220
J. HESLOP-HARRISON
plants are restricted entirely to lead-containing soils, and the intolerant
ones to leadlfree soils. Genetically the property of tolerance was found
to be completely dominant, and although a full analysis has not been
possible yet, it is conceivable that the tolerant genotypes may differ
from the normal in substitutions at single loci. In a field trial no morphological features could be shown to be correlated with tolerance, and for
that reason Wilkins has been reluctant to look upon the tolerant populations as constituting an ecotype. Whatever terminology is used, BOWever, it is evident that this is an example of genecological Zfferentiation
in the broad sense, and rather an important one in that the selective
factor has been identified with some assurance and related to a physiologicd property which should, in principle a t least, be open to precise
meaiurement .
The examples of ecological races adapted to serpentine soils also discussed in an earlier section are in some respects comparable. Kruckeberg
(1954) states that a major criterion for serpentine tolerance must be the
capacity for growth on soils of low calcium levels. A direct comparison
of the tolerances of serpentine and non-serpentine races of Phacelia
californica to calcium deficiency revealed a very clear difference ; the
normal soil race showed no growth in un-supplemented serpentine soil,
the growth being equalized with supplements equivalent to 2 tons of
gypsum per acre. NPKctreatments greatly benefited the tolerant race on
serpentine soil, but were without effect on the normal soil race on this
medium. The suggestion of Walker (1954) that serpentine races might
benefit from a capacity to accumulate calcium in preference to other
cations was also verified with the Phacelia races. At each of three soilcalcium levels the tolerant race absorbed greater amounts of calcium
and lesser amounts of magnesium, as revealed by tissue analysis. A
similar distinction between serpentine and non-serpentine races in
tolerance of calcium-deficient soils was observed in twelve other species
of annual and perennial herbs, although no serpentine ecotypes were
observed among the grasses tested.
Although there seems little doubt that the major adaptation shown
by the serpentine races is their capacity to tolerate calcium deficiency,
Kruckeberg points to the fact that serpentine ecotypes may show other
adaptive characteristics. These include in particular the capacity to
tolerate the exposure and sometimes drought which may also characterise the serpentine habitat. He concludes “that serpentine plants are
physiologically adapted to the open characteristics of serpentine communities as well as to the special soil conditions, and that the physiological adaptations of serpentine plants may have some degree of
morphological expression” This multidimensional adaptation of some
serpentine races offers a pretty example for consideration in relation to
J. HESLOP-HARRISON
plants are restricted entirely to lead-containing soils, and the intolerant
ones to leadlfree soils. Genetically the property of tolerance was found
to be completely dominant, and although a full analysis has not been
possible yet, it is conceivable that the tolerant genotypes may differ
from the normal in substitutions at single loci. In a field trial no morphological features could be shown to be correlated with tolerance, and for
that reason Wilkins has been reluctant to look upon the tolerant populations as constituting an ecotype. Whatever terminology is used, BOWever, it is evident that this is an example of genecological Zfferentiation
in the broad sense, and rather an important one in that the selective
factor has been identified with some assurance and related to a physiologicd property which should, in principle a t least, be open to precise
meaiurement .
The examples of ecological races adapted to serpentine soils also discussed in an earlier section are in some respects comparable. Kruckeberg
(1954) states that a major criterion for serpentine tolerance must be the
capacity for growth on soils of low calcium levels. A direct comparison
of the tolerances of serpentine and non-serpentine races of Phacelia
californica to calcium deficiency revealed a very clear difference ; the
normal soil race showed no growth in un-supplemented serpentine soil,
the growth being equalized with supplements equivalent to 2 tons of
gypsum per acre. NPKctreatments greatly benefited the tolerant race on
serpentine soil, but were without effect on the normal soil race on this
medium. The suggestion of Walker (1954) that serpentine races might
benefit from a capacity to accumulate calcium in preference to other
cations was also verified with the Phacelia races. At each of three soilcalcium levels the tolerant race absorbed greater amounts of calcium
and lesser amounts of magnesium, as revealed by tissue analysis. A
similar distinction between serpentine and non-serpentine races in
tolerance of calcium-deficient soils was observed in twelve other species
of annual and perennial herbs, although no serpentine ecotypes were
observed among the grasses tested.
Although there seems little doubt that the major adaptation shown
by the serpentine races is their capacity to tolerate calcium deficiency,
Kruckeberg points to the fact that serpentine ecotypes may show other
adaptive characteristics. These include in particular the capacity to
tolerate the exposure and sometimes drought which may also characterise the serpentine habitat. He concludes “that serpentine plants are
physiologically adapted to the open characteristics of serpentine communities as well as to the special soil conditions, and that the physiological adaptations of serpentine plants may have some degree of
morphological expression” This multidimensional adaptation of some
serpentine races offers a pretty example for consideration in relation to