ORGANIZED DEVELOPMENT IN PLANTS
59
sions, the hormones seem to provide the key to the onset of initiation of
organization. The availability of appropriate levels of essential hormones
to the cell at the proper time appears to be determinative in controlled
development.
V. Physiological Isolation as a Basis for the Initiation of
Organized Development
In the work on cell suspensions and especially in the publications of
Steward and his associates, there has been emphasized the importance of
cell isolation in establishing conditions that lead to the expression of the
full potentialities residing within the genome of the cell. Thus Steward
et al. (1964) stated that isolation of the cell from the correlative influences of other cells in a tissue system or organized structure is an
essential feature of the success achieved in demonstrating the wide range
of developmental potential inherent, but otherwise suppressed, in somatic
cells of higher plant tissues. In this case, tissue cells were physically
isolated from each other by spontaneous separation in an elaborate
nutrient culture medium where cell proliferation was taking place rapidly.
The same general argument was made concerning the importance of cell
isolation when the cells went on to form cell colonies of undifferentiated
callus parenchyma which developed more or less directly into typical bipolar embryos.
The suggestion has been made that "physiological isolation" of a cell
within a tissue system may produce an essentially similar effect in releasing the cell from inhibitions imposed upon it by its neighbors, and
thus allow an expression of its full potentialities, including the formation
of a whole new plant. It has been argued that one of the essential and
distinctive features of the zygote as the progenitor of a new organism is
the physical and/or physiological isolation which is achieved in its
formation. Thus, in the flowering plants the zygote is isolated in the
nutrient medium of the liquid endosperm within the embryo sac, and
this very isolation is an essential to establishing its integrity as an independent unit capable of forming a whole new organism. Some question of
the reality of this "isolation" of the zygote has been raised by detailed
structural consideration of embryonic development by Jensen (1964) in
electron-microscopic studies of cotton embryogenesis. The egg cell and
later the fertilized egg within the embryo sac are incompletely surrounded by a wall according to his analysis, and the closeness of association of the fertilized egg and adjacent synergid cells cannot be doubted.
It is interesting to consider other systems where the same sort of
physiological isolation might be construed to be the basis for the initiation of a distinctive course of events leading to specific cell differen-
59
sions, the hormones seem to provide the key to the onset of initiation of
organization. The availability of appropriate levels of essential hormones
to the cell at the proper time appears to be determinative in controlled
development.
V. Physiological Isolation as a Basis for the Initiation of
Organized Development
In the work on cell suspensions and especially in the publications of
Steward and his associates, there has been emphasized the importance of
cell isolation in establishing conditions that lead to the expression of the
full potentialities residing within the genome of the cell. Thus Steward
et al. (1964) stated that isolation of the cell from the correlative influences of other cells in a tissue system or organized structure is an
essential feature of the success achieved in demonstrating the wide range
of developmental potential inherent, but otherwise suppressed, in somatic
cells of higher plant tissues. In this case, tissue cells were physically
isolated from each other by spontaneous separation in an elaborate
nutrient culture medium where cell proliferation was taking place rapidly.
The same general argument was made concerning the importance of cell
isolation when the cells went on to form cell colonies of undifferentiated
callus parenchyma which developed more or less directly into typical bipolar embryos.
The suggestion has been made that "physiological isolation" of a cell
within a tissue system may produce an essentially similar effect in releasing the cell from inhibitions imposed upon it by its neighbors, and
thus allow an expression of its full potentialities, including the formation
of a whole new plant. It has been argued that one of the essential and
distinctive features of the zygote as the progenitor of a new organism is
the physical and/or physiological isolation which is achieved in its
formation. Thus, in the flowering plants the zygote is isolated in the
nutrient medium of the liquid endosperm within the embryo sac, and
this very isolation is an essential to establishing its integrity as an independent unit capable of forming a whole new organism. Some question of
the reality of this "isolation" of the zygote has been raised by detailed
structural consideration of embryonic development by Jensen (1964) in
electron-microscopic studies of cotton embryogenesis. The egg cell and
later the fertilized egg within the embryo sac are incompletely surrounded by a wall according to his analysis, and the closeness of association of the fertilized egg and adjacent synergid cells cannot be doubted.
It is interesting to consider other systems where the same sort of
physiological isolation might be construed to be the basis for the initiation of a distinctive course of events leading to specific cell differen-