Agriculture Reference
In-Depth Information
(a)
5
Figure 5.9 Leaf retention in the lower juvenile
branches of Fagus sylvatica (beech). The lower
branches were produced when the tree was juvenile.
Compare these with the upper branches, which grew
when the tree reached adulthood and therefore do not
show leaf retention
(b)
as Fagus sylvatica (beech) and Carpinus betulus
(hornbeam), where the phenomenon is exaggerated
(Figure 5.9).
In propagation , juvenility is related to rooting
success. Softwood cuttings are non-flowering and root
easily but as the season progresses and shoots switch
from juvenile to flowering growth, rooting becomes
more difficult. Rooting hormones may need to be
applied to stimulate rooting in semi-ripe and hardwood
cuttings for this reason. Since flowering growth often
roots less easily, flowers and flower buds must be
removed if juvenile material is not available. Adult
growth should be removed from stock plants (see
p. 133) to leave the more successful juvenile growth
for cutting.
Juvenility may also affect pest and disease
resistance . In Dutch elm disease, juvenile elm trees
are resistant but they succumb at around 15-20 years
old when they are mature. In some Citrus such as
lemon , juvenile non-fruiting stems grow vigorously and
are thorny for defence against herbivores unlike the
Figure 5.8 (a) Eucalyptus leaves. Left: juvenile leaves;
right: adult leaves. (b) Juvenile leaves in Catalpa
bignonioides 'Aurea'
are also common in conifer species, where the
complete appearance of the plant is altered by the
change in leaf form - for example, in Chamaecyparis
fletcheri and many Juniperus species such as J.
chinensis . In the genera Chamaecyparis and Thuja ,
the juvenile condition can be achieved permanently
by repeated vegetative propagation producing plants
called retinospores , which are used as decorative
features in the garden.
Juvenile growth is non-reproductive
(vegetative) growth, whereas adult growth is
reproductive (flowering).
Leaf retention is sometimes a characteristic of
juvenility and can be significant in species such
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