HETEROBLASTIC DEVELOPMENT IN PLANTÖ
1S1
Although the investigations of Diels (1906) established at an early
date that there is no obligatory association between flowering and a
particular stage of heteroblastic development, in general flowering only
begins when a particular leaf shape has been attained, and is itself
associated with further modifications of the leaves. Striking examples
of this association are provided by that fairly large morphological group
of semirosette plants, in which an elongated stem is produced in preparation for flowering. In such plants, the basal radical leaves are frequently
different from the stem or cauline leaves. Thus, in Campanula rotundifolia, the basal leaves are orbicular with long petioles, while the cauline
leaves are sessile and linear. Usually the radical leaves are more elaborate than the cauline, but in some species, e.g., Valeriana dioica, the
stem leaves are more complex. Many examples of this kind of heteroblastic development are cited by Troll (1939).
The cauline leaves are often transitional to the bracts and bracteoles
of the inflorescence. These greatly modified leaves arise from typical
leaf primordia, but development is usually restricted to the basal parts,
so that relatively simple scales are produced. Brief reference may also
be made to the scale leaves, or cataphylls, of the vegetative parts. In
some species, a few scale leaves may precede the first foliage leaves,
either in the ontogeny of the seedling, e.g., Vicia faba, or in the renewal
of growth in perennial parts, e.g., Anemone hepatica. The scale leaves
protecting the winter buds of deciduous trees are modified leaf bases or
stipules. Even the floral parts are believed by most plant morphologists
to represent modified leaves.
4. Anatomical Changes
It might well be expected that the leaf differences described above
would be associated with corresponding anatomical changes. Such changes
do indeed occur, and it was shown by Schneider (1952) for a representative range of species that the structure of primary leaves is simpler, and
corresponds in some respects, e.g., in the number of mesophyll layers,
to some intermediate stage in the development of later leaves.
Zalenski's Law. The first study of the progressive anatomical differences between the successive leaves of the shoot was carried out by
Zalenski (1904). He considered that the later leaves become increasingly
xeromorphic, as indicated by, among other features, an increase in the
amount of venation per unit area and a continued reduction in the size
of epidermal and mesophyll cells. This "law" was rediscovered independently by Yapp (1912) and has since been supported by the observations of various workers including Ashby (1948b) and Farkas and
Rajhâthy (1955). There can be little doubt that Zalenski's law fre-
1S1
Although the investigations of Diels (1906) established at an early
date that there is no obligatory association between flowering and a
particular stage of heteroblastic development, in general flowering only
begins when a particular leaf shape has been attained, and is itself
associated with further modifications of the leaves. Striking examples
of this association are provided by that fairly large morphological group
of semirosette plants, in which an elongated stem is produced in preparation for flowering. In such plants, the basal radical leaves are frequently
different from the stem or cauline leaves. Thus, in Campanula rotundifolia, the basal leaves are orbicular with long petioles, while the cauline
leaves are sessile and linear. Usually the radical leaves are more elaborate than the cauline, but in some species, e.g., Valeriana dioica, the
stem leaves are more complex. Many examples of this kind of heteroblastic development are cited by Troll (1939).
The cauline leaves are often transitional to the bracts and bracteoles
of the inflorescence. These greatly modified leaves arise from typical
leaf primordia, but development is usually restricted to the basal parts,
so that relatively simple scales are produced. Brief reference may also
be made to the scale leaves, or cataphylls, of the vegetative parts. In
some species, a few scale leaves may precede the first foliage leaves,
either in the ontogeny of the seedling, e.g., Vicia faba, or in the renewal
of growth in perennial parts, e.g., Anemone hepatica. The scale leaves
protecting the winter buds of deciduous trees are modified leaf bases or
stipules. Even the floral parts are believed by most plant morphologists
to represent modified leaves.
4. Anatomical Changes
It might well be expected that the leaf differences described above
would be associated with corresponding anatomical changes. Such changes
do indeed occur, and it was shown by Schneider (1952) for a representative range of species that the structure of primary leaves is simpler, and
corresponds in some respects, e.g., in the number of mesophyll layers,
to some intermediate stage in the development of later leaves.
Zalenski's Law. The first study of the progressive anatomical differences between the successive leaves of the shoot was carried out by
Zalenski (1904). He considered that the later leaves become increasingly
xeromorphic, as indicated by, among other features, an increase in the
amount of venation per unit area and a continued reduction in the size
of epidermal and mesophyll cells. This "law" was rediscovered independently by Yapp (1912) and has since been supported by the observations of various workers including Ashby (1948b) and Farkas and
Rajhâthy (1955). There can be little doubt that Zalenski's law fre-