FORTY YEARS O F GENECOLOGY
229
change, not the behaviour under a fixed photoperiod. Valuable reviews
of temperature and daylength effects in which ecological aspects are
taken into consideration are those of Wareing (19'56) and Chouard (1960).
Ecotypic variation in temperature responses has been studied experimentally in several species. Part of the current programme of the
Carnegie group (Hiesey et al., 1959, 1960, 1961; Milner et al., 1962) is
concerned with this aspect of racial differentiation, in Mimulw cardinalis.
In an earlier study, Hiesey (1953) has investigated the thermal responses of the aggregate species Achillea millefolium. The reactions of
cloned material of three races originally grown from seed under standard
garden conditions were followed under different combinations of day
and night temperature in growth chambers. The races were a diploid
maritime one, corresponding to the taxonomic subspecies arenicola from
a cool Californian coastal region, an interior valley diploid race native to
the hot San Joaquin Valley of California, and a high montane race
referable to Achillea lanulosa subsp. typica from an altitude of 8 700 ft in
the Sierra Nevada. It may be noted that, strictly, this last race must be
regarded as belonging to a separate ecospecies from the &st two, having
a tetraploid chromosome number, but much of the interest of the study
arises from its inclusion. The responses of the races may be summarized
as follows :
Coastal. Good growth and flowering was attained in night temperatures of 6" C in combination with 20" C or 30" C days; growth and flowering were inhibited with night temperatures of 17" C.
Interior valley. Good growth was attained with 6" or 17OC night
temperature, while performance was slightly better with day temperatures of 20" C than with 30" C. Flowering was delayed somewhat when
6" C nights were combined with 20" C days.
Sierra Nevada. Growth was favoured by 6°C nights compared with
17" C, and apparently by 30" C day temperatures compared with 20" C.
Flowering was stimulated by low night temperatures.
Hiesey relates these responses to the climates in the regions of origin.
The coastal race, occurring in a region of low night temperatures, is
clearly intolerant of 17" C nights, although capable of standing warmer
days than the average in its native area. The San Joaquin valley race
gives evidence of adaptation in its capacity for survival and satisfactory
growth in all combinations of day and night temperature, consonant,
according to Hiesey, with "the great amplitude of seasonal temperature
variation in its native habitat and its all-year activity". The Sierran
race evinces rapid development towards flowering with cool nights and
warm days, again showing adaptation to climate in its place of origin,
where during the growing season the average minimum temperatures
range between 5" C and 9" C and the average maximum from 19" C to
229
change, not the behaviour under a fixed photoperiod. Valuable reviews
of temperature and daylength effects in which ecological aspects are
taken into consideration are those of Wareing (19'56) and Chouard (1960).
Ecotypic variation in temperature responses has been studied experimentally in several species. Part of the current programme of the
Carnegie group (Hiesey et al., 1959, 1960, 1961; Milner et al., 1962) is
concerned with this aspect of racial differentiation, in Mimulw cardinalis.
In an earlier study, Hiesey (1953) has investigated the thermal responses of the aggregate species Achillea millefolium. The reactions of
cloned material of three races originally grown from seed under standard
garden conditions were followed under different combinations of day
and night temperature in growth chambers. The races were a diploid
maritime one, corresponding to the taxonomic subspecies arenicola from
a cool Californian coastal region, an interior valley diploid race native to
the hot San Joaquin Valley of California, and a high montane race
referable to Achillea lanulosa subsp. typica from an altitude of 8 700 ft in
the Sierra Nevada. It may be noted that, strictly, this last race must be
regarded as belonging to a separate ecospecies from the &st two, having
a tetraploid chromosome number, but much of the interest of the study
arises from its inclusion. The responses of the races may be summarized
as follows :
Coastal. Good growth and flowering was attained in night temperatures of 6" C in combination with 20" C or 30" C days; growth and flowering were inhibited with night temperatures of 17" C.
Interior valley. Good growth was attained with 6" or 17OC night
temperature, while performance was slightly better with day temperatures of 20" C than with 30" C. Flowering was delayed somewhat when
6" C nights were combined with 20" C days.
Sierra Nevada. Growth was favoured by 6°C nights compared with
17" C, and apparently by 30" C day temperatures compared with 20" C.
Flowering was stimulated by low night temperatures.
Hiesey relates these responses to the climates in the regions of origin.
The coastal race, occurring in a region of low night temperatures, is
clearly intolerant of 17" C nights, although capable of standing warmer
days than the average in its native area. The San Joaquin valley race
gives evidence of adaptation in its capacity for survival and satisfactory
growth in all combinations of day and night temperature, consonant,
according to Hiesey, with "the great amplitude of seasonal temperature
variation in its native habitat and its all-year activity". The Sierran
race evinces rapid development towards flowering with cool nights and
warm days, again showing adaptation to climate in its place of origin,
where during the growing season the average minimum temperatures
range between 5" C and 9" C and the average maximum from 19" C to