Agriculture Reference
In-Depth Information
Plumule emerges
Epicotyl elongates
Testa splits
Cotyledons
remain below ground
Radicle
(a)
(b)
Foliage leaf
5
Cotyledons (seed
leaves) emerge above
ground shedding testa
Hypocotyl elongates
Remains of discarded testa
Radicle
(c)
Figure 5.4
(a) Hypogeal germination in
Vicia faba
(broad bean); (b) Broad bean on left, French bean on right;
(c) epigeal germination in
Phaseolus vulgaris
(French bean)
must rapidly produce its own food supply and begin to
photosynthesize. It must therefore respond to stimuli
in its environment to establish the correct direction of
growth. Such a response is termed a
tropism
, and is
very important in the early survival of the seedling.
is followed by growth of the root system, which
must quickly take up water and minerals to enable
the shoot system to develop. A seed germinating
near the surface of a growing medium must not put
out roots that grow on to the surface and dry out but
establish roots that grow downwards to tap water
supplies. Conversely,
phototropism
enables the
shoot to grow towards a light source that provides
the energy for photosynthesis. If the light source is to
the side of the plant, a bend takes place in the stem
just below the tip as cells in the stem away from the
light grow larger than those nearer to the light source.
A
tropism
is a directional growth response to an
environmental stimulus.
Geotropism
is a directional growth response to
gravity. The emergence of the radicle from the testa
