208
F.
C. STEWARD AND
Η. Y. MOHAN RAM
Candida albicans, cell division occurs as a result of reduction of
disulphjde bonds in a glucomannan-protein complex of the yeast cell
wall with metabolically generated hydrogen by a specific enzymeprotein disulphide reductase. (This situation recalls that of keratin in
the wool of sheep, in which the form of the hair is determined by the
oxidation of SH-bonds, which in turn is regulated by copper-containing
enzymes in the sheep.) The reduction of disulphide linkages results in
the 'softening' of the wall material at a site where the new bud extrudes
as a protoplasmic sphere. (Whether some similar explanation could apply
to instances of budding in cell suspensions of carrot, cf. Steward, Mapes
and Smith, 1958, remains to be seen.) Nickerson and Falcone substantiated their findings by the observation that in a division-less mutant
there is a block to the reduction step, and the metabolically generated
hydrogen virtually 'spills over' (Nickerson, 1954; Falcone and Nickerson, 1959).
In the special cases of coenocytic structures such as those found in
the Siphonales among the algae, or the Phycornycetes among the fungi,
or in certain fibres in flowering plants, nuclear divisions occur without
the formation of septa to divide the protoplasts. One can visualize the
possibility that in these cases some localized block to the synthesis of
new wall material may operate to prevent the formation of septa,
without impairing, of course, the nuclear divisions.
Regarding cell enlargement as the second phase of cell growth, an
attempt will now be made to understand its bearing on some morphogenetic problems. Before doing so however, it may be well to refer to
some situations in which a knowledge of the factors that govern growth
by cell enlargement should be applicable.
After cells are laid down in the root apex, much growth by enlargement ensues in the extending 5 mm from the tip (Brown and Broadbent,
1950) of the root. It is the region preceding this 5 mm, in which some
cells of the piliferous layer extend outwards to form root hairs,
which is, classically, the site of geotropic responses. After each leaf
primordium and its subtending internode is laid down in the apex of
the shoot, phases of growth ensue in which cell enlargement may be the
conspicuous feature, namely elongation of the internode following upon
growth by expansion of the lamina. These regions of the shoot are again
associated with geotropism and phototropism (as in the hypocotyl of
Helianthus) and with the morphogenetic effects of light (as in the contrast
between plants with elongated and compressed internodes with erect
and rosette habits as seen in Sempervivum, or as seen in the effects upon
leaf shape of such stimuli as those due to etiolation, growth under or
above water, growth in shade or in high light intensity, etc.). The coleop
tile of grasses, as a modified foliar organ, strictly determined in its
growth by the early cessation of cell division, has provided the traditional
F.
C. STEWARD AND
Η. Y. MOHAN RAM
Candida albicans, cell division occurs as a result of reduction of
disulphjde bonds in a glucomannan-protein complex of the yeast cell
wall with metabolically generated hydrogen by a specific enzymeprotein disulphide reductase. (This situation recalls that of keratin in
the wool of sheep, in which the form of the hair is determined by the
oxidation of SH-bonds, which in turn is regulated by copper-containing
enzymes in the sheep.) The reduction of disulphide linkages results in
the 'softening' of the wall material at a site where the new bud extrudes
as a protoplasmic sphere. (Whether some similar explanation could apply
to instances of budding in cell suspensions of carrot, cf. Steward, Mapes
and Smith, 1958, remains to be seen.) Nickerson and Falcone substantiated their findings by the observation that in a division-less mutant
there is a block to the reduction step, and the metabolically generated
hydrogen virtually 'spills over' (Nickerson, 1954; Falcone and Nickerson, 1959).
In the special cases of coenocytic structures such as those found in
the Siphonales among the algae, or the Phycornycetes among the fungi,
or in certain fibres in flowering plants, nuclear divisions occur without
the formation of septa to divide the protoplasts. One can visualize the
possibility that in these cases some localized block to the synthesis of
new wall material may operate to prevent the formation of septa,
without impairing, of course, the nuclear divisions.
Regarding cell enlargement as the second phase of cell growth, an
attempt will now be made to understand its bearing on some morphogenetic problems. Before doing so however, it may be well to refer to
some situations in which a knowledge of the factors that govern growth
by cell enlargement should be applicable.
After cells are laid down in the root apex, much growth by enlargement ensues in the extending 5 mm from the tip (Brown and Broadbent,
1950) of the root. It is the region preceding this 5 mm, in which some
cells of the piliferous layer extend outwards to form root hairs,
which is, classically, the site of geotropic responses. After each leaf
primordium and its subtending internode is laid down in the apex of
the shoot, phases of growth ensue in which cell enlargement may be the
conspicuous feature, namely elongation of the internode following upon
growth by expansion of the lamina. These regions of the shoot are again
associated with geotropism and phototropism (as in the hypocotyl of
Helianthus) and with the morphogenetic effects of light (as in the contrast
between plants with elongated and compressed internodes with erect
and rosette habits as seen in Sempervivum, or as seen in the effects upon
leaf shape of such stimuli as those due to etiolation, growth under or
above water, growth in shade or in high light intensity, etc.). The coleop
tile of grasses, as a modified foliar organ, strictly determined in its
growth by the early cessation of cell division, has provided the traditional