B.Sc. II Sem. III Unit VI Double fertilization and Triple fusion

1. B.Sc.- II, Sem- III
Unit – VI
Embryology of Angiosperm
Double Fertilization and triplefusion
Dr. SwatiPundkar
AssistantProfessor
Departmnetof Botany
ShriShivajiScienceCollege, Amravati

2. DOUBLE FERTILIZATION AND TRIPLE FUSION
Double fertilization was discovered by
Nawaschin in 1898 in liliaceous plants,
Lilium martagon and Fritillaria tenella.

3. Double fertilization
• is the fusion of the embryo sac with two male gametes. It occurs in
angiosperms.
• Pollination is the deposition of pollen grains from anthers on the stigma of a
flower, facilitating the fertilization, which is one of the major events of sexual
reproduction of plants.
• Pollen contains two cells, a tube cell, which produces the pollen tube, and a
generative cell, which produces two sperm cells.
• Pollen grains are germinated on the stigma, producing the pollen tube, which
goes down the style until it reaches the micropyle in the ovule.
• Once it meets the micropyle, pollen tube bursts, releasing the two sperm cells
which are carried through the pollen tube.
• One of the haploid sperm cells is fertilized with the haploid egg cell in the
embryo sac, while the other haploid sperm cell fertilizes the central cell,
which is also found in the embryo sac of the female gametophyte.

4. • Fertilization of the two gametes is called syngamy.
• The fertilization of a sperm cell with the central cell is called triple fusion.
• The embryo sac is fertilized twice, the fertilization of gametes in angiosperms
is called double fertilization.
• After fertilization, the ovule is developed into the seed.
• The fusion of egg cell with the sperm cell produces the diploid zygote, which
is developed into the embryo.
• The zygote is divide into two cells called terminal cell and basal cell by mitosis.
• The basal cell divides to produce the suspensor, which provides nutrients
from mother plant to the growing embryo.
• The division of the terminal cell produces the proembryo.
• Further development of the embryo is arrested until the germination of the
seed.

5. Significance of Double Fertilization
1.Two products are obtained as a result of double fertilization.
2.There are chances of polyembryony, and the plant has better chances of
survival.
3.Double fertilization gives rise to an endosperm that provides nourishment to
the developing embryo.
4.It increases the viability of the seeds of angiosperms.
5.It utilizes both the male gametes produced by the pollen grains.

6. Triple fusion is the fertilization of the two polar nuclei in the embryo sac. It occurs
during the double fertilization of angiosperms. While one sperm cell fertilizes the
egg cell in the embryo sac, the other sperm cell fertilizes the two polar nuclei
found in the central cell. Since the fusion of three haploid nuclei forms a triploid
nucleus, the process is called triple fusion. The triploid nucleus serves as the
primary endosperm nucleus. It gives rise to the endosperm of the seed, which
stores nutrients of the seed.
During triple fusion, three types of endosperm development are observed:
1-Nuclear endosperm , 2-Cellular endosperm , and 3-Helobial endosperm
Liquid endosperm is developed from the nuclear endosperm development as in
coconut water. Coconut meat is an example for the cellular endosperm
development and monocots have helobial endosperm development.

7. (i)Nuclear type: It is the most common type of endosperm
found. Here PEN divides repeatedly (mitotic division) without
cytokinesis. It results in the formation of a large number of free
nuclei in the cell. A large central vacuole is formed and nuclei
get arranged at the periphery. There are more nuclei at the
chalazal and micropylar end, compared to the sides.
At this stage, cell wall formation takes place from the periphery
towards the centre and multicellular endosperm is formed.
Examples: Maize, Rice, Wheat, Cotton, Sunflower. In coconut,
the cell wall formation is incomplete resulting in the outer
multicellular solid endosperm and inner multinucleated (having
free nuclei) liquid endosperm (coconut milk).

8. (ii) Cellular type:It is not very common. In this type of development,
division of PEN (karyokinesis) is followed by cytokinesis and two cells are
formed due to transverse division, giving rise to the chalazal and
micropylar chamber. Further division is similar, which leads to the
formation of the cellular endosperm. Examples: Petunia, Balsam, Datura.
(iii) Helobial type: This type of endosperm development is common in
monocotyledons. The first division is similar to cellular endosperm and
results in a large micropylar cell and small chalazal cell. The chalazal cell
mostly does not divide further and functions as a base cell. The
micropylar cell divides further, similar to nuclear endosperm. So it is an
intermediate type, a combination of both nuclear and cellular endosperm.
Examples: Eremurus

10. Significance of Endosperm
•Endosperm is important for the growth of an embryo and stores the food reserve
•They supply nutrients and provide protection to the developing embryo
•Endospermic tissues have shown to regulate gene expression and seed
germination
•Endosperm induces signals according to environmental conditions and regulates
embryonic growth
•The endosperm contains cytokinin, which regulates cellular differentiation
•It may induce abortion of seeds from the genetically mismatched cross