cuttings in the third growth cycle on 12 hour photoperiod at
24°/19°C suggests that flower initiation might have occurred
at a later date in the 16 hour photoperiod treatment at this
temperature if the experiment had been continued. How
ever, this was precluded because the plants became too large
for the growth chambers. The fact that, in the field, some
flowering does occur during long days of summer cautions
against too ready an acceptance of the qualitative nature of
the photoperiodic response. In the field, however, the
increase in day length from spring to summer is gradual and
it is possible that the observed flowers of summer relate to a
residual effect of short days. It is well known that some very
sensitive plants (Xanthium pennsylvanicum (SALISBURY,
1963); Latium temulentum (EVANS, 1960)) initiate flowers
after exposure to a single inductive photoperiod, but the
quantity of flowers is greatly increased and the time to
clearly visible flower initiation is very much reduced with
exposure to more than one inductive photoperiod.