232
J. HESLOP-HARRISON
periods recorded by Olmsted are shown in Table VII, the strains being
ordered according to latitude of origin. The plants of the most southerly
origin show a short-day type of reaction, and those from the northern
sites are evidently accelerated in development towards flowering by
long days. The Oklahoma and New Mexico samples show diversity in
response, perhaps as Olmsted suggests because they contain both longday and day-intermediate genotypes. In respect to vegetative development, the different races revealed a consistent pattern of response:'with
increasing latitude of origin, growth was increasingly suppressed under
the shorter photoperiod. No internode elongation occurred in the
primary axis of plants originating north of 39" N latitude in 9-h days,
and none in plants originating north of 414" N in 13-h days. Plants from
southern latitudes elongated in all photoperiods.
The study of geographical variation in photoperiodic response in
Andropogon scoparius (Schizachyrium scoparium) by Olmsted's pupil
Larsen (1947) revealed a pattern of responses broadly similar to'that
observed in Bouteloua. In a comparison of samples from twelve stations
spanning a latitudinal range of Zl", plants from northern origins were
found to be accelerated towards flowering by long days; in the case of
Andropogon scoparius, however, the southern strains behaved as dayintermediate plants. Again short photoperiods marked suppressed
growth in plants from northern localities.
While Olmsted's and Larsen's results were obtained under experimental conditions which cannot be compared directly with any natural
situations, they do indicate quite clearly that the differentiation of races
adapted to different lengths of growing season in the central grasslands
of North America has involved the adjustment of photoperiodic response. The transplant experiments of McMillan (1956a, b, 1959) indicate that several other grass species show similar kinds of ecotypic
variation, periodicity being adapted to the south to north decrease in
the frost-free growing season and to a similar east to west gradient
determined by decreasing precipitation during the later part of the
summer.
Photoperiodic adaptation has also been demonstrated in tree species
which span wide latitudinal ranges. Vaartaja (1954, 1959) found variation in the responses of northern and southern races in several tree
species tested for growth rate and onset of dormancy under various
photoperiods. Samples were grown from seed derived from native
stands, and the seedlings were exposed to natural daylight for 11 h
extended by artificial light to give photoperiods of 12, 14, 16 and 18 h.
An example of Vaartaja's results is given in Table VIII, where the fresh
weights of the tops of plants of Betulapapyrijera from two localities, 17"
of latitude apart, grown for 15 weeks under various photoperiods are
J. HESLOP-HARRISON
periods recorded by Olmsted are shown in Table VII, the strains being
ordered according to latitude of origin. The plants of the most southerly
origin show a short-day type of reaction, and those from the northern
sites are evidently accelerated in development towards flowering by
long days. The Oklahoma and New Mexico samples show diversity in
response, perhaps as Olmsted suggests because they contain both longday and day-intermediate genotypes. In respect to vegetative development, the different races revealed a consistent pattern of response:'with
increasing latitude of origin, growth was increasingly suppressed under
the shorter photoperiod. No internode elongation occurred in the
primary axis of plants originating north of 39" N latitude in 9-h days,
and none in plants originating north of 414" N in 13-h days. Plants from
southern latitudes elongated in all photoperiods.
The study of geographical variation in photoperiodic response in
Andropogon scoparius (Schizachyrium scoparium) by Olmsted's pupil
Larsen (1947) revealed a pattern of responses broadly similar to'that
observed in Bouteloua. In a comparison of samples from twelve stations
spanning a latitudinal range of Zl", plants from northern origins were
found to be accelerated towards flowering by long days; in the case of
Andropogon scoparius, however, the southern strains behaved as dayintermediate plants. Again short photoperiods marked suppressed
growth in plants from northern localities.
While Olmsted's and Larsen's results were obtained under experimental conditions which cannot be compared directly with any natural
situations, they do indicate quite clearly that the differentiation of races
adapted to different lengths of growing season in the central grasslands
of North America has involved the adjustment of photoperiodic response. The transplant experiments of McMillan (1956a, b, 1959) indicate that several other grass species show similar kinds of ecotypic
variation, periodicity being adapted to the south to north decrease in
the frost-free growing season and to a similar east to west gradient
determined by decreasing precipitation during the later part of the
summer.
Photoperiodic adaptation has also been demonstrated in tree species
which span wide latitudinal ranges. Vaartaja (1954, 1959) found variation in the responses of northern and southern races in several tree
species tested for growth rate and onset of dormancy under various
photoperiods. Samples were grown from seed derived from native
stands, and the seedlings were exposed to natural daylight for 11 h
extended by artificial light to give photoperiods of 12, 14, 16 and 18 h.
An example of Vaartaja's results is given in Table VIII, where the fresh
weights of the tops of plants of Betulapapyrijera from two localities, 17"
of latitude apart, grown for 15 weeks under various photoperiods are