184
A. D. BONEY
Belidium is that by Dixon (1958a,b) for Gelidium pulchellum and a.
latifolium, both species being commonly found in the British Isles.
They are relatively small plants (erect branches 2-10 cm in length),
and are not used extensively as agarophytes, but in many ways their
biology is relevant to the larger species which are regularly harvested
as sources of agar.
Growth of the fronds is effected by a single apical cell which is
lenticular in shape, and this cuts off a segment cell on its inner face.
This then gives rise to a cell which produces in turn axial cells and the
pericentral cells, and these give rise to lateral branches of limited
growth which clothe the main axis and give the appearance of a cortex
and medulla in the maturer parts of the frond. Due to the elongation of
the medullary cells and the formation of secondary pit-connections the
main axis takes on a multiaxial appearance when seen in transverse
section. Downgrowing hyphae are formed from the outermost cells of
the medullary region, and these undergo rapid development. These
hyphae probably have a supporting function, and their position varies
with both the age of the frond and the part of the plant concerned.
The possession of lateral branches of unlimited lateral growth is one
reason for the variability in frond pattern seen over the seasons, and for
the profuse branching growth typical of Gelidium. Branches are
initiated by the transformation of a cortical cell. With G. pulchellum
this change in character of the cortical cell usually takes place at a certain
distance from the main apical cell, and a lateral branch similar to the
main axis then develops. With G. latifolium, whilst certain of the erect
branches behave in much the same way, in others a more flattened
form is found with a number of apical cells (3-8) dispersed over the
margin of the apical meristem. The rates of division of the apical cells
at the extremity of the “meristematic area” are the same, whilst
apical cells on the flanks divide at a much slower rate, with the result
that a branch primordium grows out at right angles to the main axis.
Depending on the time of year and the number of functional apical
cells, the lateral branches may either be as broad as the main axis, or
of a cylindrical nature. Under favourable environmental conditions
the lateral branch initials arise in great profusion in close proximity to
the apical cell, tending to obscure it by their development, and resulting
in a marked change in the appearance of the frond. Transitional forms
between these two extremes are also frequently observed. Adventitious lateral branches can also develop, either aa a result of frond
injury, or because of certain environmental changes.
The erect fronds grow from prostrate creeping branches fixed to the
substratum by small attachment discs, and each prostrate branch has a
A. D. BONEY
Belidium is that by Dixon (1958a,b) for Gelidium pulchellum and a.
latifolium, both species being commonly found in the British Isles.
They are relatively small plants (erect branches 2-10 cm in length),
and are not used extensively as agarophytes, but in many ways their
biology is relevant to the larger species which are regularly harvested
as sources of agar.
Growth of the fronds is effected by a single apical cell which is
lenticular in shape, and this cuts off a segment cell on its inner face.
This then gives rise to a cell which produces in turn axial cells and the
pericentral cells, and these give rise to lateral branches of limited
growth which clothe the main axis and give the appearance of a cortex
and medulla in the maturer parts of the frond. Due to the elongation of
the medullary cells and the formation of secondary pit-connections the
main axis takes on a multiaxial appearance when seen in transverse
section. Downgrowing hyphae are formed from the outermost cells of
the medullary region, and these undergo rapid development. These
hyphae probably have a supporting function, and their position varies
with both the age of the frond and the part of the plant concerned.
The possession of lateral branches of unlimited lateral growth is one
reason for the variability in frond pattern seen over the seasons, and for
the profuse branching growth typical of Gelidium. Branches are
initiated by the transformation of a cortical cell. With G. pulchellum
this change in character of the cortical cell usually takes place at a certain
distance from the main apical cell, and a lateral branch similar to the
main axis then develops. With G. latifolium, whilst certain of the erect
branches behave in much the same way, in others a more flattened
form is found with a number of apical cells (3-8) dispersed over the
margin of the apical meristem. The rates of division of the apical cells
at the extremity of the “meristematic area” are the same, whilst
apical cells on the flanks divide at a much slower rate, with the result
that a branch primordium grows out at right angles to the main axis.
Depending on the time of year and the number of functional apical
cells, the lateral branches may either be as broad as the main axis, or
of a cylindrical nature. Under favourable environmental conditions
the lateral branch initials arise in great profusion in close proximity to
the apical cell, tending to obscure it by their development, and resulting
in a marked change in the appearance of the frond. Transitional forms
between these two extremes are also frequently observed. Adventitious lateral branches can also develop, either aa a result of frond
injury, or because of certain environmental changes.
The erect fronds grow from prostrate creeping branches fixed to the
substratum by small attachment discs, and each prostrate branch has a