Agriculture Reference
In-Depth Information
When illuminated from above, auxin
produced in the shoot tip is translocated
to an area behind the tip where it is
evenly distributed across the stem.
This brings about cell enlargement and
enables the stem to grow up towards
the light
Auxin
Figure. 5.5 Germinating tomato seed showing
the simpler 'seed leaves' or cotyledons which are
produced fi rst, followed by the true leaves
Auxin
When illuminated from the side, auxin
accumulates on the shaded side of the
stem causing cell expansion on that side.
The stem therefore grows larger here
and bends towards the light.
A greater concentration of a substance called auxin
in the shaded part of the stem causes the extended
growth (Figure 5.6). Roots display positive geotropic
and negative phototropic responses while shoots
display the reverse, they are positively phototropic and
negatively geotropic.
Figure 5.6 Positive phototropism in shoots
Juvenile growth
The early growth stage or juvenile stage is a
period after germination that is capable of rapid
vegetative growth and is non-reproductive ,
that is, non-flowering. By putting its energies into
growth rather than producing seeds, the plant can
establish itself more effectively in competition with
others. Juvenile growth can be characterized by
certain physical appearances and activities that are
different from those found in the later stages or in
adult growth. Plant growth habit may differ; the
juvenile stem of Hedera helix (ivy), for example, tends
to grow horizontally and is vegetative in nature with
adventitious roots for climbing, and its energy is put
into internode extension rather than leaves. Adult
growth is vertical, with shortened internodes, large
leaves and terminal shoots bearing flowers and fruit.
Often leaf shapes vary, for example the juvenile
leaf of ivy is three-lobed while the adult leaf is more
oval, as shown in Figure 5.7. The attractive juvenile
leaves of Eucalyptus vary greatly from the adult
leaves (Figure 5.8a). In Eucalyptus gunnii they are
round and often without petioles (sessile) compared
Figure 5.7 Left: juvenile growth showing adventitious
roots and lobed leaf; right: adult growth showing
fl owers and entire leaf in Hedera helix (ivy)
with the adult plant and are much used by florists,
while in Catalpa bignonioides the juvenile leaves
are much larger making an attractive foliage display
at the expense of flowers (Figure 5.8b). Other leaf
characteristics such as colour and arrangement on
the stem may differ. Differences in juvenile growth
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