224
J. HESLOP-HARRISON
McKell et al. note that subsp. judaica characteristically passes into a
state of summer dormancy. This presumably forms its principal means
of surviving the main period of moisture stress in its native habitat,
rather than any direct adaptations to water deficiency. During the actual
growth period the main physiological feature suggesting adaptation to
soil moisture stress was the lower rate of water use ; otherwise the overall responses were not very different from those of subsp. lusitanim.
D. ADAPTATION TO LIGHT INTENSITY
A conspicuous form of environmentally correlated morphological
variation in higher plants is that apparent in leaves according t o the
intensity of illumination. Differences between sun and shade leaves of
the same genotype are a direct consequence of the morphogenetic effects
of light, and it is widely supposed that the characteristic features are in
some way adaptive, although the direct evidence is slight (Wassink et al.,
1956). I n consequence of plasticity, the sun and shade grown individuals
of some species may reveal substantia,l phenotypic differences in leaf
size and texture, but, as shown by Turesson (192213) with Lysimachia
vulgaris, such differences disappear on cultivation in standard environments. Heritable differences in leaf form between populations of a
species from open and shaded habitats have, however, been frequently
demonstrated, and this may be regarded as an example of genecological
differentiation.
Although it is the variation in the light environment which is commonly most conspicuous, “sun” and “shade” habitats do inevitably
differ also in other correlated factors, notably in temperature and humidity. There is therefore the familiar diEculty of identifying the effective selective factors, and again it must be supposed that several responses have been selected for simultaneously in the course of adaptation.
Nevertheless, there is good reason for concentrating upon light as a
major measureable differential factor in studying the adaptation of sun
and shade ecotypes, particularly since the role of light in the economy of
the plant is reasonably well understood.
An excellent study of photosynthesis efficiency in sun and shade ecotypes of Solidago virgaurea has recently been carried out by Bjorkman
and Holmgren (1963). The sampled populations were from two shaded
habitats, oak and beech forest, and two open habitats, dry open meadow
at 105 m altitude and alpine heath at 600 m. Ten plants were selected
from each environment and propagated to provide two pairs of individuals of each genotype. One pair from each clone was cultivated at low
light intensity, 3 x 104 erg/sec/cm2 and one pair at high, 15 x lo4 erglsecl
ems. Other conditions were identical : photoperiod 16 h, temperature
during light period 20.0 3 0.3’C and during dark 10.0 f 0.3” C. Air
J. HESLOP-HARRISON
McKell et al. note that subsp. judaica characteristically passes into a
state of summer dormancy. This presumably forms its principal means
of surviving the main period of moisture stress in its native habitat,
rather than any direct adaptations to water deficiency. During the actual
growth period the main physiological feature suggesting adaptation to
soil moisture stress was the lower rate of water use ; otherwise the overall responses were not very different from those of subsp. lusitanim.
D. ADAPTATION TO LIGHT INTENSITY
A conspicuous form of environmentally correlated morphological
variation in higher plants is that apparent in leaves according t o the
intensity of illumination. Differences between sun and shade leaves of
the same genotype are a direct consequence of the morphogenetic effects
of light, and it is widely supposed that the characteristic features are in
some way adaptive, although the direct evidence is slight (Wassink et al.,
1956). I n consequence of plasticity, the sun and shade grown individuals
of some species may reveal substantia,l phenotypic differences in leaf
size and texture, but, as shown by Turesson (192213) with Lysimachia
vulgaris, such differences disappear on cultivation in standard environments. Heritable differences in leaf form between populations of a
species from open and shaded habitats have, however, been frequently
demonstrated, and this may be regarded as an example of genecological
differentiation.
Although it is the variation in the light environment which is commonly most conspicuous, “sun” and “shade” habitats do inevitably
differ also in other correlated factors, notably in temperature and humidity. There is therefore the familiar diEculty of identifying the effective selective factors, and again it must be supposed that several responses have been selected for simultaneously in the course of adaptation.
Nevertheless, there is good reason for concentrating upon light as a
major measureable differential factor in studying the adaptation of sun
and shade ecotypes, particularly since the role of light in the economy of
the plant is reasonably well understood.
An excellent study of photosynthesis efficiency in sun and shade ecotypes of Solidago virgaurea has recently been carried out by Bjorkman
and Holmgren (1963). The sampled populations were from two shaded
habitats, oak and beech forest, and two open habitats, dry open meadow
at 105 m altitude and alpine heath at 600 m. Ten plants were selected
from each environment and propagated to provide two pairs of individuals of each genotype. One pair from each clone was cultivated at low
light intensity, 3 x 104 erg/sec/cm2 and one pair at high, 15 x lo4 erglsecl
ems. Other conditions were identical : photoperiod 16 h, temperature
during light period 20.0 3 0.3’C and during dark 10.0 f 0.3” C. Air