|Figure 7.6 Seeds: a
range of species, from
top – runner bean, left to
right – leek, artichoke,
tomato, lettuce, Brussels
The seed, resulting from sexual reproduction, creates a new
generation of plants that bear characteristics of both parents.
The plant must survive often through conditions that would be
damaging to a growing vegetative organism. The seed is a means
of protecting against extreme conditions of temperature and
moisture, and is thus the overwintering stage
The basic seed structure is shown in Figure 7.7. The main features
of the seed are:
- embryo, in order to survive the seed must contain a small
immature plant protected by a seed coat;
- testa, the seed coat, is formed from the outer layers of the
ovule after fertilization;
- micropyle, a weakness in the testa, marks the point of entry of
the pollen tube prior to fertilization;
- hilum, this is the point of attachment to the fruit.
The embryo consists of a radicle
, which will develop into the
primary root of the seedling, and a plumule
, which develops
into the shoot system, the two being joined by a region called
. A single seed leaf (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,
e.g. in beans, to act as the food store for the embryo.
|Figure 7.7 The structure of the seed, (a) runner bean seed just
beginning to germinate and showing developing radicle showing a
geotropic response, (b) long section of
bean seed showing structure
In some species, e.g. grasses and Ricinus (castor oil plant), the food
of the seed is found in a different tissue from the cotyledons. This
tissue is called endosperm
and is derived from the fusion of extra cell
nuclei, at the same time as fertilization. Plant food is usually stored
as the carbohydrate, starch, formed from sugars as the seed matures, e.g. in peas and beans. Other seeds, such as sunflowers, contain high
proportions of fats and oils, and proteins are often present in varying
proportions. The seed is also a rich store of nutrients that it requires
when a seedling, such as phosphate.
The seed structure may be specialized for wind dispersal, e.g. members
of the Asteraceae family, including groundsel, dandelion and thistle,
which have parachutes, as does Clematis
woody species such as lime (Tilia
), ash (Fraxinus
), and sycamore (Acer
produce winged fruit. Other seed-pods are explosive, e.g. balsam and
hairy bittercress. Organisms such as birds and mammals distribute
hooked fruits such as goosegrass and burdock, succulent types (e.g.
tomato, blackberry, elderberry), or those that are filled with protein (e.g.
dock). Dispersal mechanisms are summarized in Table 7.1.
Seeds are contained within fruits which provide a means of protection
and, often, dispersal.