BIOLOGY OF SEAWEEDS OF ECONOMIC IMl'ORTANCE
155
6-12 m below E.L.W.S.T. where the sea bed is of soft mud. The
seasonal cycle of sporophyte production shows a shore migration,
being found a t higher levels during winter, early spring and late
summer; and at low levels during spring, early summer and early
autumn. Winter sporophytes rarely survive, and depopulation rates
can be 100% in exposed areas, whereas sporophytes produced in the
spring show a higher survival rate and grow more vigorously. Plants
which survive the initial settlement and growth rarely live for more
than 3 years. Summer and autumn sporophytes rarely survive beyond
the first winter, whilst the spring sporophytes are more firmly attached
and are more robust and can survive the winter storms.
Growth of the plants show alternating periods of fast and slow
growth. The period of fast growth lasts from January until midsummer, with most rapid growth from March to June. Slow growth is
from July to December, with the slowest rate from September to the
end of the year. Early growth of the sporelings is variable in rate
depending on the habitat and time of year ; during the spring the growth
rate is more rapid than in late autumn.
Growth of the lamina shows annual cycles of fast and slow growth,
the fast growth period extending from January to July, with the growth
rate gradually increasing from January to March and remaining
fairly constant then until June. The rate slows down from June to
July, and this continues until the minimum rate is reached in October
or November, and thereafter it increases again. In the second year of
the plant's life the increase in frond length during the rapid growth
phase reaches its maximum compared with the first and third years.
Depending on where the sporophytes commence growth, so the maximum growth phase will also vary. With spring germinated sporophytes
this will be at 15 months old, whereas with sporophytes produced in the
following summer this phase is reached after 21 months. Increase in
frond length is greater the lower down the shore that the plant grows.
In exposed intertidal habitats the plants show very little further
growth after the second period of rapid growth, whereas plants immersed in rock pools at the same levels will survive well into the third
year of growth. Increases of 6" latitude generally result in a higher rate
of frond growth at the northerly limits.
Primary growth of the frond from the basal meristem follows the
course of rapid and slow growth. Secondary growth of tissues above
this region occurs by cell division and elongation of cells already formed.
Secondary growth continues throughout the year in the lower part of
the frond, but lags behind the rapid growth phase by 1-2 months.
Complete regeneration of frond tissue is only possible in plants under
155
6-12 m below E.L.W.S.T. where the sea bed is of soft mud. The
seasonal cycle of sporophyte production shows a shore migration,
being found a t higher levels during winter, early spring and late
summer; and at low levels during spring, early summer and early
autumn. Winter sporophytes rarely survive, and depopulation rates
can be 100% in exposed areas, whereas sporophytes produced in the
spring show a higher survival rate and grow more vigorously. Plants
which survive the initial settlement and growth rarely live for more
than 3 years. Summer and autumn sporophytes rarely survive beyond
the first winter, whilst the spring sporophytes are more firmly attached
and are more robust and can survive the winter storms.
Growth of the plants show alternating periods of fast and slow
growth. The period of fast growth lasts from January until midsummer, with most rapid growth from March to June. Slow growth is
from July to December, with the slowest rate from September to the
end of the year. Early growth of the sporelings is variable in rate
depending on the habitat and time of year ; during the spring the growth
rate is more rapid than in late autumn.
Growth of the lamina shows annual cycles of fast and slow growth,
the fast growth period extending from January to July, with the growth
rate gradually increasing from January to March and remaining
fairly constant then until June. The rate slows down from June to
July, and this continues until the minimum rate is reached in October
or November, and thereafter it increases again. In the second year of
the plant's life the increase in frond length during the rapid growth
phase reaches its maximum compared with the first and third years.
Depending on where the sporophytes commence growth, so the maximum growth phase will also vary. With spring germinated sporophytes
this will be at 15 months old, whereas with sporophytes produced in the
following summer this phase is reached after 21 months. Increase in
frond length is greater the lower down the shore that the plant grows.
In exposed intertidal habitats the plants show very little further
growth after the second period of rapid growth, whereas plants immersed in rock pools at the same levels will survive well into the third
year of growth. Increases of 6" latitude generally result in a higher rate
of frond growth at the northerly limits.
Primary growth of the frond from the basal meristem follows the
course of rapid and slow growth. Secondary growth of tissues above
this region occurs by cell division and elongation of cells already formed.
Secondary growth continues throughout the year in the lower part of
the frond, but lags behind the rapid growth phase by 1-2 months.
Complete regeneration of frond tissue is only possible in plants under