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

152
Table 7.4 lncrease in the length of Oaro plants during late winter
and spring - summey.
size class (m)
29/6/73 - 14/9/73
14/9/73 - 9/1/74
0.1 ~ 0.5
0.5 1.0
1 ~ 2
28.9 ± 14.4 em
23.0 ± 12.5
33.5 ± 7.4
ILL I ± 8.3
19.7 ± 4.0
lO.7 :.t ''102
(0)
Some small D. anto;r>ctica specimens gre\v in very shaded positions.
These were often sandwiched between, or attached to large composite
holdfasts, and were shaded by the fronds of large plants. These Ivere
designated "understorey plants", and their grOtl1th was compared with
tha.t of other small plants growing' in more open and better illuminated
positions in the kelp bed (Fig. 7.11). Results clearly showed that
understorey plants grew more slowly than plants in more open positions.
Host unders torey plants \"ere attached to large composite holdfasts
supporting several plants, whereas other small plants in more
open areas usually had discrete holdfasts. It could be argued.
therefore, that the relatively slow growt.h of understorey plants was
due, not so much to their shady habitat, but because they were
borne on composite holdfasts. It was pertinent t.o find out if larger
specimens (1.6-3.6 m long) which were not shaded, but nevertheless
attached to large composite holdfasts, grew more slowly than similar
sized specimens with discrete holdfasts. Results of this comparison
(Fig. 7.12) revealed no significant difference between growth rates,
It seems likely that understorey plants grow more slc,.,:ly because they
are sha.ded.
(d) Growth rate of D. anto~ctica ~ants infecte~BerP0discU8
As described previously, marked D. antarctica plants at
Tautuxu were segretated into size classes, and also according to
whether or not they were visibly infected with Herpodi8CUS. Grm>1'ch
rates of infected and uninfected plants were compared (Figs.
7013; 7.14) _ A large size class of uninfected plants
because few lC\.:cge specimens were entirely free of this parasitic
alga (Table 7.5).