180
J. HESLOP-HARRISON
involved, the pattern of lead tolerance being superimposed upon all other
systems of adaptive and non-adaptive variation within Pe&ma ovina.
3. Clinal Variation
Some of the most informative examples of infraspecific ecoclines have
been described in woody species. Thus the variation of the Scots pine,
Pinw sylvestris, throughout its European range has been the subject of
much illuminating research and some controversy (Langlet, 1934, 1936,
1959, 1963; Wright and Baldwin, 1957). In his earlier work, Langlet
described clines spanning almost 25" of latitude, involving several
morphological and physiological features such as leaf length, hardiness,
dormancy period and shoot extension rate. The 1938 international test
involving fifty-two provenances provides some particularly striking
evidence. I n the trial grown near Stockholm, Langlet recorded the
percentage of dry matter in the needles of 2-4 year seedlings in the Iate
autumn. This particular measure showed a close relationship with the
length of the growing season, assessed as the number of days with an
average temperature of 6" C or more, in the native habitats. A still closer
relationship was apparent between dry matter content and the length
of the first day of the year with an average temperature of greater than
6°C. The scatter diagram is reproduced in Fig. la: the regression is
curvilinear, and r exceeds +0*98 (Langlet, 1959). The clinal nature of
the variation is quite beyond challenge.
The trial reported upon by Wright and Baldwin (1957) involved
forty-six of the fifty-two provenances tested by Langlet, but their treatment and conclusions were radically different. The material was grown
in New Hampshire, a t a latitude of 43" N, some 14" south of Stockholm.
According to Wright and Baldwin, the evidence suggested that the
variation in Scots pine is discontinuous, and they grouped the sampled
populations into a number of regional ecotypes. Some of these - in the
northern part of the range-occupied latitudinal belts, and bore a
clinal relationship to each other ; others further south coincided with no
particular latitudinal or climatic zones. This treatment has been heavily
criticized by Langlet (1959), who pointed out that the method of grouping the different samples geographically and comparing regional means
for the various characters observed would necessarily obscure variational
continuity. His own re-plotting of the height data for 17-year trees
grown in New Hampshire in relation to day length is reproduced in Fig.
lb. There is a remarkable correspondence between the distributibn
shown in this figure and that shown in Fig. la; aa Langlet remarks,
Wright and Baldwin do seem very satisfactorily to have demonstrated
continuous variability in Scots pine, in spite of their own conclusion to
the contrary.
J. HESLOP-HARRISON
involved, the pattern of lead tolerance being superimposed upon all other
systems of adaptive and non-adaptive variation within Pe&ma ovina.
3. Clinal Variation
Some of the most informative examples of infraspecific ecoclines have
been described in woody species. Thus the variation of the Scots pine,
Pinw sylvestris, throughout its European range has been the subject of
much illuminating research and some controversy (Langlet, 1934, 1936,
1959, 1963; Wright and Baldwin, 1957). In his earlier work, Langlet
described clines spanning almost 25" of latitude, involving several
morphological and physiological features such as leaf length, hardiness,
dormancy period and shoot extension rate. The 1938 international test
involving fifty-two provenances provides some particularly striking
evidence. I n the trial grown near Stockholm, Langlet recorded the
percentage of dry matter in the needles of 2-4 year seedlings in the Iate
autumn. This particular measure showed a close relationship with the
length of the growing season, assessed as the number of days with an
average temperature of 6" C or more, in the native habitats. A still closer
relationship was apparent between dry matter content and the length
of the first day of the year with an average temperature of greater than
6°C. The scatter diagram is reproduced in Fig. la: the regression is
curvilinear, and r exceeds +0*98 (Langlet, 1959). The clinal nature of
the variation is quite beyond challenge.
The trial reported upon by Wright and Baldwin (1957) involved
forty-six of the fifty-two provenances tested by Langlet, but their treatment and conclusions were radically different. The material was grown
in New Hampshire, a t a latitude of 43" N, some 14" south of Stockholm.
According to Wright and Baldwin, the evidence suggested that the
variation in Scots pine is discontinuous, and they grouped the sampled
populations into a number of regional ecotypes. Some of these - in the
northern part of the range-occupied latitudinal belts, and bore a
clinal relationship to each other ; others further south coincided with no
particular latitudinal or climatic zones. This treatment has been heavily
criticized by Langlet (1959), who pointed out that the method of grouping the different samples geographically and comparing regional means
for the various characters observed would necessarily obscure variational
continuity. His own re-plotting of the height data for 17-year trees
grown in New Hampshire in relation to day length is reproduced in Fig.
lb. There is a remarkable correspondence between the distributibn
shown in this figure and that shown in Fig. la; aa Langlet remarks,
Wright and Baldwin do seem very satisfactorily to have demonstrated
continuous variability in Scots pine, in spite of their own conclusion to
the contrary.