V.
DETERMINING
FACTORS IN
CELL
GROWTH
201
growth induction caused by any one of these materials. Much, however,
is known about the nature of this problem.
The first point to be stressed is that 'no single substance unlocks the
door of cell division' (Steward and Shantz, 1956, p. 179). An array of
substances is involved in each of the sources that has been investigated.
Indeed one of the problems is that similar effects may be evoked by
molecules which seem to have but little in common, except the effect in
question. Furthermore, the growth response is not even a single effect,
for quite different components of the extracts combine synergistically
to produce the over-all result.
Although whole coconut milk, as a supplement to White's basal
medium, will evoke a rapid rate of growth in carrot phloem explants
for which the main nitrogen source is nitrate, a still greater response is
also achieved in the presence of casein hydrolysate as a general nitrogenous source, or by the supply of various amino acids, or even urea.
Thus the growing cells use reduced nitrogen in preference to nitrate
(Shantz and Steward, 1959). Part of the effect of the coconut milk is
therefore due to this non-specific role of its nitrogen compounds, which
can be supplemented by the use of casein hydrolysate (Steward and
Shantz, 1954, 1956, 1959).
The specific growth induction stimulus, however, is now divisible into
two main parts. One of these is called the 'neutral' fraction (Steward
and Shantz, 1959a, p. 229, cf. 1959b, pp. 392-3), because it does not
adhere to either basic resins or acidic resins, and, together with the
principal sugars, it passes through such ion-exchange columns. From
investigations not fully described here, it now appears that this material
('neutral fraction') does not alone stimulate cell division, but it creates
the conditions in which even more active cell division-promoting substances can act. The neutral fraction may, however, permit preformed
cells to enlarge.
The neutral fraction of the coconut milk contains three hexitols; one
is the sugar alcohol sorbitol, which comprises a surprisingly large part of
the dry weight of coconut milk (order of 20%); and two inositols, myoinositol and sci/Zfo-inositol, in very much smaller amounts (Pollard,
Shantz and Steward, 1959).
The so-called 'active' fraction in coconut milk, however, can be
further fractionated to yield extracts which will cause the growth
induction in the presence of the 'neutral' fraction. There is thus a
synergistic interaction of these two fractions. The components of the
active fraction work at dilutions as low as a few parts per million (more
or less) in the medium. The concept is, therefore, that the extremely
potent substances present in the so-called 'active' fraction represent the
direct stimuli to cell division, but that these require for their full effect
DETERMINING
FACTORS IN
CELL
GROWTH
201
growth induction caused by any one of these materials. Much, however,
is known about the nature of this problem.
The first point to be stressed is that 'no single substance unlocks the
door of cell division' (Steward and Shantz, 1956, p. 179). An array of
substances is involved in each of the sources that has been investigated.
Indeed one of the problems is that similar effects may be evoked by
molecules which seem to have but little in common, except the effect in
question. Furthermore, the growth response is not even a single effect,
for quite different components of the extracts combine synergistically
to produce the over-all result.
Although whole coconut milk, as a supplement to White's basal
medium, will evoke a rapid rate of growth in carrot phloem explants
for which the main nitrogen source is nitrate, a still greater response is
also achieved in the presence of casein hydrolysate as a general nitrogenous source, or by the supply of various amino acids, or even urea.
Thus the growing cells use reduced nitrogen in preference to nitrate
(Shantz and Steward, 1959). Part of the effect of the coconut milk is
therefore due to this non-specific role of its nitrogen compounds, which
can be supplemented by the use of casein hydrolysate (Steward and
Shantz, 1954, 1956, 1959).
The specific growth induction stimulus, however, is now divisible into
two main parts. One of these is called the 'neutral' fraction (Steward
and Shantz, 1959a, p. 229, cf. 1959b, pp. 392-3), because it does not
adhere to either basic resins or acidic resins, and, together with the
principal sugars, it passes through such ion-exchange columns. From
investigations not fully described here, it now appears that this material
('neutral fraction') does not alone stimulate cell division, but it creates
the conditions in which even more active cell division-promoting substances can act. The neutral fraction may, however, permit preformed
cells to enlarge.
The neutral fraction of the coconut milk contains three hexitols; one
is the sugar alcohol sorbitol, which comprises a surprisingly large part of
the dry weight of coconut milk (order of 20%); and two inositols, myoinositol and sci/Zfo-inositol, in very much smaller amounts (Pollard,
Shantz and Steward, 1959).
The so-called 'active' fraction in coconut milk, however, can be
further fractionated to yield extracts which will cause the growth
induction in the presence of the 'neutral' fraction. There is thus a
synergistic interaction of these two fractions. The components of the
active fraction work at dilutions as low as a few parts per million (more
or less) in the medium. The concept is, therefore, that the extremely
potent substances present in the so-called 'active' fraction represent the
direct stimuli to cell division, but that these require for their full effect