A biological study of Durvillaea antarctica (Chamisso) Hariot and D ...

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(iii) Cape form (Figs.3.9a,b: 5.6g-i; 5.8b-e)
As Lewis (1964:30) pointed out, the flatter and more extensive
a shore, and the more irregular its profile, then the more gentle wave
action will be, This is because much of the kinetic energy
wave is expended against the seaward edge of the shore platfdbn, and
the remaining energy is dissipated over the IJlide expanse of shore.
Impact forces are ~.,eak, and the pattern of waves approaches that seen
on a beach. Populations of D, antarctica on shores such as these,
or in ~ semishelter of harbours p contrast markedly with populations
of thonged plants on steeper, more exposed shores (Figs 5.6; 5.7'; 5.13;
Table 5.4).
Extre:i'ae cape form. specimens have ovate or ohlanceolate laminae
wi th entire margins. Honeycombing is relaJcively th in (less than 5 mm)
and rarely extends over the entire lamina. the margins of which are
distinctly undulate (Flg. 3. 9a) . Stipes are small in proportion to
the size of the lamina; most being shorter than 10 em and thinner,
than 25 rom. A 9 m specimen from Oaro ,I1as supported by a stipe 3 C'f(I
long and 10 rom thick. Cape form plants often have a shall~~ groove
\1hich runs from the base of the lamina part way down the stipe, a
feature very rarely seen on the stipes of thonged plants. The stipe
is usually recessed into the hold£ast, and pronounced flexuxes
extend right around it at its base. Flexures in the stipes of
thonged plants are typically more pronounced on the side of the stipe
facing down shore, This slight difference is possibly due to the
fact that on gently slol?ing shores, the stipes of cape form plants
are bent in almost any direc'don when the tide receeds, while the
stipes of thonged plants are invariably bent down the steep slope.
A more detailed investigation of local morphological variation
in D. antarctioa ~1as made at Tautuku. The gently sloping sandstone
slopes B,t Tautuku afford an excellent opportunity tu sample D ..
an taJ:>ctioa plants across a gradient of tolaVe force. Samples \olere
collected from six different positions on First Slope, across which
there IIla5 o. continuous but ir.regular decline in wave force (f'ig.
5.10) . The data obtained from these six samples are presented in
Table 5.2. They sho~"ed that the degree of braiding and honeycombing
of fronds decreased markedly across the slope. 1n Area 1 (the most
e:v.posed site) 30% of all specimens longer than I In were rated as
highly braided and highly honeycombed; whereas in l'..rea 10, the most
inshore posi tion, only 5% of the plan'ts \'Iere ra ted. Mean