Download SEEDS A seed is a dormant embryonic plant. All seeds have several

SEEDS
A seed is a dormant embryonic plant. All seeds have
several features in common:
SEED COAT
This is the outer layer of the seed.
Function: Prevent destruction of the seed by
dehydration or predation
The seed coat develops from tissues called the
integuments – which surround the embryo sac.
The seed coat is a multilayered tissue which can include
a hard protective mechanical layer; it is usually covered
by a thick water-impermeable cuticle.
Seed coat surfaces vary widely from smooth to striate
(with lines) to papillate (with bumps). In cotton
(Gossypium) seed coats are covered with epidermal
trichomes – source of the textile fibers.
The seed coat plays a major role in controlling
dormancy of the seed.
A seed coat typically has three characteristic marks:
1. Hilum – an elliptical (round or punctate) scar
indicating the point of attachment of the funicle
(funiculus). The funicle is a stalk that attaches the ovule
to the placenta within the fruit.
2. Micropyle – a small hole at one end of the hilum
through which the pollen tube grew prior to
fertilization.
3. Raphe – this is a ridge adjacent to the hilum –
opposite the micropyle, caused by the fusion of the
funicle to the side of the ovule.
ENDOSPERM
This is stored food tissue whose function is to supply the
embryo with nutrients while dormant and during
germination to produce the seedling. It develops from
the fusion of a sperm with two polar nuclei. It is
therefore typically triploid.
In monocots the endosperm either surrounds the
embryo or is off to one side. The single cotyledon acts as
an absorptive organ that takes in nutrients from the
endosperm and transfers them to the embryo. In
grasses this cotyledon is so highly modified it has its
own name – the scutellum.
In dicots (and a few monocots) most or all of the
endosperm is absorbed by the developing embryo
before the seed becomes dormant. These seeds develop
fleshy food-storing cotyledons (seed-leaves). Some dicots
are intermediate with stored food in the cotyledons but
a considerable amount of endosperm remains in the
seed.
Perisperm is a food storing tissue found in a few plants.
It develops from the nucellus – a maternal tissue that
surrounds the embryo. Endosperm develops little if at
all. This is characteristic in plants such as cacti,
buckwheats, spinach, beets and carnations (subclass
Caryophyllidae). Perisperm may also be the principal
food storage tissue of some monocots (order
Zingiberales – ginger, bananas).
Some very small seeds may have very little (no)
endosperm – orchids & bromeliads.
The principal foods stored in seeds are carbohydrates,
protein and lipids.
A mature seed in which endosperm is abundant (corn)
is an albuminous seed. If endosperm is sparse or absent
at maturity (beans) the seed is exalbuminous.
EMBRYO
The embryo develops from the zygote (the diploid
fertilized egg cell) via mitotic division.
The zygote first develops into an undifferentiated
globular mass of cells – the proembryo. The proembryo
is attached to the embryo sac wall by a stalk – the
suspensor. The suspensor is thought to act as a conduit
for the translocation of nutrients form the surrounding
tissue to the proembryo. The suspensor ultimately
degenerates and the proembryo undergoes a process of
irregular meristematic activity that results in a shift
from radial to bilateral symmetry. The proembyro
eventually becomes a structure with root-shoot apices at
opposite ends of an embryonic axis – the hypocotyl (the
root-shoot junction).
The mature embryo consists of:
1. Epicotyl – the primordial stem, consists of an
apical meristem and the two (1-4) cotyledons. In
some embryos the epicotyl consists of just an apical
meristem, in others it bears one or more young
leaves. The epicotyl together with its young leaves
is a plumule.
2. Hypocotyl – the stem like axis below the cotyledons
3. Radicle – the immature or embryonic root, at the
opposite end of the embryonic axis from the
epicotyl.
The amount of embryo growth and development that
occurs before dormancy is variable. Orchids and
bromeliads have an embryo which consists of a small
ball of cells with no cotyledons or radicle. The number
of leaves besides the cotyledons is variable from 2-3 to
corn which may have 6 (a fully mature corn plant often
has only 10-12 leaves).
DORMANCY
Early in the development of an angiosperm embryo –
the embryo simply stops developing. This often happens
after apical meristems and the cotyledons are formed.
Once the seed coat forms around the embryo its
metabolic activities cease. A mature seed contains only
10-40% water; the progressive and severe dessication of
the embryo and reduction of metabolic activity are
responsible for arrested growth. Germination cannot
take place until water and oxygen reach the embryo.
1. Seeds allow plants to postpone development when
conditions are unfavorable; remain dormant until the
environmental conditions are “right.”
2. Seeds permit the course of embryo development to be
synchronized with important aspects of the plants
habitat – temperature, moisture, etc.
3. The dispersal of seeds aid in the migration and
dispersal of genotypes to new habitats and offers
maximum protection of the young plant at its most
vulnerable stage.
GERMINATION
Different kinds of seeds require different internal or
external stimuli to break dormancy and germinate – the
beginning or resumption of growth of a seed. The most
common external factors are water, temperature and
light.
Many seeds with a tough or thick seed coat require
scarification – mechanical or chemical breaking of the
seed coat.
Stratification – is used for seeds that require being held
for a period of time a low temperatures (freezing) –
after ripening prevents the seeds of plants growing in
cold areas from germinating until winter has passed.
There are two types of seed germination: Epigean in
which the cotyledon(s) are pulled above ground by the
elongation of the hypocotyl; and Hypogean in which the
cotyledon(s) remain beneath the soil.
Germination results in a young plant that develops
from the seed (embryo) – the Seedling.