200
F.
C. STEWARD AND
Η. Y. MOHAN RAM
the tumour and the tumorous plant will cause the cells of carrot to
divide, whereas the tissue of the more stable parents which do not
normally form tumours will not produce this effect. Similarly, the cells
of the crown gall, which continue to divide and form a tumorous overgrowth (Fig. 2(h)), have become autotrophic for some growth-promoting
substances; these can be extracted and, when applied to carrot explants,
will cause them to grow rapidly by cell division (Steward et al., 1955;
Steward and Shantz, 1959b). All this means, therefore, that over and
above the genetic mechanism that controls how cells divide, over and
above the nutrients and vitamins which nourish them, there are certain
chemical regulatory principles which, when present at the optimal level,
promote the maximum rate of division. Obviously, these innate potentialities for cell division must be in some way controlled in the intact
plant body when growth by cell division subsides.
Thus one now recognizes that the stimulus to growth in fleshy pome
fruits, that is normally associated with the presence of a viable embryo,
must have a chemical basis not unlike that which is transmitted to
the carrot cells by the coconut milk. In some other fruits, such as the
edible banana or certain cucurbits, in which fleshy tissues are formed
parthenocarpically by cell division, even without benefit of the stimuli
that normally emanate from an embryo, there must be, as it were,
'built-in' a similar chemical stimulus. Steward and Simonds (1954) were
able to extract from the wall of the loculus of the developing banana
fruit, at the stage of incipient growth of pulp, substances which would
also stimulate division in carrot cells (Figs. 2(i) and (j)). These varied
observations, which extend to different morphological situations, have
been illustrated in a chart in a recent review by Steward and Shantz
(1959b). The implication is, however, that powerful growth regulatory
substances can intervene in plants to cause cells to grow by rapid
multiplication and proliferation, and that these substances are additional
to the familiar inorganic or organic nutrients, vitamins, or even to the
hormone (auxin or indoleacetic acid) which was already well known at
the start of these investigations.
3. Isolation and Identification of Naturally Occurring Principles
which Stimulate Cell Division in Plants
An assay system was developed (Steward et al., 1952) and used to
follow the course of fractionation of coconut milk of an alcoholic extract
of immature corn grains, and of the liquid from the vesicle of immature
Aesculus fruits. It now remains to summarize the present status of these
investigations.
It should be made clear at the outset that a complete account cannot
yet be rendered of the chemical substances which are responsible for the
F.
C. STEWARD AND
Η. Y. MOHAN RAM
the tumour and the tumorous plant will cause the cells of carrot to
divide, whereas the tissue of the more stable parents which do not
normally form tumours will not produce this effect. Similarly, the cells
of the crown gall, which continue to divide and form a tumorous overgrowth (Fig. 2(h)), have become autotrophic for some growth-promoting
substances; these can be extracted and, when applied to carrot explants,
will cause them to grow rapidly by cell division (Steward et al., 1955;
Steward and Shantz, 1959b). All this means, therefore, that over and
above the genetic mechanism that controls how cells divide, over and
above the nutrients and vitamins which nourish them, there are certain
chemical regulatory principles which, when present at the optimal level,
promote the maximum rate of division. Obviously, these innate potentialities for cell division must be in some way controlled in the intact
plant body when growth by cell division subsides.
Thus one now recognizes that the stimulus to growth in fleshy pome
fruits, that is normally associated with the presence of a viable embryo,
must have a chemical basis not unlike that which is transmitted to
the carrot cells by the coconut milk. In some other fruits, such as the
edible banana or certain cucurbits, in which fleshy tissues are formed
parthenocarpically by cell division, even without benefit of the stimuli
that normally emanate from an embryo, there must be, as it were,
'built-in' a similar chemical stimulus. Steward and Simonds (1954) were
able to extract from the wall of the loculus of the developing banana
fruit, at the stage of incipient growth of pulp, substances which would
also stimulate division in carrot cells (Figs. 2(i) and (j)). These varied
observations, which extend to different morphological situations, have
been illustrated in a chart in a recent review by Steward and Shantz
(1959b). The implication is, however, that powerful growth regulatory
substances can intervene in plants to cause cells to grow by rapid
multiplication and proliferation, and that these substances are additional
to the familiar inorganic or organic nutrients, vitamins, or even to the
hormone (auxin or indoleacetic acid) which was already well known at
the start of these investigations.
3. Isolation and Identification of Naturally Occurring Principles
which Stimulate Cell Division in Plants
An assay system was developed (Steward et al., 1952) and used to
follow the course of fractionation of coconut milk of an alcoholic extract
of immature corn grains, and of the liquid from the vesicle of immature
Aesculus fruits. It now remains to summarize the present status of these
investigations.
It should be made clear at the outset that a complete account cannot
yet be rendered of the chemical substances which are responsible for the