162
A. ALLSOPP
adventitious shoots of woody species is well established (see e.g.,
Doorenbos, 1965). Montaldi et al. (1963b) induced callus formation on
adult parts of Passiflora caerulea and observed that the adventitious
shoots produced had juvenile characteristics. Buds differentiated in vitro
on callus of stems from flowering tobacco plants, also gave rise to shoots
with juvenile characteristics. Trippi (1964) obtained a similar formation
of juvenile shoots from the callus formed on decapitated tomato stems.
The seedlings produced from nucellar embryos in polyembryonic species
of Citrus are a frequently cited example of rejuvenation (Swingle, 1932).
As mentioned in the previous section, Trippi (1963d, Trippi et al.,
1965) considers that there is a partial rejuvenation on vegetative reproduction. He believes that rejuvenation, however produced, is a consequence of a disorganization of the individual, which alters the correlations
between nutritional and hormonal substances. One can agree that the
renewal of active growth associated with the formation of adventitious
buds, or with the outgrowth of normal lateral buds, is probably associated
with some dedifferentiation of tissues and corresponding changes in the
metabolic condition.
3. The Role of Hormones
Many authors have explained heteroblastic development in terms of
changes in either the quantity or quality of one or more hormones or
similar substances. It is known that auxins can replace the correlative
effects of older leaves, and that other substances are necessary for leaf
expansion. But there is no evidence that under normal conditions the
rate of heteroblastic development is limited by either of these two
groups of substances. Various workers, e.g., Njoku (1957) and Montaldi
et al. (1963a), have shown that changes in leaf shape are mediated by
substances produced from expanded leaves, but as pointed out above
there is no reason to believe that these substances are other than the
carbohydrate produced by photosynthesis.
In studies on rejuvenation in ivy, Frank and Renner (1956) discovered that reversion occurred in adult scions not only when grafted on
seedling stocks, as described by Doorenbos (1954), but even when they
were merely placed in the same culture solution as juvenile shoots. In
explanation of these results, Frank and Renner proposed the working
hypothesis that the juvenile forms produce a youth hormone (Jugendsubstanz) which can induce the development of juvenile characteristics
in adult shoots. There is no necessity, however, to invoke a special
hormone. The results might well be explained by a disturbance of the
normal nutritional relationships or of other correlations. It is probably
significant that in Frank and Renner's experiments unrooted twigs retained adult characteristics. Furthermore, Robbins (1960) obtained
reversion on heavy pruning of arborescent ivy. The demonstration by
A. ALLSOPP
adventitious shoots of woody species is well established (see e.g.,
Doorenbos, 1965). Montaldi et al. (1963b) induced callus formation on
adult parts of Passiflora caerulea and observed that the adventitious
shoots produced had juvenile characteristics. Buds differentiated in vitro
on callus of stems from flowering tobacco plants, also gave rise to shoots
with juvenile characteristics. Trippi (1964) obtained a similar formation
of juvenile shoots from the callus formed on decapitated tomato stems.
The seedlings produced from nucellar embryos in polyembryonic species
of Citrus are a frequently cited example of rejuvenation (Swingle, 1932).
As mentioned in the previous section, Trippi (1963d, Trippi et al.,
1965) considers that there is a partial rejuvenation on vegetative reproduction. He believes that rejuvenation, however produced, is a consequence of a disorganization of the individual, which alters the correlations
between nutritional and hormonal substances. One can agree that the
renewal of active growth associated with the formation of adventitious
buds, or with the outgrowth of normal lateral buds, is probably associated
with some dedifferentiation of tissues and corresponding changes in the
metabolic condition.
3. The Role of Hormones
Many authors have explained heteroblastic development in terms of
changes in either the quantity or quality of one or more hormones or
similar substances. It is known that auxins can replace the correlative
effects of older leaves, and that other substances are necessary for leaf
expansion. But there is no evidence that under normal conditions the
rate of heteroblastic development is limited by either of these two
groups of substances. Various workers, e.g., Njoku (1957) and Montaldi
et al. (1963a), have shown that changes in leaf shape are mediated by
substances produced from expanded leaves, but as pointed out above
there is no reason to believe that these substances are other than the
carbohydrate produced by photosynthesis.
In studies on rejuvenation in ivy, Frank and Renner (1956) discovered that reversion occurred in adult scions not only when grafted on
seedling stocks, as described by Doorenbos (1954), but even when they
were merely placed in the same culture solution as juvenile shoots. In
explanation of these results, Frank and Renner proposed the working
hypothesis that the juvenile forms produce a youth hormone (Jugendsubstanz) which can induce the development of juvenile characteristics
in adult shoots. There is no necessity, however, to invoke a special
hormone. The results might well be explained by a disturbance of the
normal nutritional relationships or of other correlations. It is probably
significant that in Frank and Renner's experiments unrooted twigs retained adult characteristics. Furthermore, Robbins (1960) obtained
reversion on heavy pruning of arborescent ivy. The demonstration by