The Angiosperm Seed

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The Angiosperm Seed Develops from the ovule as a
result of double fertilisation. Contains the
embryo and stored nutrients, and is protected by
the seed coat (testa). The seed coat is derived
from the integuments. Endosperm derived from the
fused polar and sperm nuclei in the central cell
of the embryo sac, stores food external to the
embryo. Perisperm sporophyte nucellar tissue,
stores food external to the embryo. In many
dicots, the endosperm and perisperm are transient
and are absorbed by the developing embryo before
the seed becomes dormant. The mature seed then
stores its food in the cotyledons which become
fleshy. The micropyle may remain as an occluded
pore or disappear. The funiculus abscises from
the ovule, leaving a scar (hilum). In anatropous
ovules part of the funiculus remains as a
longitudinal ridge, the raphe. Aril a funiculus
outgrowth. Caruncle an integumentary
protuberance near the micropyle. Elaiosome an
oily appendage used as food by ants. Seed
Development Double fertilisation initialises seed
development. The ovule and endosperm grow and the
embryo sac enlarges to accommodate the endosperm.
Once the endosperm reaches maximum volume, the
embryo starts to grow rapidly, initially by cell
division and then by cell enlargement. In the pea
(Pisum sativum), the embryo almost fills the
embryo sac as it grows at the expense of the
endoderm. (Organic C is tranferred from the
endosperm to the embryo). Finally the seed
becomes dormant (at this stage in the pea the
suspensor disintegrates). The growing embryo sac
is a sink, absorbing water and soluble materials
from the surrounding ovular tissues (which are
digested) which are supplied by the vessels in
the funiculus. In the pea, the growth of pod and
seeds is under the control of gibberellins, auxin
and abscisic acid. Hormones mobilised synthesised
by the growing ovules, following fertilisation,
stimulate growth of the pod. Albuminous seeds
store food in the endosperm (monocotyledons) or
perisperm (Amaranthaceae, Chenopodiaceae,
Polygonaceae). Exalbuminous seeds food is stored
in the embryo no endosperm or perisperm when
mature. Most seeds show a combination of storage
tissue types. Mature Embryo Dicotyledons the two
cotyledons arise as lateral organs at the apex of
the embryo axis and are in the same position,
relative to the meristem, as foliage
leaves. Monocotyledons one cotyledon, displaces
apical meristem to a lateral position. Embryo
upright, bent or curved.
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The diagram above shows the breaking of seed
dormancy (in a generalised grass/cereal
seed). All seeds must absorb (imbibe) water to
become metabolically active, but not all require
a red light stimulus to stimulate phytochrome,
e.g. lettuce seeds require both the red light
stimulus and gibberellic acid activation by
imbibition, whereas barley seeds require only
gibberellic acid activation by imbibition. Q.1 At
what two stages on the above diagram is
hydrolysis occurring? Q.2 What is the function of
the scutellum in grass seeds? Q.3 What is the
advantage of requiring a red light stimulus to
break dormancy in certain plants? Some plants
require a cold period to break seed dormancy, and
others require damage to the seed coat to allow
water to enter. In the walnut (Juglans regia) the
nut has to decompose on the forest floor for two
years, before its hard seed coat is weakened
enough to allow water in and the germinating
embryo out!
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Endosperm Formation Endosperm is characteristic
of angiosperm seeds. There are three modes of
endosperm formation 1. Nuclear many nuclei are
formed by nuclear divisions whilst the endosperm
remains noncellular or develops cell walls later.
The centre of the cell is occupied by a large
vacuole and the nuclei are parietal. In Capsella,
endosperm nuclei accumulate at the two ends of
the embryo sac and the endosperm at the chalazal
end digests nucellar cells in front of the
advancing embryo sac. The nucellar cells
hypertrophy before digestion and become rich in
protein and nucleic acids. Their digested remains
are seen inside vesicles in large vacuoles in the
endosperm cells. In Eranthis hiemalis
(Ranunculaceae) the endosperm digests adjacent
integumentary cells. Integumentary cell nuclei
migrate into the endosperm and are broken down.
This reduces the integumentary cell layers from
11 to 7 by the time the endodermis becomes
cellular. The cell walls form from phragmoplasts
or by wall ingrowth. In Pisum, an endosperm cell
wall and middle lamella joins the endosperm from
the outer walls of the embryo. The outermost
layer becomes the aleurone layer and endosperm
cells eventually take up the central vacuolar
space. In Cocos nucifera, the central cavity does
not fill with cells, but contains coconut
milk. 2. Cellular cell wall formation begins
with the first mitosis and continues until the
endosperm stops growing. 3. Helobial the embryo
sac is divided into two unequal cells, the larger
chalazal cell usually develops noncellularly
whilst the smaller micropylar cell develops in
either manner. Occurs mainly in the
monocotyledons. Food stores contain mainly
carbohydrate, protein and lipid. Cereals 70-80
dry weight is starch, peas and beans 50 starch.
Zea contains mostly starch in the endosperm, but
the embryo is 50 oil. Rape and mustard
(Brassicaceae) are 40 oil and 30 protein soya
(Fabaceae) seeds are 20 oil and 40 protein. The
starchy endosperm may be living, with starch
grains inside amyloplasts, or nonliving with free
starch grains. Carbohydrates may also be stored
in thickened cell walls (of endosperm or
cotyledons) composed mostly of hemicelluloses.
Protein is stored in granules enclosed in
membrane derived from the tonoplast and may
consist of globulins. Oil is stored as
triglyceride in cytoplasmic granules, which may
be bound by a unit membrane or a phospholipid
monolayer (?). Seed Coat The seed coat is dry in
most angiosperms, but may have fleshy appendages,
e.g. elaiosomes, or juicy layers such as the
fleshy epidermis of pomegranate (Punica) seeds.
Many gymnosperms have fleshy seed coats.
Cuticular layers and phenolic compounds restrict
water entry into the seed. The seed coat and
associated coverings (pericarp, other floret
parts in grasses) may contain germination
inhibitors and only when these are removed will
the seed germinate. Seeds that rely on animal
ingestion to disperse them, have testa that are
resitant to digestive processes. The epidermis of
some seeds secretes mucilage and this may cause
them to stick to animals and be dispersed, or may
prevent desiccation or prevent germination in
excessive moisture by swelling and impeding
oxygen diffusion into the seed.