V.
DETERMINING
FACTORS IN
CELL GROWTH
215
growth-promoting hormone system.' In fact, the whole category of cell
division factors could act in lieu of what Thimann refers to as the
growth-promoting hormone system. The widespread presence of genetic
dwarfs and the recent use of gibberellins to induce their elongation is
open to the interpretation that elongation in the dwarf is suppressed by
an antiauxin-like substance, the effect of which is released by the
treatment with gibberellins. In the best known cases where gibberellin
acts like low temperature treatment (Lang, 1956) to release the growth
of a dormant dwarf vegetative shoot and to promote the growth of the
taller flowering shoot, it may counteract an existing inhibitor of growth
by elongation.
Substances which structurally resemble auxin may act antagonistically by competing for the site of action; 3,5-D (3,5-dichlorophenoxy
acetic acid) and indoleisobutyric acid are such root growth promoters
which inhibit shoot growth (Thimann, 1956).
Studies of the chemical regulation of growth in tobacco pith callus
(Skoog and Tsui, 1951; Miller and Skoog, 1953), have drawn attention
to the effect of adenine and 6-furfurylaminopurine which bring about
bud induction. It is suggested (Thimann, 1956) that this effect may
be due to counteracting bud inhibition caused by auxin. Recently,
Wickson and Thimann (1958) have concluded that apical dominance
depends on the interaction between a kinetin-like substance and
auxin. These authors have demonstrated the kinetin-induced reversibility of lateral bud inhibition of pea seedlings by IAA, NAA and
2,4-D. While the effects of auxins and of kinetin are commonly
interpreted at the physiological level, the work of Patau and Das (1959)
(see also Partanen, 1959) invokes these effects at a cytochemical level.
Measuring the DNA content of cells spectrophotometrically, Patau and
Das showed that resting cells of the storage organ have an average
content of DNA at the unduplicated or 2C level. When brought to
active growth and division, by the action of kinetin and auxin, the effect
was to increase the frequency of 'DNA doubling' in cells at the 4C level;
nuclei in such cells either undergo mitosis to form two 2C nuclei or
synthesize more DNA, and on division produce 4C nuclei. In the view
of Patau and Das, a balance of auxin and kinetin is necessary for DNA
synthesis and mitosis. However, generalizations should not be made
until other workers have found that this balance of kinetin and auxin
determines DNA synthesis.
When lateral buds are suppressed by apical dominance and released
by removal of the apex, similar problems arise. Naylor (1958) has
demonstrated that in Tradescantia paludosa the distal cells of the lateral
buds (which include the 'Meristeme d'attente' of the French school, see
Sect. IV D) have the 2C DNA content in their nuclei, so long as the main
DETERMINING
FACTORS IN
CELL GROWTH
215
growth-promoting hormone system.' In fact, the whole category of cell
division factors could act in lieu of what Thimann refers to as the
growth-promoting hormone system. The widespread presence of genetic
dwarfs and the recent use of gibberellins to induce their elongation is
open to the interpretation that elongation in the dwarf is suppressed by
an antiauxin-like substance, the effect of which is released by the
treatment with gibberellins. In the best known cases where gibberellin
acts like low temperature treatment (Lang, 1956) to release the growth
of a dormant dwarf vegetative shoot and to promote the growth of the
taller flowering shoot, it may counteract an existing inhibitor of growth
by elongation.
Substances which structurally resemble auxin may act antagonistically by competing for the site of action; 3,5-D (3,5-dichlorophenoxy
acetic acid) and indoleisobutyric acid are such root growth promoters
which inhibit shoot growth (Thimann, 1956).
Studies of the chemical regulation of growth in tobacco pith callus
(Skoog and Tsui, 1951; Miller and Skoog, 1953), have drawn attention
to the effect of adenine and 6-furfurylaminopurine which bring about
bud induction. It is suggested (Thimann, 1956) that this effect may
be due to counteracting bud inhibition caused by auxin. Recently,
Wickson and Thimann (1958) have concluded that apical dominance
depends on the interaction between a kinetin-like substance and
auxin. These authors have demonstrated the kinetin-induced reversibility of lateral bud inhibition of pea seedlings by IAA, NAA and
2,4-D. While the effects of auxins and of kinetin are commonly
interpreted at the physiological level, the work of Patau and Das (1959)
(see also Partanen, 1959) invokes these effects at a cytochemical level.
Measuring the DNA content of cells spectrophotometrically, Patau and
Das showed that resting cells of the storage organ have an average
content of DNA at the unduplicated or 2C level. When brought to
active growth and division, by the action of kinetin and auxin, the effect
was to increase the frequency of 'DNA doubling' in cells at the 4C level;
nuclei in such cells either undergo mitosis to form two 2C nuclei or
synthesize more DNA, and on division produce 4C nuclei. In the view
of Patau and Das, a balance of auxin and kinetin is necessary for DNA
synthesis and mitosis. However, generalizations should not be made
until other workers have found that this balance of kinetin and auxin
determines DNA synthesis.
When lateral buds are suppressed by apical dominance and released
by removal of the apex, similar problems arise. Naylor (1958) has
demonstrated that in Tradescantia paludosa the distal cells of the lateral
buds (which include the 'Meristeme d'attente' of the French school, see
Sect. IV D) have the 2C DNA content in their nuclei, so long as the main