BIOLOQY O F SEAWEEDS OF ECONOMIC IME'ORTANCE
139
Lodge (1951) observed that whilst hybrid forms between the different
species of Fucus can be obtained under experimental conditions, e.g.
in laboratory cultures, or on " cleared strips "(p. 223), they are rarely
found in closed shore communities, possibly due to a factor of competition, or to separate times of fruiting. In more open communities
hybrid populations have been recognized (Burrows and Lodge, 1951).
3. Ascophyllum
Ascophyllum nodosum is a fucoid alga which has become an important component of seaweed meals in many parts of the world. It
grows astride the middle of the foreshore, in a zone which roughly
extends from high level neap tides to low level of neap tides. Thus
methods of harvesting do not entail extensive underwater cropping.
Seaweed meals are being produced in ever-increasing quantities, so
that knowledge of the life history and seasonal behaviour of the plants
is of considerable importance. Ascophyllum plants are also harvested
as manure. It is noticeable that controlled harvesting has been instituted in Canada (MacParlane, 1964a). Ascophyllum plants seem to
grow best on shores which are sheltered from more severe wave action,
and the type of substratum is an important factor here. David (1943)
made an extensive study of the autecology of Ascophyllum on the coast
of Wales, together with similar observations on other shores in the
British Isles. The strap-shaped Ascophyllum frond bears a terminal
meristem lying in an apical groove, and has numerous short lateral
branches arising from narrow slits in the frond margin which also
contain meristematic cells. Regenerative growth of cut branches is
clearly important. Thus, when fronds were severed below the fertile
regions no regenerative growth was observed, whereas if cut in the
fertile regions of the frond new branches developed from lateral pits
which normally developed receptacles. Regenerative growth was
possible in branches up to 5 years old, whereas in much older plants
this power was lost. Where lateral meristems are lacking, as in very
young plants, regenerative growth is not observed. Branching systems
are monopodial, dichotomous or trichotomous, with plants in the
mid-part of the shore zone showing predominantly monopodial growth.
Baardseth (1955b) observed that if fronds were cut within 4-15 cm of
the base in regions which did not contain lateral meristems no regenerative growth would be observed. Frond damage of this nature
can also result from natural causes, such as mollusc browsing, fungal
parasites, overloading with epiphytes (particularly Polysiphonia
lanosa). If, however, short basal " shoots " are present which bear
apical meristems then growth can continue, and these basal shoots
H Z
139
Lodge (1951) observed that whilst hybrid forms between the different
species of Fucus can be obtained under experimental conditions, e.g.
in laboratory cultures, or on " cleared strips "(p. 223), they are rarely
found in closed shore communities, possibly due to a factor of competition, or to separate times of fruiting. In more open communities
hybrid populations have been recognized (Burrows and Lodge, 1951).
3. Ascophyllum
Ascophyllum nodosum is a fucoid alga which has become an important component of seaweed meals in many parts of the world. It
grows astride the middle of the foreshore, in a zone which roughly
extends from high level neap tides to low level of neap tides. Thus
methods of harvesting do not entail extensive underwater cropping.
Seaweed meals are being produced in ever-increasing quantities, so
that knowledge of the life history and seasonal behaviour of the plants
is of considerable importance. Ascophyllum plants are also harvested
as manure. It is noticeable that controlled harvesting has been instituted in Canada (MacParlane, 1964a). Ascophyllum plants seem to
grow best on shores which are sheltered from more severe wave action,
and the type of substratum is an important factor here. David (1943)
made an extensive study of the autecology of Ascophyllum on the coast
of Wales, together with similar observations on other shores in the
British Isles. The strap-shaped Ascophyllum frond bears a terminal
meristem lying in an apical groove, and has numerous short lateral
branches arising from narrow slits in the frond margin which also
contain meristematic cells. Regenerative growth of cut branches is
clearly important. Thus, when fronds were severed below the fertile
regions no regenerative growth was observed, whereas if cut in the
fertile regions of the frond new branches developed from lateral pits
which normally developed receptacles. Regenerative growth was
possible in branches up to 5 years old, whereas in much older plants
this power was lost. Where lateral meristems are lacking, as in very
young plants, regenerative growth is not observed. Branching systems
are monopodial, dichotomous or trichotomous, with plants in the
mid-part of the shore zone showing predominantly monopodial growth.
Baardseth (1955b) observed that if fronds were cut within 4-15 cm of
the base in regions which did not contain lateral meristems no regenerative growth would be observed. Frond damage of this nature
can also result from natural causes, such as mollusc browsing, fungal
parasites, overloading with epiphytes (particularly Polysiphonia
lanosa). If, however, short basal " shoots " are present which bear
apical meristems then growth can continue, and these basal shoots
H Z