ENDOSPERM
The evolutionary origin and
morphological nature of endosperm in angiosperms are very difficult subjects.
The formation of endosperm is a special feature
of higher plants. Endosperm is the main source of food material which surrounds
and nourishes the embryo
in the seeds of angiospermic plants. The initiation of endosperm is a
definitive characteristic of the double fertilization of angiosperms. The
endosperm development requires the fusion of two polar nucleus in the embryo sac with one sperm nuclei from
the pollen grain
Since all the three nuclei taking part in the fusion are haploid, the endosperm
becomes triploid (3n ).In gymnosperms, the endosperm is haploid (n) and forms a
continuation of the female gametophyte In gymnosperms the
nutritive material of the seed is
present before fertilization.
In general the endosperm is
triploid but haploid, tetraploid and polyploid endosperms are also found in
higher taxa. Generally the endosperm nucleus divides after the division of the
oospore, but in many cases the endosperm is formed even before the first
division of the oospore. The endosperm
formation is suppressed in two families,
namely Orchidaceae and Podostemonaceae
In some seeds the endosperm is completely
absorbed at maturity (e.g., pea ,bean etc ), The fleshy
cotyledons nourish the embryo as it germinates. In plants like wheat, others,
endosperm is present until seed germination. In
monocotyledon. ntrients are stored in the cotyledon and endosperm tissue.
In coconut,
there is liquid endosperm which contains
important growth substances. Endosperm accounts for the economic importance
of cereal grains and
oilseeds.
Development of Endosperm
The formation of endosperm is a special feature of higher plants.Endosperm acts as a special
nutritive tissue.The development of endosperm usually begins before embryo
development. The formation of endosperm is initiated by mitotic divisions of
primary endosperm nucleus (3n). This primary endosperm nucleus (PEN) undergoes
nuclear divisions and endosperm is developed.It fills up the space of
nucellus.Endosperm formation occurs usually prior to the zygote or oospore
divisions.
Types of endosperm formation:
Depending upon the development of
endosperm they are grouped into three types as below (a) Nuclear type, (b) Cellular type and (c)
Helobial type.
a)Nuclear Type:
The nuclear type of endosperm formation
is the most common type and reported in about of 56%angiospermic families(116
families). It is found in maize, wheat, rice, sunflower, etc plants . In this
case, the first division is nuclear and
several other following divisions are
unaccompanied by wall formation. The nuclei may either remain free or in later
stages, they may become separated by walls. As divisions progress, the nuclei
are being pushed towards the periphery, thus a large central vacuole is formed.
Often the nuclei are especially aggregated at the micropylar and chalazal ends
of the embryo sac and form only a thin
layer at the sides. The endosperm nuclei in the chalazal end of the embryo sac
are larger than that in the micropylar end. The number of free
nuclear divisions varies in different plants. It is found in families like Palmae, Aracaceae
The endosperm development in coconut
( Cocos nucifera - Palmae ) is
special mention. In coconut the primary endosperm nucleus undergoes a number of
free nuclear divisions. In young
fruit of about 50 mm long the
embryo sac remains filled with a watery fluid or milk containing free nuclei
and fine cytoplasmic particles. In later
stage when the fruit becomes about 100 mm in length the liquid shows additional
free nuclei, and several cells , Later on these cells and free nuclei get
arranged at the periphery of the cavity, and layers of cellular endosperm are
formed,
In Areca catecheu or the
development of the endosperm is similar to that of coconut. In Areca nut the embryo sac cavity is small and it is
completely filled up by the endosperm tissue.It later becomes very hard.
(b) Cellular Type:
The cellular type of endosperm is
found 72 families mostly of dicotyledons. It is seen in families like
Anonaceae, Acanthaceae, Scrophulariaceae Ietc In this type, the first and other
following divisions are accompanied by
wall formation and thus the sac is divided into several chambers. In this sac some of cells may contain more than one nucleus. The first
wall is usually transverse but sometimes vertical or oblique divisions are also
carried out. In cellular endosperm formation the plane of division is not
constant. The endosperm become cellular from its beginning. Another important
feature in cellular endosperm is formation of some special cells namely
haustoria. The haustoria are formed at micropylar end or chalazal end or at both positions .These
haustoria penetrate the nucellar tissue to obtain food. It is commonly found in
Annona, Balsum, Datura, Magnolia, Petunia
,Impatiens, Blepharis etc plants. The occurrence of endosperm haustoria at both
ends is universal future of Scrophulariaceae family. This type of endosperm has
been further divided into several subtypes
( c ) Helobial Type:
The members of the order Helobiales
shows this type. The helobial type of endosperm formation is found in 17
families. This type is intermediate between the nuclear and the cellular types.
In this type the first division is followed by a transverse wall resulting in a
micropylar and chalazal chamber. The embryo sac forms two compartments .The further
divisions of endosperm formations are generally free nuclear. They may be seen in the micropylar chamber only. It is found in
members like Vallisnrria , Limnophyton
Eremurus is an example of a
typical Helobial type endosperm. In this
case the primary endosperm nucleus
divides tranversely forming two chambers, a large micropylar and a small
chalazal. Later on free nuclear divisions occur in both chambers. Due to such
divisions, when four nuclei are formed in the chalazal chamber, eight nuclei
are produced in the micropylar chamber. The number of nuclei are increased
rapidly and finally micropylar chamber has considerably a large number of
nuclei. In older ovules, the chalazal chamber begins to degenerate. The cell formation takes place in the micropylar
chamber. Later , the chalazal chamber is almost crushed and finally only a few disorganized nuclei are seen.
Nature of Endosperm
The main function of endosperm is
storage of food reserve. Endosperm cells are thin walled,large isodiametric and without pits. The plasmodesmata are clearly seen
in endosperm cell wall. Endosperm cell shows starch grains and also secretes
some enzymes. In families like Annonaceae, Rubiaceae,
Myristiceae,Palmae endosprm is irregularly ridged and furrowed. It is termed as
ruminate endosperm. This rumination is developed due to invagination of other
tissues
There is relation
between embryo and endosperm nucleus. Due to embryo growth there is decrease in
quantity of endosperm and nucleus. During embryo development endosperm is
utilized as source of food. It later on results into three types of seeds in
angiosperms.
a)
Ex-albuminous
:In plants like gram, pea, bean, tamarind etc.endosperm tissue and nucleus is
completely used by developing embryo.These seed are called ex-albuminous or
non-endospermic. They have thick cotyledons to store reserve food.
b)
Albuminous
:In plants like wheat, castor, maize, rice, palms etc. have large amount of endosperm tissue.
This tissue is not completely used by development of embryo.
Hence these seed are called albuminous or endospermic. Endosperm
nourishes embryo as well as growing seedling.
c)
Perispermic:In
some plants endosperm develops completely and fills the entire nucellus area.In
these seeds nucellus is found as thin layered nutritive tissue called
perisperm.It acts as food storage region e.g. Nymphea, Mirabilis
Importance of Endosperm
1.
In
general the endosperm is triploid but haploid, tetraploid and polyploid
endosperms are also found in higher taxa Endosperm formation is distinct
feature of higher plants.
2.
Endosperm
tissue is rich in reserve food in the form of proteins, lipids and
carbohydrates. These food materials are
utilized during seed germination
3.
The endosperm also serve as a source of growth promoting substances viz zeatin
(cytokinin) is extracted from maize endosperm.
Similarly the liquid endosperm in coconut (coconut milk) is used in tissue
culture nutrient medium
4.
In some angiospermic plants certain chemicals
present in endosperm helps for seed dormancy condition.
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