132
A. ALLSOPP
quently holds, but leaf development is so sensitive to environmental
changes that it is scarcely surprising that exceptions have been reported,
e.g., by Daoud and Brown (1956).
S. Reversion in Adult Plants
Although primary and juvenile leaves are characteristic of the early
stages of ontogeny, leaves with similar features are also found in localized parts of the adult plant. Thus, in many plants, reduced leaves,
similar to or even simpler than the primary leaves, are frequently produced on flowering shoots or on the weak terminal growth at the end of
the growing season. It is quite widely found that the first few leaves
produced on the outgrowth of lateral buds have juvenile characteristics.
Similarly adventitious shoots, whether formed by roots, stems, or leaves,
begin their development by the production of primary leaf forms. Less
frequently certain plants with highly specialized adult leaves, have a
marked tendency towards a reversion to the juvenile form, e.g., Acacia
heterophylla and A. melanoxylon, in which bipinnate juvenile leaves
occur together with typical phyllodes in adult parts of the plant. As discovered independently by Nordhausen (1912) and Schramm (1912), the
well-known differences between sun and shade leaves correspond to the
differences between adult and juvenile leaves, respectively of the same
plant.
The effects of the position of the leaves in the plant on their morphological features are particularly complex in large woody plants. Thus,
Jentys-Szaferowa (1955) concluded that the leaf shape of Betula is affected by at least six factors, including the developmental stage of the
tree, the occurrence of the leaves on short shoots, long shoots or adventitious shoots, their location in the crown of the tree, and the presence or
absence of inflorescences on the short shoots. Critchfield (1960) also
found that in Populus trichocarpa, and probably in many other plants,
the leaves of short shoots have the typical adult form, while the leaves of
long shoots retain some juvenile features.
B. Changes in Stem Characteristics
The changes in leaf shape described in Section ΙΙ,Α are associated
with important changes in the stem. Several workers have investigated
changes in the relative lengths of the internodes during plant ontogeny
(e.g., Troll, 1937; Von Maltzahn, 1957a,b), but apart from the conspicuous change in the semirosette plants, mentioned above, the internode length does not appear to be an important factor in heteroblastic
development. Changes in stem thickness are evidently of greater importance.
A. ALLSOPP
quently holds, but leaf development is so sensitive to environmental
changes that it is scarcely surprising that exceptions have been reported,
e.g., by Daoud and Brown (1956).
S. Reversion in Adult Plants
Although primary and juvenile leaves are characteristic of the early
stages of ontogeny, leaves with similar features are also found in localized parts of the adult plant. Thus, in many plants, reduced leaves,
similar to or even simpler than the primary leaves, are frequently produced on flowering shoots or on the weak terminal growth at the end of
the growing season. It is quite widely found that the first few leaves
produced on the outgrowth of lateral buds have juvenile characteristics.
Similarly adventitious shoots, whether formed by roots, stems, or leaves,
begin their development by the production of primary leaf forms. Less
frequently certain plants with highly specialized adult leaves, have a
marked tendency towards a reversion to the juvenile form, e.g., Acacia
heterophylla and A. melanoxylon, in which bipinnate juvenile leaves
occur together with typical phyllodes in adult parts of the plant. As discovered independently by Nordhausen (1912) and Schramm (1912), the
well-known differences between sun and shade leaves correspond to the
differences between adult and juvenile leaves, respectively of the same
plant.
The effects of the position of the leaves in the plant on their morphological features are particularly complex in large woody plants. Thus,
Jentys-Szaferowa (1955) concluded that the leaf shape of Betula is affected by at least six factors, including the developmental stage of the
tree, the occurrence of the leaves on short shoots, long shoots or adventitious shoots, their location in the crown of the tree, and the presence or
absence of inflorescences on the short shoots. Critchfield (1960) also
found that in Populus trichocarpa, and probably in many other plants,
the leaves of short shoots have the typical adult form, while the leaves of
long shoots retain some juvenile features.
B. Changes in Stem Characteristics
The changes in leaf shape described in Section ΙΙ,Α are associated
with important changes in the stem. Several workers have investigated
changes in the relative lengths of the internodes during plant ontogeny
(e.g., Troll, 1937; Von Maltzahn, 1957a,b), but apart from the conspicuous change in the semirosette plants, mentioned above, the internode length does not appear to be an important factor in heteroblastic
development. Changes in stem thickness are evidently of greater importance.