136
A. ALLSOPP
types in the New Zealand flora, Cockayne (1911) concluded that the
juvenile stages resemble the ancestral adult types, and that the xeromorphic stages evolved later in response to a change to a semidesert
climate in the Upper Pliocene. In some New Zealand plants, e.g., Pseudopanax crassifolium and Plagianthus betulinus, there is even a third stage,
with mesomorphic leaves, corresponding to present climatic conditions.
1. Homoblastic Forms
Goebel also postulated that, in general, homoblastic forms with modified leaves have originated secondarily from heteroblastic forms by an
extension of the leaf modification to the juvenile stages. Thus, in the
monocotyledonous family Liliaceae, Semele androgyna is strikingly
heteroblastic with large green primary leaves and only small scale leaves
in the adult regions, while in all species investigated of the related genus
Asparagus only scale leaves are present from the earliest stages.
2. Neoteny
In contrast to the homoblastic forms, in which distinct juvenile stages
have been eliminated from the ontogeny, there are many cases of neoteny
in which it is the later stages that have been discarded so that the plant
becomes reproductive while still at a juvenile stage of development. The
three genera of the family of water ferns, Marsileaceae, provide an interesting example of progressive neoteny. Marsilea itself with quadrifoliate adult leaves is believed to have originated from other ferns with
considerably more elaborate leaves. During sporeling development several types of leaves are produced in succession; first an acicular cotyledon followed by undivided spatulate leaves, then bifid or bifoliate leaves,
and finally quadrifid or quadrifoliate leaves. Regnellidium is more reduced and its leaves never advance beyond the bilobed condition, while
in Pilularia all the leaves are subulate like the cotyledon.
Neoteny accompanied by further modification of the juvenile stages
can have far-reaching phylogenetic consequences. It has obviously been
of the greatest importance in the evolution of higher plants and animals.
As a major example from the plant kingdom, it may be mentioned that
Takhtajan (1954) has adduced considerable evidence that herbaceous
angiosperms are neotenous derivatives from woody ancestors. Evidence
of paedomorphosis is found also in certain dicotyledonous woods (Carlquist, 1962).
The above brief survey indicates that there is a close relationship between phylogeny and ontogeny, but it scarcely requires emphasis that
whatever the phylogenetic interpretation of the ontogeny of any particular plant, the ontogenetic changes within that individual still require
a causal explanation.
A. ALLSOPP
types in the New Zealand flora, Cockayne (1911) concluded that the
juvenile stages resemble the ancestral adult types, and that the xeromorphic stages evolved later in response to a change to a semidesert
climate in the Upper Pliocene. In some New Zealand plants, e.g., Pseudopanax crassifolium and Plagianthus betulinus, there is even a third stage,
with mesomorphic leaves, corresponding to present climatic conditions.
1. Homoblastic Forms
Goebel also postulated that, in general, homoblastic forms with modified leaves have originated secondarily from heteroblastic forms by an
extension of the leaf modification to the juvenile stages. Thus, in the
monocotyledonous family Liliaceae, Semele androgyna is strikingly
heteroblastic with large green primary leaves and only small scale leaves
in the adult regions, while in all species investigated of the related genus
Asparagus only scale leaves are present from the earliest stages.
2. Neoteny
In contrast to the homoblastic forms, in which distinct juvenile stages
have been eliminated from the ontogeny, there are many cases of neoteny
in which it is the later stages that have been discarded so that the plant
becomes reproductive while still at a juvenile stage of development. The
three genera of the family of water ferns, Marsileaceae, provide an interesting example of progressive neoteny. Marsilea itself with quadrifoliate adult leaves is believed to have originated from other ferns with
considerably more elaborate leaves. During sporeling development several types of leaves are produced in succession; first an acicular cotyledon followed by undivided spatulate leaves, then bifid or bifoliate leaves,
and finally quadrifid or quadrifoliate leaves. Regnellidium is more reduced and its leaves never advance beyond the bilobed condition, while
in Pilularia all the leaves are subulate like the cotyledon.
Neoteny accompanied by further modification of the juvenile stages
can have far-reaching phylogenetic consequences. It has obviously been
of the greatest importance in the evolution of higher plants and animals.
As a major example from the plant kingdom, it may be mentioned that
Takhtajan (1954) has adduced considerable evidence that herbaceous
angiosperms are neotenous derivatives from woody ancestors. Evidence
of paedomorphosis is found also in certain dicotyledonous woods (Carlquist, 1962).
The above brief survey indicates that there is a close relationship between phylogeny and ontogeny, but it scarcely requires emphasis that
whatever the phylogenetic interpretation of the ontogeny of any particular plant, the ontogenetic changes within that individual still require
a causal explanation.