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JOHN G. TORREY
VIII. The Role of Hormones in the Initiation of Organization
In summarizing what has been discussed above, two alternative conclusions suggest themselves: (1) auxins, kinins, and gibberellins are essential in the in vitro systems as rather generalized (and perhaps artificial) stimulants to cell division and enlargement and make it possible
for the cell population to proceed with its intrinsic developmental program; (2) these hormones and their specific release to the cells in their
normal in vivo site are an integral and essential pail of the genetically
controlled program of development for organizing cell populations. Such
hormones can also be supplied in vitro by appropriately balanced concentrations and thereby affect development. If the first alternative is the
correct one, then one must look to as yet undiscovered intrinsic control
mechanisms for development in which pattern determination is controlled
essentially independent of all externally supplied materials including
growth substances (although it may be limited by them). If the second
alternative is true, then in the manipulation of hormone levels in vitro
one may be exercising directly part of the machinery under genetic control used by plant cells in determining their own patterns of development.
It could be said that the evidence from embryo development, from
plant tissue culture, and from cell suspension studies points in the direction of the first conclusion, i.e., that the nutrient medium, including the
appropriate hormone levels, simply establishes the essential conditions
which permit the full expression of the intrinsic control mechanisms
leading to development of the organized plant structure. Thus a single
somatic cell in the appropriate milieu proceeds to form a multicellular
mass—perhaps in a rather precise pattern of cell lineages (e.g., a linear
filamentous embryo as in wild carrot or a random pattern to a globular
mass as in cotton). Only when a multicellular mass is produced does
organized development occur, i.e., the formation of cotyledons and the
initiation of meristems. It may well be, and the evidence favors the view
but is exceedingly sparse, that as a multicellular system forms, specific
sites of synthesis, especially of hormones such as auxin and kinin, arise
where they did not exist before, gradients within tissues occur, and,
therefore, patterns of cell division and enlargement under hormonal
control determine specific form and structure. The environment can be
involved as well, acting on the plant by influencing the rates of synthesis
or destruction of the hormones at specific sites and influencing in a
general way the homeostatic balance of hormones within the plant.
The author believes it is true that many research workers have tended
to take the latter answer to be the true one, i.e., that in manipulating
the balance of externally provided hormone levels one is, in fact, operat-
JOHN G. TORREY
VIII. The Role of Hormones in the Initiation of Organization
In summarizing what has been discussed above, two alternative conclusions suggest themselves: (1) auxins, kinins, and gibberellins are essential in the in vitro systems as rather generalized (and perhaps artificial) stimulants to cell division and enlargement and make it possible
for the cell population to proceed with its intrinsic developmental program; (2) these hormones and their specific release to the cells in their
normal in vivo site are an integral and essential pail of the genetically
controlled program of development for organizing cell populations. Such
hormones can also be supplied in vitro by appropriately balanced concentrations and thereby affect development. If the first alternative is the
correct one, then one must look to as yet undiscovered intrinsic control
mechanisms for development in which pattern determination is controlled
essentially independent of all externally supplied materials including
growth substances (although it may be limited by them). If the second
alternative is true, then in the manipulation of hormone levels in vitro
one may be exercising directly part of the machinery under genetic control used by plant cells in determining their own patterns of development.
It could be said that the evidence from embryo development, from
plant tissue culture, and from cell suspension studies points in the direction of the first conclusion, i.e., that the nutrient medium, including the
appropriate hormone levels, simply establishes the essential conditions
which permit the full expression of the intrinsic control mechanisms
leading to development of the organized plant structure. Thus a single
somatic cell in the appropriate milieu proceeds to form a multicellular
mass—perhaps in a rather precise pattern of cell lineages (e.g., a linear
filamentous embryo as in wild carrot or a random pattern to a globular
mass as in cotton). Only when a multicellular mass is produced does
organized development occur, i.e., the formation of cotyledons and the
initiation of meristems. It may well be, and the evidence favors the view
but is exceedingly sparse, that as a multicellular system forms, specific
sites of synthesis, especially of hormones such as auxin and kinin, arise
where they did not exist before, gradients within tissues occur, and,
therefore, patterns of cell division and enlargement under hormonal
control determine specific form and structure. The environment can be
involved as well, acting on the plant by influencing the rates of synthesis
or destruction of the hormones at specific sites and influencing in a
general way the homeostatic balance of hormones within the plant.
The author believes it is true that many research workers have tended
to take the latter answer to be the true one, i.e., that in manipulating
the balance of externally provided hormone levels one is, in fact, operat-