236
J. HE S L 0 P-H A R R I S 0 N
IX. It is evident that in response to photoperiod the latitudinal variation follows the same pattern as that discussed above for various grass
and tree species.
TABLE IX
Phenology of Oxyria digyna Plants from Four Populations in
the "Arctic" a,nd "Alpine" Growth Chambers. [Data of Mooney
and Billings, 1961.1
Latitude of
Percentage flowering after:
population
4 weeks
t ? weeks
16 weeks
Arctic
Alpine
Arctic
Alpine
Arctic
Alpine
38" 42' N
20
60
80
100
100
100
41" 20' N
30
30
100
80
100
100
48' 42' N
10
0
90
20
100
60
69" 60' N
30
0
100
0
100
0
( e ) The northern populations are substantially less tolerant of high
temperatures than are the southern ones. This is illustrated by the
summer death rate curves of the northern and southern populations
compared in Fig. 7.
In discussing the general performance of plants from the different
populations, Mooney and Billings conclude that the evidence indicates a
close adjustment to the specific light climate of the habitat through adaptations in growth and flowering response, perennating bud formation,
I
I
I
I
I
Date
FIQ. 7. Summer death rate of plants of two northern and two southern populations of
OXYT~U digyna under greenhouse conditions in Durham, North Carolina.
61" 23' N; -.-. -. 68" 56' N; --------lo 20' N; - - - - - - - - 48" 42' N. From
Mooney and Billings (1961).