V. DETERMINING
FACTORS IN
CELL GROWTH
205
(Steward, Pollard, Witkop and Patchett, 1958) can neutralize the
response of carrot cells to coconut milk. The interpretation of this
strange effect will be given when the effect of the coconut milk stimulus
on protein synthesis is considered. Moreover, the hydroxy-d-proline and
allo-hydroxyprolines and their compounds are less active as inhibitors
of the coconut milk growth stimulus (loc. cit.).
As indicated therefore, growth by cell division is not merely a function
of the nuclear apparatus with its DNA and its self-duplication properties
nor is it fostered only by nutrients, inorganic and organic, or vitamins of
the known kind. For, in addition, this phenomenon in plants may be
brought under the regulatory control of a wide range of substances.
Some of these substances act as primary stimulants of cells, others act
synergistically with them, and some act as stimulators or inhibitors so
that they in turn may hold the key to the pace or cessation of cell
division. Therefore, for the interpretation of morphogenesis, one can
now see great potentialities, for the intrinsic ability of cells to grow
and divide may be controlled by extrinsic factors. Thus growth stimuli
which are traceable to environmental effects may be mediated by
demonstrable chemical machinery that acts through the incidence or
suppression of cell division.
5. Kinetin and the Kinins
The essential contributions in this field have been made in Professor
Skoog's laboratory. Using tobacco pith for their test system, Skoog and
his collaborators have discovered chemical substances that will cause
these mature cells to divide. These substances have been called 'kinins',
and the particular, or parent, substance of this class has been named
'kinetin'. As yet, Skoog and his collaborators have not isolated pure
chemical compounds which produce these effects from natural sources,
or at least from sources that would indicate that they occur as such in
intact plants or organs, though the natural occurrence of compounds
which produce equivalent effects follows from the general trend of
recent work in this field of growth substances. However, a substance
has been isolated, purified, and synthesized and shown to produce these
effects in the tobacco assay system; this substance came from the
treatment of nucleic acid preparations. First from aged samples of
nucleic acid and later from autoclaved samples of nucleic acid, Skoog
and his collaborators have isolated the substance which was later
identified as 6-furfurylaminopurine, and this is now given the common
name of'kinetin' (Miller etal, 1955a, 1955b; Skoog and Miller, 1957).
It is characteristic of kinetin, however, that its activity is shown only
in the presence of indoleacetic acid. The effects attributed above to
whole coconut milk, corn extract, and Aesculus extract do not, however,
FACTORS IN
CELL GROWTH
205
(Steward, Pollard, Witkop and Patchett, 1958) can neutralize the
response of carrot cells to coconut milk. The interpretation of this
strange effect will be given when the effect of the coconut milk stimulus
on protein synthesis is considered. Moreover, the hydroxy-d-proline and
allo-hydroxyprolines and their compounds are less active as inhibitors
of the coconut milk growth stimulus (loc. cit.).
As indicated therefore, growth by cell division is not merely a function
of the nuclear apparatus with its DNA and its self-duplication properties
nor is it fostered only by nutrients, inorganic and organic, or vitamins of
the known kind. For, in addition, this phenomenon in plants may be
brought under the regulatory control of a wide range of substances.
Some of these substances act as primary stimulants of cells, others act
synergistically with them, and some act as stimulators or inhibitors so
that they in turn may hold the key to the pace or cessation of cell
division. Therefore, for the interpretation of morphogenesis, one can
now see great potentialities, for the intrinsic ability of cells to grow
and divide may be controlled by extrinsic factors. Thus growth stimuli
which are traceable to environmental effects may be mediated by
demonstrable chemical machinery that acts through the incidence or
suppression of cell division.
5. Kinetin and the Kinins
The essential contributions in this field have been made in Professor
Skoog's laboratory. Using tobacco pith for their test system, Skoog and
his collaborators have discovered chemical substances that will cause
these mature cells to divide. These substances have been called 'kinins',
and the particular, or parent, substance of this class has been named
'kinetin'. As yet, Skoog and his collaborators have not isolated pure
chemical compounds which produce these effects from natural sources,
or at least from sources that would indicate that they occur as such in
intact plants or organs, though the natural occurrence of compounds
which produce equivalent effects follows from the general trend of
recent work in this field of growth substances. However, a substance
has been isolated, purified, and synthesized and shown to produce these
effects in the tobacco assay system; this substance came from the
treatment of nucleic acid preparations. First from aged samples of
nucleic acid and later from autoclaved samples of nucleic acid, Skoog
and his collaborators have isolated the substance which was later
identified as 6-furfurylaminopurine, and this is now given the common
name of'kinetin' (Miller etal, 1955a, 1955b; Skoog and Miller, 1957).
It is characteristic of kinetin, however, that its activity is shown only
in the presence of indoleacetic acid. The effects attributed above to
whole coconut milk, corn extract, and Aesculus extract do not, however,