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A. ALLSÔPP
duction of flowers or other reproductive organs. Apart from this evident
relationship with the progressive maturation of the plant, there is still
no general agreement as to the causation of heteroblastic development.
A number of varied hypotheses have been advanced, some supported
by apparently convincing evidence. But as indicated in recent articles
(Allsopp, 1964a,b), it seems evident that several more or less distinct
types of morphological change have been included within the general
concept of heteroblastic development, with the result that conflicting
evidence and hypotheses are to be expected if this diversity is not
appreciated. There is also the additional complication that the normal
ontogenetic changes may be confused with those directly induced by the
environment. Heteroblastic development can be modified by the environment, but is not dependent on environmental changes and will proceed
quite normally in any suitable constant environment.
Some degree of heteroblastic development is found in all groups of
plants, but there have still been relatively few studies on nonvascular
plants, although their ontogeny is considerably more varied than that
of the vascular plants. The latter are comparatively uniform and at
present provide more suitable material for a broad survey of heteroblastic development.
II. Morphological Characteristics
Most recent investigations of heteroblastic development in vascular
plants aim at relating the observed morphological differences to internal
physiological changes. This objective presupposes a sound knowledge of
the morphological aspects of heteroblastic development as well as of
relevant physiological processes. Since in recent work there has perhaps
been a tendency to overlook certain morphological information, an outline of some of the principal morphological findings may not be inappropriate.
The morphological changes characteristic of heteroblastic development
have been described in detail and with varying emphasis by a considerable number of authors (e.g., Goebel, 1889, 1898; Troll, 1937, 1938;
Ashby, 1948a; Takhtajan, 1954; Allsopp, 1965b). Heteroblastic development is found to affect almost all parts of the plant but is most evident
in its effects on leaf morphology.
A. Changes in Leaf Morphology
Although heteroblastic leaf development has been known since the
earliest studies of plant ontogeny, it is only recently that it has become
apparent that the changes characteristic of seedlings or sporelings may
be different in nature from those encountered in somewhat later stages
(Allsopp, 1964a,b). In some plants, differences affecting the later stages
A. ALLSÔPP
duction of flowers or other reproductive organs. Apart from this evident
relationship with the progressive maturation of the plant, there is still
no general agreement as to the causation of heteroblastic development.
A number of varied hypotheses have been advanced, some supported
by apparently convincing evidence. But as indicated in recent articles
(Allsopp, 1964a,b), it seems evident that several more or less distinct
types of morphological change have been included within the general
concept of heteroblastic development, with the result that conflicting
evidence and hypotheses are to be expected if this diversity is not
appreciated. There is also the additional complication that the normal
ontogenetic changes may be confused with those directly induced by the
environment. Heteroblastic development can be modified by the environment, but is not dependent on environmental changes and will proceed
quite normally in any suitable constant environment.
Some degree of heteroblastic development is found in all groups of
plants, but there have still been relatively few studies on nonvascular
plants, although their ontogeny is considerably more varied than that
of the vascular plants. The latter are comparatively uniform and at
present provide more suitable material for a broad survey of heteroblastic development.
II. Morphological Characteristics
Most recent investigations of heteroblastic development in vascular
plants aim at relating the observed morphological differences to internal
physiological changes. This objective presupposes a sound knowledge of
the morphological aspects of heteroblastic development as well as of
relevant physiological processes. Since in recent work there has perhaps
been a tendency to overlook certain morphological information, an outline of some of the principal morphological findings may not be inappropriate.
The morphological changes characteristic of heteroblastic development
have been described in detail and with varying emphasis by a considerable number of authors (e.g., Goebel, 1889, 1898; Troll, 1937, 1938;
Ashby, 1948a; Takhtajan, 1954; Allsopp, 1965b). Heteroblastic development is found to affect almost all parts of the plant but is most evident
in its effects on leaf morphology.
A. Changes in Leaf Morphology
Although heteroblastic leaf development has been known since the
earliest studies of plant ontogeny, it is only recently that it has become
apparent that the changes characteristic of seedlings or sporelings may
be different in nature from those encountered in somewhat later stages
(Allsopp, 1964a,b). In some plants, differences affecting the later stages