V.
DETERMINING FACTORS IN
CELL GROWTH
207
significant feature of this work is that, over and above the supply of
necessary nutrients which are furnished exogenously, the growth by
cell division is regulated by a combination of chemical molecules which,
in their effects, interact to promote the growth process. The substances
which were most effective in Braun's work were naphthaleneacetic acid
(NAA) and meso-inositol, though the effect was accentuated by sources
of nitrogen. Thus Braun (1958) states (loc. cit., p. 347), 'The moderately
fast-growing tumour cell required that the basic medium be supplemented with glutamine, meso-inositol and a cell enlargement factor
(NAA) to achieve a growth rate comparable to that of the fully altered,
rapidly growing type of tumour cell.' The most remarkable feature is
that, here also, the induction and persistence of growth by cell division
is regulated by chemical means.
B. Stimuli to Growth by Cell Enlargement
Because growth by cell enlargement contributes in such a large
measure to increase in volume, the term 'growth' has often been used as
though it were synonymous with cell enlargement. The great attention
focused on the role of auxins, and now of gibberellins, which act predominantly upon this phase of growth, tends to give credence to this
approach. Although division and enlargement are but two stages that
characterize cell growth, they are distinct in nature and are affected by
different stimuli. One can easily visualize that cells may multiply (in
such areas as young embryos and the apices of shoot and root) with the
minimum of enlargement, but one can hardly conceive growth being
exclusively by cell enlargement without cell division, or multiplication
of protoplasts in coenocytes.
However, cell enlargement and cell division have proved separable in
recent work on certain micro-organisms (Nickerson, 1948). Filamentous
growth resulting from lack of cross wall formation ensues in many rodshaped bacteria due to certain deficiencies in the metabolites, caused
by treatment with a range of chemical substances and even with X- and
other radiations (Webb, 1953, 1959; Deering and Setlow, 1957). All
these variously related inhibitory stimuli must be in some way blocking
the synthesis of a cellular component. It is interesting that these microorganisms in which such filamentous growth has been induced have the
same dry weight and total DNA content as those of the control cultures
in which normal divisions are uninterrupted (Falcone and Nickerson,
1959).
In the quest for an enzymatic explanation of cell division, there has
emerged a valuable finding by the investigations of Nickerson and
Falcone (1956). These authors have demonstrated that in the yeast
DETERMINING FACTORS IN
CELL GROWTH
207
significant feature of this work is that, over and above the supply of
necessary nutrients which are furnished exogenously, the growth by
cell division is regulated by a combination of chemical molecules which,
in their effects, interact to promote the growth process. The substances
which were most effective in Braun's work were naphthaleneacetic acid
(NAA) and meso-inositol, though the effect was accentuated by sources
of nitrogen. Thus Braun (1958) states (loc. cit., p. 347), 'The moderately
fast-growing tumour cell required that the basic medium be supplemented with glutamine, meso-inositol and a cell enlargement factor
(NAA) to achieve a growth rate comparable to that of the fully altered,
rapidly growing type of tumour cell.' The most remarkable feature is
that, here also, the induction and persistence of growth by cell division
is regulated by chemical means.
B. Stimuli to Growth by Cell Enlargement
Because growth by cell enlargement contributes in such a large
measure to increase in volume, the term 'growth' has often been used as
though it were synonymous with cell enlargement. The great attention
focused on the role of auxins, and now of gibberellins, which act predominantly upon this phase of growth, tends to give credence to this
approach. Although division and enlargement are but two stages that
characterize cell growth, they are distinct in nature and are affected by
different stimuli. One can easily visualize that cells may multiply (in
such areas as young embryos and the apices of shoot and root) with the
minimum of enlargement, but one can hardly conceive growth being
exclusively by cell enlargement without cell division, or multiplication
of protoplasts in coenocytes.
However, cell enlargement and cell division have proved separable in
recent work on certain micro-organisms (Nickerson, 1948). Filamentous
growth resulting from lack of cross wall formation ensues in many rodshaped bacteria due to certain deficiencies in the metabolites, caused
by treatment with a range of chemical substances and even with X- and
other radiations (Webb, 1953, 1959; Deering and Setlow, 1957). All
these variously related inhibitory stimuli must be in some way blocking
the synthesis of a cellular component. It is interesting that these microorganisms in which such filamentous growth has been induced have the
same dry weight and total DNA content as those of the control cultures
in which normal divisions are uninterrupted (Falcone and Nickerson,
1959).
In the quest for an enzymatic explanation of cell division, there has
emerged a valuable finding by the investigations of Nickerson and
Falcone (1956). These authors have demonstrated that in the yeast