FORTY YEARS O F GENECOLOGY
219
truly will the terminus have been reached. Fortunately, however, when
it is difference betweenhabitats which is under consideration, the situation may be simpler, because very often it is possible to identify the
major determinants. “Adaptation” is still likely to be a holistic process,
but it is feasible to measure plant responses to the major environmental
variables singly and in interaction in various simple patterns, and obviously this can be done in a comparative manner with populations of
different provenqnces.
It needs no emphasis that this type of study only becomes useful as a
basis for extrapolation to the natural situation when the conditions of
the experiments themselves can be related in some meaningful manner
to those in nature. Merely to match the list of morphological differentiae
observed between ecotypically differentiated populations with another
of physiological differentiae demonstrated under some experimental
condition or the other is obviously not enough. It is only when a functional difference can be shown to be associated with a differentially
acting environmental factor that we are beginning to approach the
understanding of a causal sequence. Yet even here it may still not be
possible to say with assurance that the selective agent and the specific
function upon which it impinges have been identified. The possibility
remains that the “real” target of selection is an associated or secondary
response, or a function for which the one studied acts as a governor or
time-keeper .
B. EDAPHIC ADAPTATION
There is no particular reason for supposing that the adaptation of
plants to their rooting media will involve processes or principles different from those governing adaptation to the sub-aerial environment, but
it so happens that there are examples of clear-cut patterns of adaptation
to soil type which should permit a more precise kind of analysis than can
be given to most examples of adaptation to climate or biotic influence.
Such a case is the tolerance of grass populations to soils contaminated
with heavy metals, briefly described in a foregoing section (Bradshaw,
1952; Wilkins, 1957, 1960a, b). Wilkins compares this example to industrial melanism in moths : in both situations the selective factor can
be identified precisely and its impact can be shown to be severe. In the
case of Pestuca ovina, lead is highly toxic to normal plants, yet on mine
spoil heaps in the British Isles tolerant populations occur on soils containing up to 4% lead. Using an assay method based upon the measurement of extension growth in the roots of tillers grown in glass tubes in
culture solutions under standard environments Wilkins (1957, 1960b)
has obtained evidence suggesting t.hat three types are present in the
species : intolerant, medium tolerant and highly tolerant. The tolerant
219
truly will the terminus have been reached. Fortunately, however, when
it is difference betweenhabitats which is under consideration, the situation may be simpler, because very often it is possible to identify the
major determinants. “Adaptation” is still likely to be a holistic process,
but it is feasible to measure plant responses to the major environmental
variables singly and in interaction in various simple patterns, and obviously this can be done in a comparative manner with populations of
different provenqnces.
It needs no emphasis that this type of study only becomes useful as a
basis for extrapolation to the natural situation when the conditions of
the experiments themselves can be related in some meaningful manner
to those in nature. Merely to match the list of morphological differentiae
observed between ecotypically differentiated populations with another
of physiological differentiae demonstrated under some experimental
condition or the other is obviously not enough. It is only when a functional difference can be shown to be associated with a differentially
acting environmental factor that we are beginning to approach the
understanding of a causal sequence. Yet even here it may still not be
possible to say with assurance that the selective agent and the specific
function upon which it impinges have been identified. The possibility
remains that the “real” target of selection is an associated or secondary
response, or a function for which the one studied acts as a governor or
time-keeper .
B. EDAPHIC ADAPTATION
There is no particular reason for supposing that the adaptation of
plants to their rooting media will involve processes or principles different from those governing adaptation to the sub-aerial environment, but
it so happens that there are examples of clear-cut patterns of adaptation
to soil type which should permit a more precise kind of analysis than can
be given to most examples of adaptation to climate or biotic influence.
Such a case is the tolerance of grass populations to soils contaminated
with heavy metals, briefly described in a foregoing section (Bradshaw,
1952; Wilkins, 1957, 1960a, b). Wilkins compares this example to industrial melanism in moths : in both situations the selective factor can
be identified precisely and its impact can be shown to be severe. In the
case of Pestuca ovina, lead is highly toxic to normal plants, yet on mine
spoil heaps in the British Isles tolerant populations occur on soils containing up to 4% lead. Using an assay method based upon the measurement of extension growth in the roots of tillers grown in glass tubes in
culture solutions under standard environments Wilkins (1957, 1960b)
has obtained evidence suggesting t.hat three types are present in the
species : intolerant, medium tolerant and highly tolerant. The tolerant