BIOLOUY OF SEAWEEDS OF ECONOMIC IMPORTANCE
137
between 069-0.84 cmlweek, and in Scotland between 0-57-0-93 cm/
week. The length/dichotomy relationship for F . serratus differs over the
geographical range as for F . vesiculosus, but in the reverse order. The
greater number of dichotomies per length are observed on the Devon
coast, and the smallest number on Scottish shores. A similar inequality
of growth potential was observed, with one arm of the fork greater than
the other, but with increasing age of the plant this inequality gradually
diminishes. This length/dichotomy relationship is important with
regard to growth after harvesting.
I n both species the various phases of reproduction are of a protracted nature. In Fucus serratus initiation of receptacles commences
in May in Devon, and this continues until July. Release of gametes
commences at the end of July, and continues through until early
autumn, and then wanes. The sterile period lasts through the winter
months. The phasing is later in the Isle of Man, with initiation of
receptacles commencing in late summer, release of gametes in late
autumn, and the sterile period lasting from March to July. The times
of fruiting have been observed to be more protracted on other shores.
In Fucus vesiculosus receptacle initiation commences in December,
but release of gametes is not observed until May on the Devon coast
and is preceded by one month in the Isle of Man. I n this last locality
initiation of new receptacles commences almost as soon as the fruiting
of the same year is completed. Completion of fruiting in both species
is followed by a wholesale defoliation of the spent fertile branches. The
maximum weight of the plants is reached during the fruiting period.
Plants of both species rarely survive for more than 3 years on the
shore, and the depopulation rate of marked plants may be as much as
83 to 90% at the end of the 3rd year. A particularly high mortality
rate was observed with very young plants in the earliest stages of
germination, mainly due to the browsing action of molluscs (p. 223).
Studies were also made on growth after cutting and on repopulation of
cleared areas. Repopulation studies with Fucus vesiczclosus on areas
cleared of plants showed that after long periods the plant population
was only slightly less than the original population before clearance.
When the fronds are removed by cutting a marked proliferation of new
branches is observed in F . vesiculosus. This is due to a loss of apical
control, and Burrows (1966) observed this same sort of proliferation
from cut plants of Fucus spiralis, with new branches always appearing
from the midrib region. Knight and Parke (1960) observed that
proliferation only occurred in cut fronds which had borne an apical
cell, and that with increasing age of the frond the amount of proliferated
frondage decreased markedly. Sauvageau (1920) observed that the
A.P.B.-3
11
137
between 069-0.84 cmlweek, and in Scotland between 0-57-0-93 cm/
week. The length/dichotomy relationship for F . serratus differs over the
geographical range as for F . vesiculosus, but in the reverse order. The
greater number of dichotomies per length are observed on the Devon
coast, and the smallest number on Scottish shores. A similar inequality
of growth potential was observed, with one arm of the fork greater than
the other, but with increasing age of the plant this inequality gradually
diminishes. This length/dichotomy relationship is important with
regard to growth after harvesting.
I n both species the various phases of reproduction are of a protracted nature. In Fucus serratus initiation of receptacles commences
in May in Devon, and this continues until July. Release of gametes
commences at the end of July, and continues through until early
autumn, and then wanes. The sterile period lasts through the winter
months. The phasing is later in the Isle of Man, with initiation of
receptacles commencing in late summer, release of gametes in late
autumn, and the sterile period lasting from March to July. The times
of fruiting have been observed to be more protracted on other shores.
In Fucus vesiculosus receptacle initiation commences in December,
but release of gametes is not observed until May on the Devon coast
and is preceded by one month in the Isle of Man. I n this last locality
initiation of new receptacles commences almost as soon as the fruiting
of the same year is completed. Completion of fruiting in both species
is followed by a wholesale defoliation of the spent fertile branches. The
maximum weight of the plants is reached during the fruiting period.
Plants of both species rarely survive for more than 3 years on the
shore, and the depopulation rate of marked plants may be as much as
83 to 90% at the end of the 3rd year. A particularly high mortality
rate was observed with very young plants in the earliest stages of
germination, mainly due to the browsing action of molluscs (p. 223).
Studies were also made on growth after cutting and on repopulation of
cleared areas. Repopulation studies with Fucus vesiczclosus on areas
cleared of plants showed that after long periods the plant population
was only slightly less than the original population before clearance.
When the fronds are removed by cutting a marked proliferation of new
branches is observed in F . vesiculosus. This is due to a loss of apical
control, and Burrows (1966) observed this same sort of proliferation
from cut plants of Fucus spiralis, with new branches always appearing
from the midrib region. Knight and Parke (1960) observed that
proliferation only occurred in cut fronds which had borne an apical
cell, and that with increasing age of the frond the amount of proliferated
frondage decreased markedly. Sauvageau (1920) observed that the
A.P.B.-3
11