H E T E R O B L A S T I C D E V E L O P M E N T I N P L A N T S
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very distinctive juvenile and adult stages, with the trifoliate foliage
leaves of the former reduced to spines in the latter. It was found that
the longer photoperiods promoted the development of the adult stage,
this effect being even more marked with the supplementary low intensity
light than in the long days at the higher light intensity. It is clear that
the results are a genuine effect of photoperiod rather than merely the
result of an increase in the total amount of light available for photosynthesis.
It is not known how the photoperiod causes this response, although
in view of comparable effects of changes in humidity and temperature,
some important change in metabolism might be expected. It is also to
be expected that as with the reproductive response, different species will
react differently to photoperiod.
2. The Effects of Temperature
Since temperature can affect many different processes, it is scarcely
surprising that different responses to temperature changes have been
reported. In general, it might be assumed that increases in growth and
respiration rates would result in a depletion of carbohydrate reserves,
with corresponding morphological effects. In some cases a reversion to
a more juvenile type of structure might ensue. Conversely, at a stage
when development is limited mainly by growth processes there might
even be a stimulation of heteroblastic changes.
As an example of reversion to a more juvenile condition of the leaves,
in Ipomoea, Njoku (1957) observed that plants grown at higher temperatures had a greater rate of leaf production and on a nodal basis produced
less deeply lobed leaves than the control plants. In Ranunculus hirtus,
Fisher (1960) found that similar effects were produced by relatively
high temperature, low light intensity, or high humidity. Rumball (1961)
induced the formation of adult foliage from juvenile shoots of Podocarpus dacryoides by immersing them at weekly intervals in water at
42°C. This effect of temperature is presumably of a different nature
from that obtained in the long exposure in the other species cited above.
Sustained high temperatures promote the development of the expanded
leaf form in the heterophyllous aquatic species Ranunculus flabellaris
(Bostrack and Millington, 1962).
3. Effects of Water Supply
It is known from the work of many early investigators that a high
atmospheric humidity favors the retention of the juvenile stage in many
heteroblastic plants, especially when the adult stages are strongly
xeromorphic. An abundant water supply to the roots, however, does not
restrict heteroblastic development in normal atmospheres.
155
very distinctive juvenile and adult stages, with the trifoliate foliage
leaves of the former reduced to spines in the latter. It was found that
the longer photoperiods promoted the development of the adult stage,
this effect being even more marked with the supplementary low intensity
light than in the long days at the higher light intensity. It is clear that
the results are a genuine effect of photoperiod rather than merely the
result of an increase in the total amount of light available for photosynthesis.
It is not known how the photoperiod causes this response, although
in view of comparable effects of changes in humidity and temperature,
some important change in metabolism might be expected. It is also to
be expected that as with the reproductive response, different species will
react differently to photoperiod.
2. The Effects of Temperature
Since temperature can affect many different processes, it is scarcely
surprising that different responses to temperature changes have been
reported. In general, it might be assumed that increases in growth and
respiration rates would result in a depletion of carbohydrate reserves,
with corresponding morphological effects. In some cases a reversion to
a more juvenile type of structure might ensue. Conversely, at a stage
when development is limited mainly by growth processes there might
even be a stimulation of heteroblastic changes.
As an example of reversion to a more juvenile condition of the leaves,
in Ipomoea, Njoku (1957) observed that plants grown at higher temperatures had a greater rate of leaf production and on a nodal basis produced
less deeply lobed leaves than the control plants. In Ranunculus hirtus,
Fisher (1960) found that similar effects were produced by relatively
high temperature, low light intensity, or high humidity. Rumball (1961)
induced the formation of adult foliage from juvenile shoots of Podocarpus dacryoides by immersing them at weekly intervals in water at
42°C. This effect of temperature is presumably of a different nature
from that obtained in the long exposure in the other species cited above.
Sustained high temperatures promote the development of the expanded
leaf form in the heterophyllous aquatic species Ranunculus flabellaris
(Bostrack and Millington, 1962).
3. Effects of Water Supply
It is known from the work of many early investigators that a high
atmospheric humidity favors the retention of the juvenile stage in many
heteroblastic plants, especially when the adult stages are strongly
xeromorphic. An abundant water supply to the roots, however, does not
restrict heteroblastic development in normal atmospheres.