166
A. ALLSOPP
lated. This regulation is based on paramutation which is directed, and
potentially reversible, by way of feedback from the cytoplasm. Paramutation determines the paragenetic state of the chromosomes and with
this the "individuality" of the somatic cell.
As a sequel to the formulation of the "operon" and "repressor" hypothesis by Jacob and Monod (1961), much attention has been paid to
the role of the intracellular environment in the regulation of gene
activity. It may well be that environmentally controlled gene action is
responsible for some types of heteroblastic development, but a more
direct effect of metabolic changes cannot be ruled out. Further genetic
and biochemical studies are clearly of the utmost importance to a deeper
understanding of plant ontogeny in general and heteroblastic development in particular.
IV. Conclusions
Although heteroblastic development still poses many problems, certain
general conclusions may be drawn from the evidence considered above.
In the first place, heteroblastic development can be modified by environmental changes, but is an inherent characteristic of the plant and not
dependent on such changes. It is also certain that more than one type
of ontogenetic change has been included within the general concept of
heteroblastic development.
The most widespread type of heteroblastic development affects the
seedlings and sporelings of almost all plants. It is now virtually established that the increase in size and frequently in complexity of successive
primary leaves is correlated with the primary enlargement, "strengthening," of the axis, particularly of the apical meristem. This strengthening
is in turn dependent on the nutritional status of the plant and undergoes
a reversion under unfavorable conditions.
Another kind of heteroblastic development is particularly prominent
after the early stages of primary strengthening have been completed,
although the two processes usually overlap. This second type appears to
be determined by changes in the physiological condition of the plant,
which for convenience may be termed "physiological aging." In some
plants, the physiological aging is gradual, while in others there is a more
or less abrupt change from a juvenile to an adult stage.
Physiological aging is perhaps the most controversial aspect of heteroblastic development. Many authors consider that the morphological
changes associated with physiological aging are the result of an inherent
aging of the apical meristem. This author considers that there is no
convincing evidence of aging of the meristems in heteroblastic development. The results are more simply explained by a response of the apex
to the physiological condition of the plant. In work with Marsilea, it
A. ALLSOPP
lated. This regulation is based on paramutation which is directed, and
potentially reversible, by way of feedback from the cytoplasm. Paramutation determines the paragenetic state of the chromosomes and with
this the "individuality" of the somatic cell.
As a sequel to the formulation of the "operon" and "repressor" hypothesis by Jacob and Monod (1961), much attention has been paid to
the role of the intracellular environment in the regulation of gene
activity. It may well be that environmentally controlled gene action is
responsible for some types of heteroblastic development, but a more
direct effect of metabolic changes cannot be ruled out. Further genetic
and biochemical studies are clearly of the utmost importance to a deeper
understanding of plant ontogeny in general and heteroblastic development in particular.
IV. Conclusions
Although heteroblastic development still poses many problems, certain
general conclusions may be drawn from the evidence considered above.
In the first place, heteroblastic development can be modified by environmental changes, but is an inherent characteristic of the plant and not
dependent on such changes. It is also certain that more than one type
of ontogenetic change has been included within the general concept of
heteroblastic development.
The most widespread type of heteroblastic development affects the
seedlings and sporelings of almost all plants. It is now virtually established that the increase in size and frequently in complexity of successive
primary leaves is correlated with the primary enlargement, "strengthening," of the axis, particularly of the apical meristem. This strengthening
is in turn dependent on the nutritional status of the plant and undergoes
a reversion under unfavorable conditions.
Another kind of heteroblastic development is particularly prominent
after the early stages of primary strengthening have been completed,
although the two processes usually overlap. This second type appears to
be determined by changes in the physiological condition of the plant,
which for convenience may be termed "physiological aging." In some
plants, the physiological aging is gradual, while in others there is a more
or less abrupt change from a juvenile to an adult stage.
Physiological aging is perhaps the most controversial aspect of heteroblastic development. Many authors consider that the morphological
changes associated with physiological aging are the result of an inherent
aging of the apical meristem. This author considers that there is no
convincing evidence of aging of the meristems in heteroblastic development. The results are more simply explained by a response of the apex
to the physiological condition of the plant. In work with Marsilea, it