V.
DETERMINING FACTORS IN CELL GROWTH
229
ability to grow and divide over 100-150 years. The pith cells of Ferrocactus wislizenii may live and increase their size for over a century, but
they may lose their ability to divide. Other instances can be seen in the
parenchyma cells of the xylem rays, deep in the annual rings of
old trees (e.g. Sequoia sempervirens), which may stay alive for nearly
100 years (MacDougal and Smith, 1927). Although MacDougal, his coworkers, Fritzche (1910), and others inferred the longevity of these
parenchyma cells by microscopic observation, they did not show experimentally the ability of such cells to revert to proliferative growth.
However, Barker (1953) demonstrated that cells of the perimedullary
sheath and of the pith region of the bass wood (Tilia americana) trees,
approximately 50 years old, can be restored to active growth by tissue
culture methods. Barker believes that this latent ability to proliferate,
in such old cells, is inhibited while the tissue is in contact with the plant,
but the inhibition is released when they are severed from the rest of the
plant and are cultured.
When cells revert from organized to unorganized growth in the
formation of plant tumours, they also acquire a greater degree of
autonomous and potentially unlimited growth. Conversely, when such
conspicuously unlimited growth is produced under tissue culture conditions (viz. the carrot callus of Gautheret, originally started no doubt
from the cambium but transformed during its continuous subculture
over a quarter of a century to the familiar 'habituated' culture) the
growth which occurs is usually unorganized.
Some organs of perennation offer exceptions or intermediate conditions. The cells of the potato tuber, stored at 8-10° C, will withstand
long storage and remain capable of a recrudescence of growth, as in
wound healing and as in their response to growth factors (Steward
et al. 9 1941). However, if the tubers are stored at + I
o C, their chemical
composition changes, as shown by the familiar conversion of starch to
sugar, and then physiological activity as shown by respiration may
increase markedly. But it is of greater consequence that these cells
lose, by this treatment, their ability to resynthesize protein from their
soluble nitrogen compounds and, accompanying this change, is a
complete loss of the ability to divide, as shown by wound periderm
formation (Steward et al., 1943). Protein synthesis is here the tangible
sign that the cells can renew or replace their own protoplasm.
The climacteric in a fruit, as e.g. in banana, is another crisis in cellular development. The edible variety of banana fruit normally grows
parthenocarpically in response to certain cell division stimuli that have
already been referred to. But parts of even the mature, but pre-climacteric fruit, may be induced to show a marked recrudescence of growth
under treatments with chemical growth regulators (e.g. 2,4-D i coconut
DETERMINING FACTORS IN CELL GROWTH
229
ability to grow and divide over 100-150 years. The pith cells of Ferrocactus wislizenii may live and increase their size for over a century, but
they may lose their ability to divide. Other instances can be seen in the
parenchyma cells of the xylem rays, deep in the annual rings of
old trees (e.g. Sequoia sempervirens), which may stay alive for nearly
100 years (MacDougal and Smith, 1927). Although MacDougal, his coworkers, Fritzche (1910), and others inferred the longevity of these
parenchyma cells by microscopic observation, they did not show experimentally the ability of such cells to revert to proliferative growth.
However, Barker (1953) demonstrated that cells of the perimedullary
sheath and of the pith region of the bass wood (Tilia americana) trees,
approximately 50 years old, can be restored to active growth by tissue
culture methods. Barker believes that this latent ability to proliferate,
in such old cells, is inhibited while the tissue is in contact with the plant,
but the inhibition is released when they are severed from the rest of the
plant and are cultured.
When cells revert from organized to unorganized growth in the
formation of plant tumours, they also acquire a greater degree of
autonomous and potentially unlimited growth. Conversely, when such
conspicuously unlimited growth is produced under tissue culture conditions (viz. the carrot callus of Gautheret, originally started no doubt
from the cambium but transformed during its continuous subculture
over a quarter of a century to the familiar 'habituated' culture) the
growth which occurs is usually unorganized.
Some organs of perennation offer exceptions or intermediate conditions. The cells of the potato tuber, stored at 8-10° C, will withstand
long storage and remain capable of a recrudescence of growth, as in
wound healing and as in their response to growth factors (Steward
et al. 9 1941). However, if the tubers are stored at + I
o C, their chemical
composition changes, as shown by the familiar conversion of starch to
sugar, and then physiological activity as shown by respiration may
increase markedly. But it is of greater consequence that these cells
lose, by this treatment, their ability to resynthesize protein from their
soluble nitrogen compounds and, accompanying this change, is a
complete loss of the ability to divide, as shown by wound periderm
formation (Steward et al., 1943). Protein synthesis is here the tangible
sign that the cells can renew or replace their own protoplasm.
The climacteric in a fruit, as e.g. in banana, is another crisis in cellular development. The edible variety of banana fruit normally grows
parthenocarpically in response to certain cell division stimuli that have
already been referred to. But parts of even the mature, but pre-climacteric fruit, may be induced to show a marked recrudescence of growth
under treatments with chemical growth regulators (e.g. 2,4-D i coconut