254
F. C. STEWARD
AND Η. Y. MOHAN RAM
tissue cultures are those of Skoog and his collaborators. Starting with
the pith of Nicotiana, which readily forms a callus composed of large
cells, Miller et al. (1955a, b) discovered the role played by 6-substituted
amino purines (particularly 6-furfurylamino purine or kinetin) upon
cell divisions within these giant cells (Plate 2, (1)). This led to further
studies of the formative effect that could be induced in this system by
balanced control of such compounds as kinetin (which acts primarily
upon cell division) and various auxins (which act primarily upon cell
enlargement), and these also with general (caseinhydrolysate) or particular (tyrosine, tryptophan) sources of organic nitrogen (Plate 2, (4)).
The important point is that the tobacco pith cells which have the same
'built-in', or genetically established capacity to grow may express this
in very different ways according to the exogenous stimuli they receive.
These workers have successively stimulated growth which ranged from
rapidly growing callus to degrees of organ formation (mostly modified
shoots) by critical control of the balance between the growth factors
that are employed. Thus, the regions which seem to exert formative
effects upon growth and differentiation (as e.g. the quiescent region in
the root) may well turn out to be centres for the production and secretion, in balanced amount, of such growth-regulating substances that act
through the control of cell division and cell enlargement.
Braun has also reached a somewhat similar position in the interpretation of tumours. Receptive cells, prepared to different degrees by a
wound healing type of response, can now be caused to grow in culture
by a balanced application of growth substances in a manner strangely
reminiscent of the autonomous tumour cells. The degree of transformation is conditioned by the combination of growth substances (Braun,
1957). But it seems that the chemical causation of the autonomy of
the true tumour still eludes description, notwithstanding the claims of
Klein (1954), Thomas and Klein* (1959), de Ropp (1947) and Camus
et al. (1951). But even in the tumour cells the totipotency which is
suppressed in the tumour may be recovered. Once incited by the crown
gall organism, the tobacco callus tissue will grow indefinitely in an
undifferentiated way, with no ability to organize tissues and organs, on
a simple basal nutrient medium. One may, therefore, assume that the
crown gall tumour is a permanently altered tissue in which the new cells
continue to be of the same kind.
There occurs, however, a complex and morphologically distinct type
of overgrowth or teratoma (Plate 2, (3b)), which has a capacity to
produce abnormal buds and leaves when on the host plant (Smith,
1920) as well as in continuous growth in culture (Braun, 1959). A tumour
* These claims have now been retracted by Klein and Braun (1960).
F. C. STEWARD
AND Η. Y. MOHAN RAM
tissue cultures are those of Skoog and his collaborators. Starting with
the pith of Nicotiana, which readily forms a callus composed of large
cells, Miller et al. (1955a, b) discovered the role played by 6-substituted
amino purines (particularly 6-furfurylamino purine or kinetin) upon
cell divisions within these giant cells (Plate 2, (1)). This led to further
studies of the formative effect that could be induced in this system by
balanced control of such compounds as kinetin (which acts primarily
upon cell division) and various auxins (which act primarily upon cell
enlargement), and these also with general (caseinhydrolysate) or particular (tyrosine, tryptophan) sources of organic nitrogen (Plate 2, (4)).
The important point is that the tobacco pith cells which have the same
'built-in', or genetically established capacity to grow may express this
in very different ways according to the exogenous stimuli they receive.
These workers have successively stimulated growth which ranged from
rapidly growing callus to degrees of organ formation (mostly modified
shoots) by critical control of the balance between the growth factors
that are employed. Thus, the regions which seem to exert formative
effects upon growth and differentiation (as e.g. the quiescent region in
the root) may well turn out to be centres for the production and secretion, in balanced amount, of such growth-regulating substances that act
through the control of cell division and cell enlargement.
Braun has also reached a somewhat similar position in the interpretation of tumours. Receptive cells, prepared to different degrees by a
wound healing type of response, can now be caused to grow in culture
by a balanced application of growth substances in a manner strangely
reminiscent of the autonomous tumour cells. The degree of transformation is conditioned by the combination of growth substances (Braun,
1957). But it seems that the chemical causation of the autonomy of
the true tumour still eludes description, notwithstanding the claims of
Klein (1954), Thomas and Klein* (1959), de Ropp (1947) and Camus
et al. (1951). But even in the tumour cells the totipotency which is
suppressed in the tumour may be recovered. Once incited by the crown
gall organism, the tobacco callus tissue will grow indefinitely in an
undifferentiated way, with no ability to organize tissues and organs, on
a simple basal nutrient medium. One may, therefore, assume that the
crown gall tumour is a permanently altered tissue in which the new cells
continue to be of the same kind.
There occurs, however, a complex and morphologically distinct type
of overgrowth or teratoma (Plate 2, (3b)), which has a capacity to
produce abnormal buds and leaves when on the host plant (Smith,
1920) as well as in continuous growth in culture (Braun, 1959). A tumour
* These claims have now been retracted by Klein and Braun (1960).