HETEROBLASTIC DEVELOPMENT IN PLANTS
163
Stoutemyer and Britt (1961) that reversion occurs more readily on
budding than grafting is also in line with a nutritional interpretation.
The effect of the juvenile shoots in Frank and Renner 's culture solution
experiments could conceivably result from a stimulation of the rooting
of the adult twigs, particularly since Hess (1957) demonstrated that
juvenile forms of Hedera helix contain a rooting cofactor which is
absent from the adult phase. In further studies on ivy, Robbins (1957b)
discovered that treatment of the arborescent form Hedera canariensis
variegata var. arborescens with gibberellic acid (GA) resulted in the
development of branches with juvenile characteristics. Similar results
were obtained later (Robbins, 1960) with arborescent forms of Hedera
helix. In the discussion of his results Robbins suggested that GA may be
comparable with the youth hormone of Frank and Renner. Working
with Hedera canariensis, Goodin and Stoutemyer (1961) confirmed
Robbing results, but found that although there was virtually complete
reversion at 70°F, at 50° F there was almost no reversion although
reasonably good growth was still maintained. Stoutemyer et al. (1961)
established, in addition, that the morphological reversion is associated
with an improved rooting capacity of cuttings from the reverted shoots.
Other workers have studied the effects of GA on heteroblastic development in different plants, with apparently conflicting results. Scurfield
and Moore (1958) found that seedlings of Eucalyptus species when
treated with GA developed the adult type of foliage earlier than did the
control plants. Njoku (1958), however, obtained a prolongation of the
juvenile phase of seedlings of Ipomoea not only with GA, which was
most effective, but also with various other growth substances. A striking
example of reversion following GA treatment was described by Trippi
(1963e) for Acacia melanoxylon in which 3-year-old plants bearing
phyllodes reverted to the production of pinnate leaves. The effects of
GA in promoting the bolting of semi-rosette plants are now well known
(see review by Sachs, 1965). In the heterophyllous water plant Proserpinaca palustris, the effects of GA are also similar to those of long days
(Wallenstein and Albert, 1963).
Gibberellic acid, as the gibberellin most frequently utilized, has thus
extensive effects on heteroblastic development and allied aspects of
morphogenesis, but its behavior is not consistently that of a juvenility
hormone. In Marsilea, for example, it may even have two different
effects on the ontogeny of a single leaf, promoting leaf segmentation
but retarding differentiation (Section III,C,1).
4. Conclusions
The evidence discussed above demonstrates that there are important
physiological changes during plant ontogeny. In this sense, there is
indeed a physiological aging of the plant. But there is no convincing
163
Stoutemyer and Britt (1961) that reversion occurs more readily on
budding than grafting is also in line with a nutritional interpretation.
The effect of the juvenile shoots in Frank and Renner 's culture solution
experiments could conceivably result from a stimulation of the rooting
of the adult twigs, particularly since Hess (1957) demonstrated that
juvenile forms of Hedera helix contain a rooting cofactor which is
absent from the adult phase. In further studies on ivy, Robbins (1957b)
discovered that treatment of the arborescent form Hedera canariensis
variegata var. arborescens with gibberellic acid (GA) resulted in the
development of branches with juvenile characteristics. Similar results
were obtained later (Robbins, 1960) with arborescent forms of Hedera
helix. In the discussion of his results Robbins suggested that GA may be
comparable with the youth hormone of Frank and Renner. Working
with Hedera canariensis, Goodin and Stoutemyer (1961) confirmed
Robbing results, but found that although there was virtually complete
reversion at 70°F, at 50° F there was almost no reversion although
reasonably good growth was still maintained. Stoutemyer et al. (1961)
established, in addition, that the morphological reversion is associated
with an improved rooting capacity of cuttings from the reverted shoots.
Other workers have studied the effects of GA on heteroblastic development in different plants, with apparently conflicting results. Scurfield
and Moore (1958) found that seedlings of Eucalyptus species when
treated with GA developed the adult type of foliage earlier than did the
control plants. Njoku (1958), however, obtained a prolongation of the
juvenile phase of seedlings of Ipomoea not only with GA, which was
most effective, but also with various other growth substances. A striking
example of reversion following GA treatment was described by Trippi
(1963e) for Acacia melanoxylon in which 3-year-old plants bearing
phyllodes reverted to the production of pinnate leaves. The effects of
GA in promoting the bolting of semi-rosette plants are now well known
(see review by Sachs, 1965). In the heterophyllous water plant Proserpinaca palustris, the effects of GA are also similar to those of long days
(Wallenstein and Albert, 1963).
Gibberellic acid, as the gibberellin most frequently utilized, has thus
extensive effects on heteroblastic development and allied aspects of
morphogenesis, but its behavior is not consistently that of a juvenility
hormone. In Marsilea, for example, it may even have two different
effects on the ontogeny of a single leaf, promoting leaf segmentation
but retarding differentiation (Section III,C,1).
4. Conclusions
The evidence discussed above demonstrates that there are important
physiological changes during plant ontogeny. In this sense, there is
indeed a physiological aging of the plant. But there is no convincing