V. DETERMINING FACTORS IN
CELL GROWTH
247
may play a part, biochemically, in the 'division of labour' in the
vegetative apex and that photoperiodically-induced changes therein
must alter its function. They also point especially to the loss of capacity
for growth in length and the shift to reproductive instead of vegetative
buds and the loss of apical dominance as the visible concomitants of
the flowering response.
While the events of flowering emerge from a changed pattern of
growth in the shoot apex, the actions which 'trigger them off' clearly
emanate from the subjacent tissue and are transmitted to the formative
regions of the apex. In such responses to temperature as vernalization,
it may suffice if the stimulus affects a very localized region. Indeed,
many biennial plants, such as Brassica can be irreversibly switched from
the vegetative to the reproductive state by a low temperature treatment.
However, Curtis and Chang (1930) showed that this temperature treatment need only include the apex and its immediately subjacent tissue.
It is in this area that the stimulus of cold treatment may be replaced
by gibberellin in those cases (Lang, 1959) where flowering requires the
elevation of an axis from an otherwise dwarf shoot.
Where the floral induction stimulus involves light, leaves are the
organs of perception. However, in the parasitic leafless dodder plant
(Cuscuta reflexa), an excised stem tip produced flowers and fruits when
the cultures were kept in either total darkness or were exposed to a
shortday treatment (Baldev, 1959). Since in nature, flowering of the
host does not bring about floral induction in the parasite, it is assumed
that the flowering response of Cuscuta is independent ofthat of the host.
Obviously, under culture conditions, it is the leafless stem tip that
perceives the photostimulus. One cannot enter here upon a discussion of
this important problem, with its contrasted and distinctive high and low
light effects, the decisive importance of the duration of darkness and
the now well recognized role of red light to stimulate and 'far red' light
to reverse the stimulus. A lucid recent summary has been given by
Lang (1959). It is apparent, however, that the light stimulus must be
transferred to the active centres of growth by some substance or
substances which can change the distribution of their growth. The apex
is dependent on subjacent tissues (leaves, in particular), for both
stimuli and metabolites. It is, therefore, of interest that the main
morphogenetic stimuli (photoperiod, night temperature) which cause
such profound changes in the apex do cause metabolic changes in the
tissues of the shoot. Significantly enough, metabolic changes due to
photoperiod and night temperature are detectable in that area of
metabolism that connects carbohydrate metabolism and nitrogen
metabolism. In the peppermint plant (Mentha piperita), the balance
between glutamine and asparagine and amongst certain keto acids is
CELL GROWTH
247
may play a part, biochemically, in the 'division of labour' in the
vegetative apex and that photoperiodically-induced changes therein
must alter its function. They also point especially to the loss of capacity
for growth in length and the shift to reproductive instead of vegetative
buds and the loss of apical dominance as the visible concomitants of
the flowering response.
While the events of flowering emerge from a changed pattern of
growth in the shoot apex, the actions which 'trigger them off' clearly
emanate from the subjacent tissue and are transmitted to the formative
regions of the apex. In such responses to temperature as vernalization,
it may suffice if the stimulus affects a very localized region. Indeed,
many biennial plants, such as Brassica can be irreversibly switched from
the vegetative to the reproductive state by a low temperature treatment.
However, Curtis and Chang (1930) showed that this temperature treatment need only include the apex and its immediately subjacent tissue.
It is in this area that the stimulus of cold treatment may be replaced
by gibberellin in those cases (Lang, 1959) where flowering requires the
elevation of an axis from an otherwise dwarf shoot.
Where the floral induction stimulus involves light, leaves are the
organs of perception. However, in the parasitic leafless dodder plant
(Cuscuta reflexa), an excised stem tip produced flowers and fruits when
the cultures were kept in either total darkness or were exposed to a
shortday treatment (Baldev, 1959). Since in nature, flowering of the
host does not bring about floral induction in the parasite, it is assumed
that the flowering response of Cuscuta is independent ofthat of the host.
Obviously, under culture conditions, it is the leafless stem tip that
perceives the photostimulus. One cannot enter here upon a discussion of
this important problem, with its contrasted and distinctive high and low
light effects, the decisive importance of the duration of darkness and
the now well recognized role of red light to stimulate and 'far red' light
to reverse the stimulus. A lucid recent summary has been given by
Lang (1959). It is apparent, however, that the light stimulus must be
transferred to the active centres of growth by some substance or
substances which can change the distribution of their growth. The apex
is dependent on subjacent tissues (leaves, in particular), for both
stimuli and metabolites. It is, therefore, of interest that the main
morphogenetic stimuli (photoperiod, night temperature) which cause
such profound changes in the apex do cause metabolic changes in the
tissues of the shoot. Significantly enough, metabolic changes due to
photoperiod and night temperature are detectable in that area of
metabolism that connects carbohydrate metabolism and nitrogen
metabolism. In the peppermint plant (Mentha piperita), the balance
between glutamine and asparagine and amongst certain keto acids is