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A. ALLSOPP
In a great many plants, morphological changes are not restricted to
the primary or seedling leaves, but also affect the later leaves. Often
these changes are gradual and may then represent a direct continuation
of the processes initiated in the seedling, e.g., the progressive segmentation of much-divided leaves as in Delphinium. But frequently the
morphological modifications are different from those characteristic of
the primary leaves. Thus, in many divided leaves, an increase in the
depth of lobing, as distinct from the increase in number of segments,
may affect only the later leaves, e.g., Ipornoea, Seriecio. It is also evident
that several kinds of change may proceed simultaneously either in
synchrony or at different rates, e.g., as illustrated by Ashby (1948a) ;
in sugar beet the ratio of petiole length to lamina length rises to a
maximum and then diminishes, while the angle between lamina base and
petiole increases continuously.
2. Juvenile and Adult Leaves
In contrast to the gradual ontogenetic changes of the type outlined
above, which are characteristic of most herbaceous plants and of the
seedlings of woody species, in certain plants, particularly woody forms,
there is a fairly sharp distinction between juvenile and adult stages.
Although the only consistent distinction lies in the ability of the adult
stage to form reproductive organs, there are often more or less pronounced differences in leaf morphology between these two stages. These
differences are probably not essentially unlike those discussed in the
previous section, but either a more abrupt morphological transition, or
a longer duration and greater stability of the two stages, does justify
the recognition of juvenile and adult forms.
In a majority of plants the juvenile and adult leaves are both ordinary
foliage leaves which differ only in shape or other morphological details,
e.g., Hedera and Eucalyptus species. In holly, Ilex aquifolium the juvenile leaves are dentate with many prickles whereas leaves from the adult
zone are almost or quite entire. The most striking differences occur in
species where the juvenile leaves are still foliage leaves, while the adult
leaves have undergone extensive modification to form phyllodes, spines,
tendrils, etc., as in the classical examples of Acacia, Ulex europaeus, and
Lathyrus aphaca, respectively.
3. Other Leaf Modifications
The changes in leaf morphology considered in Section ΙΙ,Α,Ι and 2
are characteristic of the ordinary vegetative ontogeny of the species
cited. Other modifications of the leaves are more closely linked with
special events such as flowering or seasonal periodicity.
A. ALLSOPP
In a great many plants, morphological changes are not restricted to
the primary or seedling leaves, but also affect the later leaves. Often
these changes are gradual and may then represent a direct continuation
of the processes initiated in the seedling, e.g., the progressive segmentation of much-divided leaves as in Delphinium. But frequently the
morphological modifications are different from those characteristic of
the primary leaves. Thus, in many divided leaves, an increase in the
depth of lobing, as distinct from the increase in number of segments,
may affect only the later leaves, e.g., Ipornoea, Seriecio. It is also evident
that several kinds of change may proceed simultaneously either in
synchrony or at different rates, e.g., as illustrated by Ashby (1948a) ;
in sugar beet the ratio of petiole length to lamina length rises to a
maximum and then diminishes, while the angle between lamina base and
petiole increases continuously.
2. Juvenile and Adult Leaves
In contrast to the gradual ontogenetic changes of the type outlined
above, which are characteristic of most herbaceous plants and of the
seedlings of woody species, in certain plants, particularly woody forms,
there is a fairly sharp distinction between juvenile and adult stages.
Although the only consistent distinction lies in the ability of the adult
stage to form reproductive organs, there are often more or less pronounced differences in leaf morphology between these two stages. These
differences are probably not essentially unlike those discussed in the
previous section, but either a more abrupt morphological transition, or
a longer duration and greater stability of the two stages, does justify
the recognition of juvenile and adult forms.
In a majority of plants the juvenile and adult leaves are both ordinary
foliage leaves which differ only in shape or other morphological details,
e.g., Hedera and Eucalyptus species. In holly, Ilex aquifolium the juvenile leaves are dentate with many prickles whereas leaves from the adult
zone are almost or quite entire. The most striking differences occur in
species where the juvenile leaves are still foliage leaves, while the adult
leaves have undergone extensive modification to form phyllodes, spines,
tendrils, etc., as in the classical examples of Acacia, Ulex europaeus, and
Lathyrus aphaca, respectively.
3. Other Leaf Modifications
The changes in leaf morphology considered in Section ΙΙ,Α,Ι and 2
are characteristic of the ordinary vegetative ontogeny of the species
cited. Other modifications of the leaves are more closely linked with
special events such as flowering or seasonal periodicity.