224
F. C. STEWARD AND Η. Y. MOHAN RAM
tangled, homogenous weft of cellulose fibrils embedded in amorphous
material which could presumably enlarge by plastic flow. The later
formed wall, established after the sporelings have acquired orientation,
is strongly directed, being formed by successive layers in which the
cellulose fibrils spiral around the 'cell' and, indeed, the direction of the
spiral may shift in successive layers (Steward and Mühlethaler, 1953).
The sporelings of Valonia ventricosa cannot be made to grow into whole
plants until they contact and attach themselves to a receptive surface
(e.g. marble or calcium carbonate). After attachment, they acquire
polarity and cut off rhizoidal cells by extremely curved walls (the
'Uhrglaswande' of those who first studied these forms), and these (Fig.
3(a)) grow downward into the substratum (curiously similar growth
forms can be seen in cultured free cells of certain angiosperms, Fig. 3(e)).
However, again the very first formed wall on the spherical Valonia
aplanospore is seen, under the electron microscope, to be composed of a
tangled unoriented weft of cellulose fibrils. But, as the growth proceeds
and especially after the sporeling is attached, cellulose is laid down in
successive lamellae in which the fibril direction shifts abruptly from
layer to layer apparently through 60° or 120°, so that the first direction
is repeated at every fourth layer. For a detailed account of the regulation
of polarity in the germination of spores and zygotes of a variety of
lower plants by unilateral light and other gradients, the reader is
referred to Jaffe(1958).
These examples show that marked polarity may exist in a spherical
cell or sporeling even without successive cleavage planes, and this suggests that the states conducive to symmetrical and polarized growth
respectively differ in the way the cellulose wall is laid down. The stimulus to asymmetrical growth was in the one case (V. ocellata) mediated
by light, and in the other (V. ventricosa) by contact with a surface, and
therefore it may also involve gravity.
These examples suggest that some morphogenetic effects may be
mediated by external stimuli acting upon the submicroscopic structures
of cellulose walls, which in turn will induce asymmetry in the way cells
grow. It is interesting, therefore, to recall that the cotton hair is an
outstanding case in point. In the cotton hair the successive concentric
cellulose lamellae are definitely known to be due to asymmetric conditions during growth—in this case induced by diurnal stimuli (Kerr,
1937). When grown under constant and uniform conditions of light and
temperature, the walls of the hairs are homogeneous and free from
laminations (Anderson and Kerr, 1938). Thus one could look with
confidence to the effects of some growth-regulating substances which
act upon the submicroscopic configuration of the cellulose wall for
explanation of certain morphogenetic effects.
F. C. STEWARD AND Η. Y. MOHAN RAM
tangled, homogenous weft of cellulose fibrils embedded in amorphous
material which could presumably enlarge by plastic flow. The later
formed wall, established after the sporelings have acquired orientation,
is strongly directed, being formed by successive layers in which the
cellulose fibrils spiral around the 'cell' and, indeed, the direction of the
spiral may shift in successive layers (Steward and Mühlethaler, 1953).
The sporelings of Valonia ventricosa cannot be made to grow into whole
plants until they contact and attach themselves to a receptive surface
(e.g. marble or calcium carbonate). After attachment, they acquire
polarity and cut off rhizoidal cells by extremely curved walls (the
'Uhrglaswande' of those who first studied these forms), and these (Fig.
3(a)) grow downward into the substratum (curiously similar growth
forms can be seen in cultured free cells of certain angiosperms, Fig. 3(e)).
However, again the very first formed wall on the spherical Valonia
aplanospore is seen, under the electron microscope, to be composed of a
tangled unoriented weft of cellulose fibrils. But, as the growth proceeds
and especially after the sporeling is attached, cellulose is laid down in
successive lamellae in which the fibril direction shifts abruptly from
layer to layer apparently through 60° or 120°, so that the first direction
is repeated at every fourth layer. For a detailed account of the regulation
of polarity in the germination of spores and zygotes of a variety of
lower plants by unilateral light and other gradients, the reader is
referred to Jaffe(1958).
These examples show that marked polarity may exist in a spherical
cell or sporeling even without successive cleavage planes, and this suggests that the states conducive to symmetrical and polarized growth
respectively differ in the way the cellulose wall is laid down. The stimulus to asymmetrical growth was in the one case (V. ocellata) mediated
by light, and in the other (V. ventricosa) by contact with a surface, and
therefore it may also involve gravity.
These examples suggest that some morphogenetic effects may be
mediated by external stimuli acting upon the submicroscopic structures
of cellulose walls, which in turn will induce asymmetry in the way cells
grow. It is interesting, therefore, to recall that the cotton hair is an
outstanding case in point. In the cotton hair the successive concentric
cellulose lamellae are definitely known to be due to asymmetric conditions during growth—in this case induced by diurnal stimuli (Kerr,
1937). When grown under constant and uniform conditions of light and
temperature, the walls of the hairs are homogeneous and free from
laminations (Anderson and Kerr, 1938). Thus one could look with
confidence to the effects of some growth-regulating substances which
act upon the submicroscopic configuration of the cellulose wall for
explanation of certain morphogenetic effects.