228
J. HESLOP-HARRISON
tions in a suitably protected state of dormancy can obviously be to close
to an ideal solution, necessitating the minimum of direct sdaptive adjustments of the normal prgcesses of growth and nutrition by ensuring
that they progress during the most favourable period of the year.
The adjustment of developmental periodicity to the particular requirements of a local climate is a matter of time-keeping, and it may be
supposed that where survival depends on re-timing, selective pressures
will develop to bring this about. Since seasonal variations in temperature and daylength provide the most reliable clocks in the plant environment, thermal and photoperiodic responses will thus necessarily
become a target for selection, and variation in these responses is therefore to be expected in widely ranging species. We may speak of “photoperiodic” or “thermoperiodid’ races or ecotypes in these species, but the
adaptation is not of course to the photoperiod or thermoperiod in the
sense that these factors are themselves selective; it is that adaptation to
local climate has been achieved by the modification of photoperiodic or
thermoperiodic reactions. This kind of adjustment by itself may not be
adequate to ensure success if more or less inimical conditions prevail
even during the most favourable period of the year. Survival may then
require the acquisition of characteristics more directly adaptive, as well
as the adjustment of periodicity.
As might be expected, the best analysed examples of climatic control
of developmental periodicity have cdncerned bred strains of economic
plants, particularly the cereals and biennial root crops. b o n g perennial
species, forage grasses have been extensively investigated, and the
work of Cooper (1951,1952) on Lolium and Ryle (1963a, b) and Ryle and
Langer (1963a, b) on Phleum is particularly relevant to the problem of
analysing the environmental control of developmental cycles in ecotypes. The work of Wareing (1950a, b, 1951, 1953, 1954) on the photoperiodic responses of woody species is similarly of great genecological
significance.
For the perennial plant of the temperate regions, it is established that
temperature and photoperiod - separately or in conjunction - may
affect the duration of bud dormancy, the period of leaf formation and
stem extension growth, the timing of flowering and fruiting, the cessation of stem extension, and the onset of leaf-fall and resting bud formation. Response to temperature may be immediate, or inductive, as in the
case of vernalization; and the reaction may be to the diurnal as well as
the annual cycle of temperature change. Photoperiodic effects are mostly
inductive. The usual physiological classification of species into “long
day”, “short day”, “day intermediate” and “day neutral’’ with regard
to flower initiation has little direct ecological significance since the important factor in nature is the response to the yearly cycle of daylength
J. HESLOP-HARRISON
tions in a suitably protected state of dormancy can obviously be to close
to an ideal solution, necessitating the minimum of direct sdaptive adjustments of the normal prgcesses of growth and nutrition by ensuring
that they progress during the most favourable period of the year.
The adjustment of developmental periodicity to the particular requirements of a local climate is a matter of time-keeping, and it may be
supposed that where survival depends on re-timing, selective pressures
will develop to bring this about. Since seasonal variations in temperature and daylength provide the most reliable clocks in the plant environment, thermal and photoperiodic responses will thus necessarily
become a target for selection, and variation in these responses is therefore to be expected in widely ranging species. We may speak of “photoperiodic” or “thermoperiodid’ races or ecotypes in these species, but the
adaptation is not of course to the photoperiod or thermoperiod in the
sense that these factors are themselves selective; it is that adaptation to
local climate has been achieved by the modification of photoperiodic or
thermoperiodic reactions. This kind of adjustment by itself may not be
adequate to ensure success if more or less inimical conditions prevail
even during the most favourable period of the year. Survival may then
require the acquisition of characteristics more directly adaptive, as well
as the adjustment of periodicity.
As might be expected, the best analysed examples of climatic control
of developmental periodicity have cdncerned bred strains of economic
plants, particularly the cereals and biennial root crops. b o n g perennial
species, forage grasses have been extensively investigated, and the
work of Cooper (1951,1952) on Lolium and Ryle (1963a, b) and Ryle and
Langer (1963a, b) on Phleum is particularly relevant to the problem of
analysing the environmental control of developmental cycles in ecotypes. The work of Wareing (1950a, b, 1951, 1953, 1954) on the photoperiodic responses of woody species is similarly of great genecological
significance.
For the perennial plant of the temperate regions, it is established that
temperature and photoperiod - separately or in conjunction - may
affect the duration of bud dormancy, the period of leaf formation and
stem extension growth, the timing of flowering and fruiting, the cessation of stem extension, and the onset of leaf-fall and resting bud formation. Response to temperature may be immediate, or inductive, as in the
case of vernalization; and the reaction may be to the diurnal as well as
the annual cycle of temperature change. Photoperiodic effects are mostly
inductive. The usual physiological classification of species into “long
day”, “short day”, “day intermediate” and “day neutral’’ with regard
to flower initiation has little direct ecological significance since the important factor in nature is the response to the yearly cycle of daylength