44 
PACIFIC SCIENCE, VoL VI, January, 1952 
phenomena associated with tidal inundation 
provide physiological limits to the vertical 
distribution of littoral species. Each change 
in environmental conditions is a barrier which 
cannot be surmounted by certain species, 
either in establishment as young plants or 
animals, or in development. 
Reaction to the causal factors is shown in 
one way by Hormosira banksii. This alga plays 
a prominent part in the littoral ecology of 
Rangitoto because its vertical range is from 
M.L.W.S.T. to E.(L.)H.W.N.T. The young 
plant, or "button,” is found throughout this 
range and up to 1 foot above the upper limit. 
Mature plants show considerable varia- 
100 5S SO 85 80 75 
PERCENTAGE OF ORIGINAL WEIGHT 
A 
B 
Fig. 8. Loss of weight of Hormosira hanksii thalli 
during emergence ("exposure”). Mean percentage loss 
of weight per hour (under identical conditions) of: 
A, numbers of plants of Hormosira hanksii from top of 
range; B, main Hormosira consociation; c, Corallina- 
Hormosira association; and D, of special form (see text). 
8A, in laboratory; 8B, in sunlight. 
bility. The size of the plants, with regard to 
the size of the receptacles (and, to a lesser 
degree, to the number thereof), is propor- 
tional to their height above E.L.W.S.T. (The 
average diameter of the receptacles ranged 
from 1.50 centimetres in plants near the 
upper limit of the species to 0.75 centimetre 
in plants near the lower limit.) Further, the 
thickness of the wall of the receptacle was 
found to be inversely proportional to the 
diameter. (The average thickness of the walls 
ranged from 0.11 centimetre in plants near 
the upper limit of the species to 0.14 centi- 
metre in plants near the lower limit.) Finally, 
the rate of water loss from plants during 
emergence was found to be inversely pro- 
portional to their height on the shore (Fig. 8). 
On the basis of these measurements, the rate 
of water loss is, therefore, inversely propor- 
tional to the volume of the water-filled, hol- 
low interiors of the receptacles. In support of 
this, determinations of the above quantities 
were also made with samples of a special, 
large form of Hormosira (d in Fig. 8), which 
is found back from the shore in those low 
scoria areas where tidal seepage occurs. The 
receptacles of these plants were found to have 
an average diameter of 1.70 centimetres and 
an average wall-thickness of 0.10 centimetre, 
and their rate of water loss was the lowest 
recorded. 
It seems possible, therefore, that the rela- 
‘ tively large vertical range of Hormosira hanksii 
depends on a considerable tolerance of dif- 
ferences in submergence-emergence relations 
in the "button” stage, together with morpho- 
logical variation in the adult in relation to the 
degree of desiccation undergone at different 
levels. 
Other Factors 
In the foregoing section, the fundamental 
nature of zonation as a function of the tidal 
factors has been discussed. The final pattern 
of distribution may be considered as the 
product of the reaction of the basic vertical 
zonation with a set of variables. 
