HETEROBLASTIC DEVELOPMENT IN PLANTS
137
B. The Role of Primary Enlargement
1. Primary Leaves as Arrested Organs
Goebel (1889, 1898, 1908) was the first to propose a causal explanation, based on experimental studies, as opposed to a phylogenetic interpretation of heteroblastic development. He pointed out that, except
in plants with greatly modified adult leaves, there is no essential difference in the developmental pattern of early and later leaves. The greater
morphological simplicity of the primary leaves was explained as merely
the result of an early arrest in their development, an arrest which was
related to the limited nutritional powers of the developing seedling. Successive leaves were considered to continue their development for progressively longer periods until finally the full course of development was
completed and the adult leaf form produced. In support of this view,
Goebel showed that the progression to mature leaves is retarded in plants
grown under unfavorable conditions. Furthermore, under such conditions,
it is often possible to obtain a reversion from the mature leaf type to
primary leaves.
It may be noted that this interpretation of the transformations undergone by successive leaves carries the implication that such leaves when
arranged in sequence should illustrate, although in a somewhat modified
way, stages in the development of a single adult leaf. This rule, recognized by Goebel, has apparently been rediscovered independently by a
number of later workers, e.g., by Brown (1944) in Delphinium, and by
Stephens (1944a) in cotton. It is well illustrated by Tropaeolum, in
which the peltate orbicular adult leaf is preceded by lobed forms, both
as stages in its individual development and as fully expanded primary
leaves (Ashby, 1948a; Takhtajan, 1954). As mentioned in Section
II,A,4, some of the anatomical differences between primary and later
leaves can also be referred to the earlier arrest of development in the
former.
2. Leaf Shape and Axial Changes
Although the interpretation of primary leaves as arrested structures
was based on experimental studies, it might with equal justification have
been based on purely morphological observations. In fact, morphological
studies have led to the recognition of a correlation between leaf shape
and the primary thickening of the axis. Thus, Troll (1938) accepted
GoebeFs view that the primary leaves are arrested in development, but
added that the progressive extension of the development is related to
the obconical development of the seedling axis (see Section ΙΙ,Β,Ι).
He considered that the leaf primordia produced on a larger apex are
137
B. The Role of Primary Enlargement
1. Primary Leaves as Arrested Organs
Goebel (1889, 1898, 1908) was the first to propose a causal explanation, based on experimental studies, as opposed to a phylogenetic interpretation of heteroblastic development. He pointed out that, except
in plants with greatly modified adult leaves, there is no essential difference in the developmental pattern of early and later leaves. The greater
morphological simplicity of the primary leaves was explained as merely
the result of an early arrest in their development, an arrest which was
related to the limited nutritional powers of the developing seedling. Successive leaves were considered to continue their development for progressively longer periods until finally the full course of development was
completed and the adult leaf form produced. In support of this view,
Goebel showed that the progression to mature leaves is retarded in plants
grown under unfavorable conditions. Furthermore, under such conditions,
it is often possible to obtain a reversion from the mature leaf type to
primary leaves.
It may be noted that this interpretation of the transformations undergone by successive leaves carries the implication that such leaves when
arranged in sequence should illustrate, although in a somewhat modified
way, stages in the development of a single adult leaf. This rule, recognized by Goebel, has apparently been rediscovered independently by a
number of later workers, e.g., by Brown (1944) in Delphinium, and by
Stephens (1944a) in cotton. It is well illustrated by Tropaeolum, in
which the peltate orbicular adult leaf is preceded by lobed forms, both
as stages in its individual development and as fully expanded primary
leaves (Ashby, 1948a; Takhtajan, 1954). As mentioned in Section
II,A,4, some of the anatomical differences between primary and later
leaves can also be referred to the earlier arrest of development in the
former.
2. Leaf Shape and Axial Changes
Although the interpretation of primary leaves as arrested structures
was based on experimental studies, it might with equal justification have
been based on purely morphological observations. In fact, morphological
studies have led to the recognition of a correlation between leaf shape
and the primary thickening of the axis. Thus, Troll (1938) accepted
GoebeFs view that the primary leaves are arrested in development, but
added that the progressive extension of the development is related to
the obconical development of the seedling axis (see Section ΙΙ,Β,Ι).
He considered that the leaf primordia produced on a larger apex are