230
J. HESLOP-HARRISON
24' C. The rapid development of the Sierran race compared with that
from the Californian coast can be related to the much shorter growing
season. Hiesey encountered marked variation between individuals in
this study, but comments that the differences between the races overshadowed the within-race variation.
Clinal variation in the chilling requirements for bud break in Acer
rubrum has been demonstrated by Perry and Wang Chi Wu (1960). the
red maple has an extremely wide latitude range in North America, from
southern Florida to Canada. In the northern part of the range, the frostfree season is less than 100 days, and in the southern no frosts are experienced at all. Chilling responses were studied by Perry and Wang
Chi Wu in progenies reared from seed samples collected at eleven stations spanning the greater part of the latitudinal range. Plants raised
from parents from southern Florida showed no chilling requirement, and
bud break was in fact delayed by chilling treatments. Progeny from
parents from the northern part of New York State mostly failed to
break dormancy without vernalization, and where growth did occur it
was abnormal. Plants from intermediate latitudes showed intermediate
reactions. Among the samples grown without chilling at Gainesville,
Florida, a strong correlation (r = 0.96) was observed between date .of
bud-break and the duration of the frost-free season at the site of origin.
This study reveals clear evidence of a clinal variation in the red maple
in dormancy periods and in response to chilling. The experiments so far
reported are not, however, adequate to elucidate fully the relationship
between latitude, dormancy period and cold requirement. This would
require an analysis of the responses of the populations from the different
latitudes to a range of treatments, along the lines marked out in Olmsted's full study of dormancy in the sugar maple (1951).
Ecotypic variation in respect to chilling requirement for the induction
of flowering has been demonstrated by Ketellapper (1960) in Phalaris
tuberosa. Plants were grown from samples of caryopses collected from
twelve localities throughout the Mediterranean range. Flowering time
under a greenhouse temperature of 17-19" C without effective previous
vernalization was observed, and also flowering time following chilling at
4' C at the three-leaved stage for periods of 0 , 2 , 4 , 6 and 8 weeks, with
subsequent growth in the greenhouse. A high inverse correlation
(r = - 0.94) was found between the percentage of plants flowering without
cold treatment in each sample of c. 75 and the weeks of cold treatment
required for full induction to flower. The duration of cold treatment
required for full induction was also found to be highly correlated
(r = - 0.95) with the average temperature of the coldest month in the
locality of origin. Ketellapper is, however, dubious as to the exact role of
the cold requirement as a regulating factor for flowering time in the
J. HESLOP-HARRISON
24' C. The rapid development of the Sierran race compared with that
from the Californian coast can be related to the much shorter growing
season. Hiesey encountered marked variation between individuals in
this study, but comments that the differences between the races overshadowed the within-race variation.
Clinal variation in the chilling requirements for bud break in Acer
rubrum has been demonstrated by Perry and Wang Chi Wu (1960). the
red maple has an extremely wide latitude range in North America, from
southern Florida to Canada. In the northern part of the range, the frostfree season is less than 100 days, and in the southern no frosts are experienced at all. Chilling responses were studied by Perry and Wang
Chi Wu in progenies reared from seed samples collected at eleven stations spanning the greater part of the latitudinal range. Plants raised
from parents from southern Florida showed no chilling requirement, and
bud break was in fact delayed by chilling treatments. Progeny from
parents from the northern part of New York State mostly failed to
break dormancy without vernalization, and where growth did occur it
was abnormal. Plants from intermediate latitudes showed intermediate
reactions. Among the samples grown without chilling at Gainesville,
Florida, a strong correlation (r = 0.96) was observed between date .of
bud-break and the duration of the frost-free season at the site of origin.
This study reveals clear evidence of a clinal variation in the red maple
in dormancy periods and in response to chilling. The experiments so far
reported are not, however, adequate to elucidate fully the relationship
between latitude, dormancy period and cold requirement. This would
require an analysis of the responses of the populations from the different
latitudes to a range of treatments, along the lines marked out in Olmsted's full study of dormancy in the sugar maple (1951).
Ecotypic variation in respect to chilling requirement for the induction
of flowering has been demonstrated by Ketellapper (1960) in Phalaris
tuberosa. Plants were grown from samples of caryopses collected from
twelve localities throughout the Mediterranean range. Flowering time
under a greenhouse temperature of 17-19" C without effective previous
vernalization was observed, and also flowering time following chilling at
4' C at the three-leaved stage for periods of 0 , 2 , 4 , 6 and 8 weeks, with
subsequent growth in the greenhouse. A high inverse correlation
(r = - 0.94) was found between the percentage of plants flowering without
cold treatment in each sample of c. 75 and the weeks of cold treatment
required for full induction to flower. The duration of cold treatment
required for full induction was also found to be highly correlated
(r = - 0.95) with the average temperature of the coldest month in the
locality of origin. Ketellapper is, however, dubious as to the exact role of
the cold requirement as a regulating factor for flowering time in the