160
A. ALLSOPP
b. Experimental Studies. Various experimental studies have been made
on the physiological aging of the apical meristem. Although woody plants,
as mentioned above, show a pronounced stability when propagated as
cuttings or grafts, some investigations have demonstrated that a reversion
to the juvenile condition can occur. Thus, Doorenbos (1954) grafted
adult shoots of ivy {Hedera helix) on 2-year-old seedlings and seedling
shoots on adult plants. The juvenile parts were not affected by the adult
partner, but the new growth from the adult parts, whether stock or
scion, had juvenile characteristics. This reversion can evidently occur
only under certain environmental conditions for Stoutemyer and Britt
(1961) failed to obtain reversion of adult scions of Hedera helix and
H. canariensis in plants grown outdoors under relatively cool conditions.
On the other hand, growth of the scions was largely juvenile when the
grafted plants were maintained in a glasshouse at 80°F. The amount of
mature tissue originally present in the scion was also important. Reversion occurred much more readily after budding than after ordinary
branch grafting.
In studies on apical senescence in Alaska pea (Pisum sativum), Lockhart and Gottschall (1961) grafted stem tips from young plants on older
stems and vice versa. The young tips resumed normal growth, but there
was no rejuvenation of old tips, which grew only as much as tips left
intact on plants of corresponding age.
Habermann (1962) carried out similar experiments with sunflower
(Helianthus annuus). Seedling tips were grafted on to stocks of various
age and tips from plants of increasing age were grafted on seedlings. In
the first type of graft there was accelerated aging of the meristem, in
that the total amount of vegetative development of the various grafted
plants remained approximately constant, i.e., the total number of nodes
remained the same. In the grafts of older meristems, however, there
was a prolongation of the vegetative life span in that they produced
a greater number of nodes than they would had they remained in their
original position. Habermann considers that these results support the
hypothesis that there is an aging of apical meristems. There is certainly
evidence of apical changes, but it seems more likely that there is an
effect of the stock on apical strengthening of the scion rather than a
physiological aging. If we accept this view, the strengthening of the
seedling apex is accelerated by the older and larger stocks, and conversely a seedling stock is unable to maintain the normal rate of enlargement of an older apex.
Lewin et al. (1963a,b) studied the effects of grafting on leaf shape
in Passifiora caerulea. Mature apices were grafted both on seedlings and
on apical regions of adult plants, and seedling apices were grafted at the
base or apex of adult plants. The mature apices, which normally form
A. ALLSOPP
b. Experimental Studies. Various experimental studies have been made
on the physiological aging of the apical meristem. Although woody plants,
as mentioned above, show a pronounced stability when propagated as
cuttings or grafts, some investigations have demonstrated that a reversion
to the juvenile condition can occur. Thus, Doorenbos (1954) grafted
adult shoots of ivy {Hedera helix) on 2-year-old seedlings and seedling
shoots on adult plants. The juvenile parts were not affected by the adult
partner, but the new growth from the adult parts, whether stock or
scion, had juvenile characteristics. This reversion can evidently occur
only under certain environmental conditions for Stoutemyer and Britt
(1961) failed to obtain reversion of adult scions of Hedera helix and
H. canariensis in plants grown outdoors under relatively cool conditions.
On the other hand, growth of the scions was largely juvenile when the
grafted plants were maintained in a glasshouse at 80°F. The amount of
mature tissue originally present in the scion was also important. Reversion occurred much more readily after budding than after ordinary
branch grafting.
In studies on apical senescence in Alaska pea (Pisum sativum), Lockhart and Gottschall (1961) grafted stem tips from young plants on older
stems and vice versa. The young tips resumed normal growth, but there
was no rejuvenation of old tips, which grew only as much as tips left
intact on plants of corresponding age.
Habermann (1962) carried out similar experiments with sunflower
(Helianthus annuus). Seedling tips were grafted on to stocks of various
age and tips from plants of increasing age were grafted on seedlings. In
the first type of graft there was accelerated aging of the meristem, in
that the total amount of vegetative development of the various grafted
plants remained approximately constant, i.e., the total number of nodes
remained the same. In the grafts of older meristems, however, there
was a prolongation of the vegetative life span in that they produced
a greater number of nodes than they would had they remained in their
original position. Habermann considers that these results support the
hypothesis that there is an aging of apical meristems. There is certainly
evidence of apical changes, but it seems more likely that there is an
effect of the stock on apical strengthening of the scion rather than a
physiological aging. If we accept this view, the strengthening of the
seedling apex is accelerated by the older and larger stocks, and conversely a seedling stock is unable to maintain the normal rate of enlargement of an older apex.
Lewin et al. (1963a,b) studied the effects of grafting on leaf shape
in Passifiora caerulea. Mature apices were grafted both on seedlings and
on apical regions of adult plants, and seedling apices were grafted at the
base or apex of adult plants. The mature apices, which normally form