seaweeds. 19? 
high water was found by sighting across from the reference point 
to the horizon, and adjusting a slider on a graduated vertical scale 
into line with this. Sighting to the horizon involves an error of not 
more than half-an-inch (1.2 cm.) in the more distant readings taken 
(i.e. about 100 yards or metres). By this method the vertical height 
of any rock could be measured to within an inch or so. Several 
readings were taken at different points on the rocks for each 
species of seaweed. The zoning seemed to be similar at all points 
on the rocks measured. The zones merged into one another, but 
were on the whole very well defined. The readings were taken 
as far as possible on gently inclined rocks, and no account was 
taken of seaweeds growing in rock pools, or even in hollows which 
might form temporary rock pools, because the seaweeds in the 
pools were often entirely different from those on the rocks near 
Thus in a rock pool high up on the shore, in the zone of Fucus 
ceranoides, there were flourishing specimens of Fucus serratus and 
also of Halidrys siliquosa, which are usually found below the low 
water of the neap tides. 
The following Table shows the extreme readings, that is those 
taken on the extreme edges of each zone, and also mean readings 
taken from the middle of the zones, where each species was at its 
thickest. 
Vertical Distances Below H. W. S. T. 
Feet. Metres. 
Spring Tide ... High Water ... 0 = 0 
,, ... Low Water ... 13 = 4-0 
Neap Tide ... High Water ... 3 = 0-9 
,, ... Low Water ... 9 = 2-7 
Species of Seaweed. 
Upper Limit. 
Lower Limit. 
Mean Reading. 
ft. ins. mtrs. 
ft. ins. mtrs. 
ft. ins. mtrs. 
Fucus ceranoides ■ 
1 9 = 0-5 
3 0 = 0-9 
2 6 = 0-8 
Ascopliyllum nodosum 
2 3 = 0-7 
7 3=2-2 
3 6=1-0 
to 4-ft. to L2 
Fucus vesiculasus - 
4 9=1-4 
8 6 = 2-6 
5 6=1-7 
Fucus serratus 
Halidrys siliquosa 
Laminarias - - 
5 9=1-7 
11 6 = 3-5 
12 9 = 3-9 
12 0 = 3-7 
9 0 = 2-7 
These readings have been plotted graphically on the diagram 
(Text-fig. 27). It appears that the time during which the seaweeds 
