164
A. ALLSOPP
evidence of an inherent aging of the apical meristems. There is more
justification for the view that the activity of the meristem is modified
by the physiological condition of the rest of the plant. In herbaceous
plants, the meristem is particularly responsive to alterations in the
nutritional or general physiological status of the plant. Even in woody
plants, where the changes in meristematic activity associated with the
adult phase are apparently stable, a reversion to the juvenile phase
can often be induced by appropriate treatments.
There is also no evidence that specific hormones are responsible for
the differences between juvenile and adult phases. The physiological
differences described are possibly no more than the inevitable consequences of the pattern of plant growth with its correlated increase in
photosynthetic capacity, but constantly diminishing ability to maintain
sufficient nitrogen supplies to meristematic and other tissues. Without
further investigations, however, the possibility of the occurrence of other
maturation and aging processes cannot be entirely rejected.
G. Genetical Aspects
Whatever the part played by the environment in the ontogeny of an
individual, the complete range of its developmental potentialities are
ultimately determined by the inherited genotype. Studies on genetically
determined developmental differences between different species or mutants might therefore be expected to make important contributions
towards an understanding of heteroblastic development. It is also possible
that there may be a genetic basis for the differences between successive
phases in individual development.
1. Differences between Species and Individuals
In many groups of plants, morphological differences at the vegetative
level appear to be determined largely by differences in the duration or
rate of heteroblastic development, the juvenile leaves of one species
being comparable to the adult leaves of another. Differences of this
kind are particularly marked in the neotenous and allied genera mentioned in Section ΙΙΙ,Α. Similar, but less extreme, differences are encountered in many relatively unspecialized genera.
Stephens (1944a-d) made a thorough study of the effects of genotype
on heteroblastic leaf development in cotton, Gossypiurn species. It was
concluded leaf development is basically similar in the different species,
but is modified by differences in the alleles of a multiple allele system.
The entire-leaf species are considered to remain in a protracted juvenile
stage. In certain New World amphidiploid types, and in the wild diploids
G. thurberi and G. anomalum, development is arrested at an intermediate stage, while in Asiatic cultivated types development is accel-
A. ALLSOPP
evidence of an inherent aging of the apical meristems. There is more
justification for the view that the activity of the meristem is modified
by the physiological condition of the rest of the plant. In herbaceous
plants, the meristem is particularly responsive to alterations in the
nutritional or general physiological status of the plant. Even in woody
plants, where the changes in meristematic activity associated with the
adult phase are apparently stable, a reversion to the juvenile phase
can often be induced by appropriate treatments.
There is also no evidence that specific hormones are responsible for
the differences between juvenile and adult phases. The physiological
differences described are possibly no more than the inevitable consequences of the pattern of plant growth with its correlated increase in
photosynthetic capacity, but constantly diminishing ability to maintain
sufficient nitrogen supplies to meristematic and other tissues. Without
further investigations, however, the possibility of the occurrence of other
maturation and aging processes cannot be entirely rejected.
G. Genetical Aspects
Whatever the part played by the environment in the ontogeny of an
individual, the complete range of its developmental potentialities are
ultimately determined by the inherited genotype. Studies on genetically
determined developmental differences between different species or mutants might therefore be expected to make important contributions
towards an understanding of heteroblastic development. It is also possible
that there may be a genetic basis for the differences between successive
phases in individual development.
1. Differences between Species and Individuals
In many groups of plants, morphological differences at the vegetative
level appear to be determined largely by differences in the duration or
rate of heteroblastic development, the juvenile leaves of one species
being comparable to the adult leaves of another. Differences of this
kind are particularly marked in the neotenous and allied genera mentioned in Section ΙΙΙ,Α. Similar, but less extreme, differences are encountered in many relatively unspecialized genera.
Stephens (1944a-d) made a thorough study of the effects of genotype
on heteroblastic leaf development in cotton, Gossypiurn species. It was
concluded leaf development is basically similar in the different species,
but is modified by differences in the alleles of a multiple allele system.
The entire-leaf species are considered to remain in a protracted juvenile
stage. In certain New World amphidiploid types, and in the wild diploids
G. thurberi and G. anomalum, development is arrested at an intermediate stage, while in Asiatic cultivated types development is accel-