H E T E R O B L A S T I C D E V E L O P M E N T IN P L A N T S
165
erated so that the stages characteristic of the two previous types are
completed in the seedling and later leaves become increasingly dissected.
As a further example of the effect of the genotype on the rate of
heteroblastic development, mention may be made of the work of Paxman
(1956) who studied changes in the shape of successive leaves in a series
of diallele crosses between five varieties of Nicotiana rustica. The rate
of change in leaf shape, as in cotton, was shown to vary according to
the genotype.
There have recently been several detailed studies of a leaf-shape
mutant in the tomato (Lycopersicon escidentum). In the single-gene
mutant, lanceolate (La/La+), the leaves never attain the fully pinnate
condition of the normal (La
+ /La
+
) plant. Homozygous mutants (La/La)
are even simpler. In the present context it is of interest that the apical
meristem of the lanceolate phenotype is smaller than that of the normal,
with fewer and larger cells (Mathan and Jenkins, 1962). In later work
(Stettler, 1964), diploid, triploid, and tetraploid plants with various
proportions of the La allele were compared. Each increase in the proportion of the La allele resulted in a corresponding reduction in leaf complexity. In reverse order, the changes in leaf shape were similar to the
sequence of leaf shapes in the first 6 or 8 leaves of normal plants. It was
also found (Mathan and Cole, 1964) that the La allele leads to increases
in the activity of the oxidative enzymes tyrosinase, laccase, peroxidase,
and catalase. Since treatment with phenylboric acid simulated the effect
of the lanceolate gene, with regard to both the induction of a lanceolate
leaf and the increase in activity of the four oxidative enzymes, it was
suggested that the changes in form result from the increased enzyme
activity (Mathan, 1965).
2. Ontogenetic Changes
In many examples of heteroblastic development the changes are gradual and progressive, but in a considerable number of species there is a
relatively abrupt switch from a juvenile to an adult type of growth.
Furthermore, such changes are often very stable and can only be reversed
with some difficulty. These considerations have led several authors,
notably Brink (1962), to interpret heteroblastic development as an
expression of differential gene activity.
It is clear that gene mutations cannot be responsible for heteroblastic
development. But cytoplasmic changes are not the only alternative.
Brink considers that the chromosome may serve a paragenetic as well as
a genetic function, the former being primarily concerned with the development of the individual and the latter with heredity. It is suggested
that the chromosome, in addition to its unchanging gene complement,
possesses other more labile components by which gene action is regu-
165
erated so that the stages characteristic of the two previous types are
completed in the seedling and later leaves become increasingly dissected.
As a further example of the effect of the genotype on the rate of
heteroblastic development, mention may be made of the work of Paxman
(1956) who studied changes in the shape of successive leaves in a series
of diallele crosses between five varieties of Nicotiana rustica. The rate
of change in leaf shape, as in cotton, was shown to vary according to
the genotype.
There have recently been several detailed studies of a leaf-shape
mutant in the tomato (Lycopersicon escidentum). In the single-gene
mutant, lanceolate (La/La+), the leaves never attain the fully pinnate
condition of the normal (La
+ /La
+
) plant. Homozygous mutants (La/La)
are even simpler. In the present context it is of interest that the apical
meristem of the lanceolate phenotype is smaller than that of the normal,
with fewer and larger cells (Mathan and Jenkins, 1962). In later work
(Stettler, 1964), diploid, triploid, and tetraploid plants with various
proportions of the La allele were compared. Each increase in the proportion of the La allele resulted in a corresponding reduction in leaf complexity. In reverse order, the changes in leaf shape were similar to the
sequence of leaf shapes in the first 6 or 8 leaves of normal plants. It was
also found (Mathan and Cole, 1964) that the La allele leads to increases
in the activity of the oxidative enzymes tyrosinase, laccase, peroxidase,
and catalase. Since treatment with phenylboric acid simulated the effect
of the lanceolate gene, with regard to both the induction of a lanceolate
leaf and the increase in activity of the four oxidative enzymes, it was
suggested that the changes in form result from the increased enzyme
activity (Mathan, 1965).
2. Ontogenetic Changes
In many examples of heteroblastic development the changes are gradual and progressive, but in a considerable number of species there is a
relatively abrupt switch from a juvenile to an adult type of growth.
Furthermore, such changes are often very stable and can only be reversed
with some difficulty. These considerations have led several authors,
notably Brink (1962), to interpret heteroblastic development as an
expression of differential gene activity.
It is clear that gene mutations cannot be responsible for heteroblastic
development. But cytoplasmic changes are not the only alternative.
Brink considers that the chromosome may serve a paragenetic as well as
a genetic function, the former being primarily concerned with the development of the individual and the latter with heredity. It is suggested
that the chromosome, in addition to its unchanging gene complement,
possesses other more labile components by which gene action is regu-