FORTY Y E A R S O F GENECOLOGY
227
al., 1962) have been examining photosynthetic efficiency in various
artificial environments. As reported so far, the work is principally of
interest as a study in comparative physioJogy, since it is not always
clear in what way the differences observed can be associated with adaptation to habitat. Two examples of responses observed in experiment
which may be related to ecotypic adaptation have, however, been
recorded (Hiesey et al., 1960, p. 317). A clone from a high-altitude Yosemite population revealed a saturating light intensity for photosynthesis
1-5 times higher than one from a sea level habitat at Los Trancos; it is
suggested that this may be related to a difference in average light intensity in the two habitats. Furthermore, the Yosemite plants were found
to make less effective use of high CO, concentrations, photosynthesising
at optimum temperatures and at light saturation, than plants from Los
Trancos, and the possibility is mentioned that this may be concerned
with the 15% reduction in CO, in the atmosphere a t the altitude of the
Yosemite race compared with that at sea level. Evidence concerning the
effects of pre-conditioning comparable with that available for Solidago
virgaurea seems not yet to have been obtained, but the first of these
observations, so far as it goes, does suggest a similar pattern of racial
adaptation to variation in available light to that demonstrated in
Solidago by Bjorkman and Holmgren.
E. ADAPTATION TO CLIMATE
It is evident from the pattern of variation in many of the wide-ranging
species discussed in Section I that infiaspecific differentiation must frequently be dominated by the selective effects of regional climates. The
analysis of these effects is beset with formidable difficulties. Those
arising from the mrdtifactorial nature of climatic differences have already
been mentioned. In addition there is the complication that a diversity of
solutions is available to plant populations for adaptation to climatic
differences. In the example of habitat adaptation just reviewed, one
pervasive environmental factor can be identified as probably having had
the dominant selective influence ; survival depends absolutely upon the
capacity to adapt to it. Where habitats differ in a multiplicity of seasonally varying climatic factors at one and the same time, adaptation in
several features may be required to permit survival, while continued
existence may equally well be ensured by more than one pattern of
response.
Undoubtedly the most powerful means available to the higher plant
for adaptation to regionally varying climates is the ability to adjust the
developmental cycle. This property is of the greatest significance when a
species spans a range of climates differing in the pattern of incidence of
unfavourable seasons - hot and dry, or cold. To survive adverse condi-
227
al., 1962) have been examining photosynthetic efficiency in various
artificial environments. As reported so far, the work is principally of
interest as a study in comparative physioJogy, since it is not always
clear in what way the differences observed can be associated with adaptation to habitat. Two examples of responses observed in experiment
which may be related to ecotypic adaptation have, however, been
recorded (Hiesey et al., 1960, p. 317). A clone from a high-altitude Yosemite population revealed a saturating light intensity for photosynthesis
1-5 times higher than one from a sea level habitat at Los Trancos; it is
suggested that this may be related to a difference in average light intensity in the two habitats. Furthermore, the Yosemite plants were found
to make less effective use of high CO, concentrations, photosynthesising
at optimum temperatures and at light saturation, than plants from Los
Trancos, and the possibility is mentioned that this may be concerned
with the 15% reduction in CO, in the atmosphere a t the altitude of the
Yosemite race compared with that at sea level. Evidence concerning the
effects of pre-conditioning comparable with that available for Solidago
virgaurea seems not yet to have been obtained, but the first of these
observations, so far as it goes, does suggest a similar pattern of racial
adaptation to variation in available light to that demonstrated in
Solidago by Bjorkman and Holmgren.
E. ADAPTATION TO CLIMATE
It is evident from the pattern of variation in many of the wide-ranging
species discussed in Section I that infiaspecific differentiation must frequently be dominated by the selective effects of regional climates. The
analysis of these effects is beset with formidable difficulties. Those
arising from the mrdtifactorial nature of climatic differences have already
been mentioned. In addition there is the complication that a diversity of
solutions is available to plant populations for adaptation to climatic
differences. In the example of habitat adaptation just reviewed, one
pervasive environmental factor can be identified as probably having had
the dominant selective influence ; survival depends absolutely upon the
capacity to adapt to it. Where habitats differ in a multiplicity of seasonally varying climatic factors at one and the same time, adaptation in
several features may be required to permit survival, while continued
existence may equally well be ensured by more than one pattern of
response.
Undoubtedly the most powerful means available to the higher plant
for adaptation to regionally varying climates is the ability to adjust the
developmental cycle. This property is of the greatest significance when a
species spans a range of climates differing in the pattern of incidence of
unfavourable seasons - hot and dry, or cold. To survive adverse condi-