214
J. HESLOP-HARRISON
ecotypes of Solidago were conditioned to various levels of .irradiation.
Plants of the “sun” ecotype showed adaptive change, the photosynthetic
rate at light saturation being higher following growth in high light intensities. Plants from the “shade” ecotype were incapable of adapting
to the highest light intensity used, and the leaves were in fact damaged.
Bjorkman and Holmgren’s observations on Solidago virgaurea suggest
very clearly that the range of developmental flexibility in this species is
somewhat limited. In respect to the capacity for adapting to different
light environments, it would seem that the genotypes tested would not
be flexible enough to span more than a part of the observed tolerance
range of the species; the breadth of this evidently depends upon the
differentiation of ecotypes. There are, however, cases of species which
seem to be capable of colonizing unusual habitats either by producing
modified phenotypes (developmental flexibility) or by evolving ecotypes
(genecological differentiation). Turesson (1922b) mentions the example
of the forma nana of Succisa pratensis. This is an extreme dwarf form,
attaining a stature of no more than 8.5 cm, occurring in the upper part
of salt marshes around the coast of Scania. Samples transplanted from
various populations differed in their behaviour under cultivation. All
increased in height to some extent, but some populations proved to contain individuals fully capable of achieving the stature of plants from
inland populations, whilst others were composed exclusively of hereditary dwarfs.
Examples like Succisa pratensis f. nana raise the general problem of
the relative values of developmental flexibility and genecological
differentiation as devices permitting the extension of ecological range. It
is obvious that some direct phenotypic adaptability will always be of
value in buffering a plant against minor modulations of environment,
either spatial or chronological. Why should tolerance not be extended
indefinitely in this manner? Since the evidence shows that genecological
differentiation is the “preferred” means of adapting to extreme habitats,
it follows that there must be inherent disadvantages in direct phenotypic
modification. Turesson’s own conclusion (1 922b) probably covers the
essential point : “The question may be made clearer by the assumption
that the same characteristics which in one form of the species (the resulting modification) requires the exposure to an environmental factor of
high intensity in order to become developed, may in another form (the
hereditary variation) result as a response to a very much lower intensity
of this factor. . . . It is conceivable that the habitat responsible for the
development of the characteristic in question may at the same time act
as a limiting factor upon general development in the case of the modification, while no such limiting action results in the hereditary variation
because of the promptness with which it responds to this same habitat
J. HESLOP-HARRISON
ecotypes of Solidago were conditioned to various levels of .irradiation.
Plants of the “sun” ecotype showed adaptive change, the photosynthetic
rate at light saturation being higher following growth in high light intensities. Plants from the “shade” ecotype were incapable of adapting
to the highest light intensity used, and the leaves were in fact damaged.
Bjorkman and Holmgren’s observations on Solidago virgaurea suggest
very clearly that the range of developmental flexibility in this species is
somewhat limited. In respect to the capacity for adapting to different
light environments, it would seem that the genotypes tested would not
be flexible enough to span more than a part of the observed tolerance
range of the species; the breadth of this evidently depends upon the
differentiation of ecotypes. There are, however, cases of species which
seem to be capable of colonizing unusual habitats either by producing
modified phenotypes (developmental flexibility) or by evolving ecotypes
(genecological differentiation). Turesson (1922b) mentions the example
of the forma nana of Succisa pratensis. This is an extreme dwarf form,
attaining a stature of no more than 8.5 cm, occurring in the upper part
of salt marshes around the coast of Scania. Samples transplanted from
various populations differed in their behaviour under cultivation. All
increased in height to some extent, but some populations proved to contain individuals fully capable of achieving the stature of plants from
inland populations, whilst others were composed exclusively of hereditary dwarfs.
Examples like Succisa pratensis f. nana raise the general problem of
the relative values of developmental flexibility and genecological
differentiation as devices permitting the extension of ecological range. It
is obvious that some direct phenotypic adaptability will always be of
value in buffering a plant against minor modulations of environment,
either spatial or chronological. Why should tolerance not be extended
indefinitely in this manner? Since the evidence shows that genecological
differentiation is the “preferred” means of adapting to extreme habitats,
it follows that there must be inherent disadvantages in direct phenotypic
modification. Turesson’s own conclusion (1 922b) probably covers the
essential point : “The question may be made clearer by the assumption
that the same characteristics which in one form of the species (the resulting modification) requires the exposure to an environmental factor of
high intensity in order to become developed, may in another form (the
hereditary variation) result as a response to a very much lower intensity
of this factor. . . . It is conceivable that the habitat responsible for the
development of the characteristic in question may at the same time act
as a limiting factor upon general development in the case of the modification, while no such limiting action results in the hereditary variation
because of the promptness with which it responds to this same habitat