BIOLOGY O F SEAWEEDS OF ECONOMIC IMPORTANCE
167
of onset of fruiting due to the age of frond tissue and environmental
factors, it appears that actual initiation of the reproductive tissue is
controlled by some agency within the plant, but whether this is of a
hormonal nature is not known. L. saccharina plants on the British
coasts commence fruiting in their 3rd year, and sporangium production
commences in July, attaining a maximum in early August, and is
completed by the end of the same month. On the Norwegian coast
fruiting plants are found from the middle of May to the end of September. Tikhovskaya (1940) observed maximum growth of L. saccharina
in the Barents Sea in April, showing a sharp drop in June and a slight
increase in October. This fall in growth rate occurs at a time when the
nitrate and phosphate content of the sea is at its lowest.
Annual fluctuation in weight of the sporophytes of L. saccharina
have been described by Kireeva and Schapova (1933, 1938) and
Tikhovskaya (1940) from the Russian coast, and by Parke (1948) for
south-west England. On Russian shores maximum weight is reached
in August, and minimum weight in March, whereas on the British
coast maximum weight is reached in June or July, and minimal
values in December or January. Kuznetzov (1966) observed that L.
saccharina grew faster than L. digitata in sheltered localities bordering
the Barents Sea, but that L. digitata was the more abundant in exposed
localities. For that region growth of L. digitata blade is observed up to
July, whereas stipe and haptera grow throughout the summer, and since
the haptera attain maximum development in autumn, the plants will
be more able to withstand severe autumnal gales.
More recent work on Laminaria digitata in Norway (Sundene, 1964)
has shown that growth of the new blade is an overall process, but with
more extensive cell production from the basal meristem over the year,
and with considerable growth above this region between January and
June. Between spring and summer there is a steady slowing down of
blade growth above the basal region, and new growth commences
again in September. For south and west Norway the rapid growth
period is from February to April, and slow-growth is observed between
late summer and autumn. During the phase of rapid growth the blade
elongates at the rate of 0-03 cm/day. By contrast, in north Norway the
maximum growth period is from March to May, it then slows rather
abruptly, declining through summer and autumn and finally ceases in
mid-winter. The growth rhythms of plants from south and north
Norway are fundamentally distinct; on southern shores the growth is
slowed down by increased sea temperatures, whereas on northern shores
the rate falls with onset of winter darkness.
Depopulation studies on sporophytes of Laminaria digitata and
167
of onset of fruiting due to the age of frond tissue and environmental
factors, it appears that actual initiation of the reproductive tissue is
controlled by some agency within the plant, but whether this is of a
hormonal nature is not known. L. saccharina plants on the British
coasts commence fruiting in their 3rd year, and sporangium production
commences in July, attaining a maximum in early August, and is
completed by the end of the same month. On the Norwegian coast
fruiting plants are found from the middle of May to the end of September. Tikhovskaya (1940) observed maximum growth of L. saccharina
in the Barents Sea in April, showing a sharp drop in June and a slight
increase in October. This fall in growth rate occurs at a time when the
nitrate and phosphate content of the sea is at its lowest.
Annual fluctuation in weight of the sporophytes of L. saccharina
have been described by Kireeva and Schapova (1933, 1938) and
Tikhovskaya (1940) from the Russian coast, and by Parke (1948) for
south-west England. On Russian shores maximum weight is reached
in August, and minimum weight in March, whereas on the British
coast maximum weight is reached in June or July, and minimal
values in December or January. Kuznetzov (1966) observed that L.
saccharina grew faster than L. digitata in sheltered localities bordering
the Barents Sea, but that L. digitata was the more abundant in exposed
localities. For that region growth of L. digitata blade is observed up to
July, whereas stipe and haptera grow throughout the summer, and since
the haptera attain maximum development in autumn, the plants will
be more able to withstand severe autumnal gales.
More recent work on Laminaria digitata in Norway (Sundene, 1964)
has shown that growth of the new blade is an overall process, but with
more extensive cell production from the basal meristem over the year,
and with considerable growth above this region between January and
June. Between spring and summer there is a steady slowing down of
blade growth above the basal region, and new growth commences
again in September. For south and west Norway the rapid growth
period is from February to April, and slow-growth is observed between
late summer and autumn. During the phase of rapid growth the blade
elongates at the rate of 0-03 cm/day. By contrast, in north Norway the
maximum growth period is from March to May, it then slows rather
abruptly, declining through summer and autumn and finally ceases in
mid-winter. The growth rhythms of plants from south and north
Norway are fundamentally distinct; on southern shores the growth is
slowed down by increased sea temperatures, whereas on northern shores
the rate falls with onset of winter darkness.
Depopulation studies on sporophytes of Laminaria digitata and