Agriculture Reference
In-Depth Information
Fused testa (seed
coat) and pericarp
(fruit wall)
Starchy endosperm
Single cotyledon
Plumule
Radicle
Figure 8.13
Structure of a monocotyledonous seed
Zea mays
(sweetcorn). The kernel is actually a fruit enclosing
the seed within
up water and nutrients, and a
plumule
, which
develops into the shoot system, bearing leaves for
photosynthesizing and fl owers for seed and fruit
production. The region between the cotyledons
and the radicle is the
hypocotyl
while the short
length of stem between the cotyledons and the
shoot is termed the
epicotyl
. A single
cotyledon
will be found in monocotyledons, while two are
present as part of the embryo of dicotyledons. The
cotyledons may occupy a large part of the seed,
such as in
Vicia faba
(broad bean), and act as the
food store for the embryo. In other seeds, such
as
Zea mays
(sweetcorn), the single cotyledon
remains small and the food store is provided by
another tissue called the
endosperm
which is not
part of the embryo (Figure 8.13).
X
The
testa
, also known as the seed coat, is formed
from the outer layers of the ovule
after fertilization.
It is waterproof and airtight and may contain
germination inhibitors which enable seeds to stay
dormant over winter.
X
The
micropyle
: this is a weakness in the testa
where
water uptake occurs triggering germination.
X
The
hilum
:
this is a scar on the testa where the
seed was attached to
the
fruit.
is often stored as the carbohydrate starch,
formed from sugars as the seed matures - for
example, in peas and beans. Other seeds, such
as sunflowers, contain high proportions of
fats and oils, and proteins are often present in
varying proportions. These substances store
energy in a very concentrated form, which is
released through the process of respiration
when the seed germinates, fuelling rapid growth
(see Chapter 9). The seed is also a rich store of
nutrients, such as phosphate, which it requires
for seedling growth (see p. 168). This explains
why seeds are such a useful foodstuff for
humans too.
8
The fruit
The development of a fruit involves either the
expansion of the ovary into a juicy
succulent
structure,
or the tissues becoming hard and
dry
. Fruits provide
a means of protection and often a means of dispersal
for the seeds they contain and may also contribute to
delayed germination through dormancy. Some dry fruits
split to release their seeds (described as
dehiscent
)
while others rely on the fruit coat being broken down
to release the seeds (described as
indehiscent
) (Figure
8.14). Some methods of dispersal include:
X
Explosive
or self-dispersed: the fruit splits open
propelling the seeds into the air, e.g.
Cytisus
(broom),
Lupinus
(lupin),
Lathyrus odoratus
(sweet
pea),
Erysimum
(wallfl ower),
Lunaria annua
(honesty),
Cardamine hirsuta
(hairy bittercress) and
Geranium
.
Food storage in seeds
In some species, such as
grasses
and
Ricinus
communis
(castor oil plant), the food of the
seed
is found in a different tissue from the
cotyledons, which is called the endosperm.
Plant food in either cotyledons or endosperm
