V.
DETERMINING
FACTORS IN CELL
GROWTH
213
C. Inhibitors of Cell Division and Cell Enlargement
The knowledge of specific factors that stimulate growth by cell
division and cell enlargement has also led to the recognition of the
occurrence of inhibitory substances and mechanisms which act upon
these aspects of growth (thus conveying the inherent possibility of a
regulatory control of growth). In fact, any growth-promoting substance
may be an inhibitor at one concentration and a stimulant at another
(usually lower) concentration (e.g. 2,4-D). Furthermore, a substance
may be stimulatory at a given level of concentration in one organ and
inhibitory at the same level in another. As Stern (1956) has pointed out,
'the same stimulant or inhibitor may evoke quite different responses,
even in contiguous cells, because of the distinctive metabolic poise
acquired by each species of cell as a result of differentiation.' Thus the
age of an organ or tissue may also determine the extent of inhibition
(Thimann and Bonner, 1948). It is well known that auxins promote the
growth of shoots but inhibit the growth of lateral buds and roots at the
same concentration. However, at lower concentrations auxin is somewhat stimulatory to root growth, especially in inducing cell divisions in
lateral meristems (Torrey, 1953). In root tips of onion, 3 ppm of IAA
inhibits the divisions in the meristematic cells but stimulates the same
in the more mature cells (Stern, 1956). Also, different levels of auxin
(both applied and native) may either promote or prevent the abscission
of leaves or of fruits, which is a phenomenon involving renewed growth in
the region of the abscission. For detailed recent information on the role
of auxin in abscission, one may refer to the work of Rossetter and
Jacobs (1953); Jacobs (1955, 1958); Addicott and Lynch (1955);
Wetmore and Jacobs (1953) and Biggs and Leopold (1958).
The wide range of known mitotic poisons or inhibitors will not be
discussed here (see Brächet, 1957, for details). Maleic hydrazide seems
to be an example of a synthetic growth regulant that acts specifically by
the suppression of cell division (Greulach and Atchison, 1950, 1953;
Darlington and McLeish, 1951; Greulach and Haesloop, 1954). This and
other mitotic depressants, e.g. 2,3,5-triiodobenzoic acid (TIBA), may
act by interfering with the site of sulphydryl-containing substances
(Leopold and Price, 1957). 2,4-dinitrophenol not only inhibits cell
division in the root (Hopkins, 1952), but also prevents vascular tissue
differentiation (Torrey, 1953).
Again a special range of substances are those which control growth of
cells in the embryo and seed. An important step in the development of a
seed is the onset of dormancy in the embryo, and this may be prolonged
in varying degrees.
It is now well understood that dormancy may be prolonged and
DETERMINING
FACTORS IN CELL
GROWTH
213
C. Inhibitors of Cell Division and Cell Enlargement
The knowledge of specific factors that stimulate growth by cell
division and cell enlargement has also led to the recognition of the
occurrence of inhibitory substances and mechanisms which act upon
these aspects of growth (thus conveying the inherent possibility of a
regulatory control of growth). In fact, any growth-promoting substance
may be an inhibitor at one concentration and a stimulant at another
(usually lower) concentration (e.g. 2,4-D). Furthermore, a substance
may be stimulatory at a given level of concentration in one organ and
inhibitory at the same level in another. As Stern (1956) has pointed out,
'the same stimulant or inhibitor may evoke quite different responses,
even in contiguous cells, because of the distinctive metabolic poise
acquired by each species of cell as a result of differentiation.' Thus the
age of an organ or tissue may also determine the extent of inhibition
(Thimann and Bonner, 1948). It is well known that auxins promote the
growth of shoots but inhibit the growth of lateral buds and roots at the
same concentration. However, at lower concentrations auxin is somewhat stimulatory to root growth, especially in inducing cell divisions in
lateral meristems (Torrey, 1953). In root tips of onion, 3 ppm of IAA
inhibits the divisions in the meristematic cells but stimulates the same
in the more mature cells (Stern, 1956). Also, different levels of auxin
(both applied and native) may either promote or prevent the abscission
of leaves or of fruits, which is a phenomenon involving renewed growth in
the region of the abscission. For detailed recent information on the role
of auxin in abscission, one may refer to the work of Rossetter and
Jacobs (1953); Jacobs (1955, 1958); Addicott and Lynch (1955);
Wetmore and Jacobs (1953) and Biggs and Leopold (1958).
The wide range of known mitotic poisons or inhibitors will not be
discussed here (see Brächet, 1957, for details). Maleic hydrazide seems
to be an example of a synthetic growth regulant that acts specifically by
the suppression of cell division (Greulach and Atchison, 1950, 1953;
Darlington and McLeish, 1951; Greulach and Haesloop, 1954). This and
other mitotic depressants, e.g. 2,3,5-triiodobenzoic acid (TIBA), may
act by interfering with the site of sulphydryl-containing substances
(Leopold and Price, 1957). 2,4-dinitrophenol not only inhibits cell
division in the root (Hopkins, 1952), but also prevents vascular tissue
differentiation (Torrey, 1953).
Again a special range of substances are those which control growth of
cells in the embryo and seed. An important step in the development of a
seed is the onset of dormancy in the embryo, and this may be prolonged
in varying degrees.
It is now well understood that dormancy may be prolonged and