Fertilization And Embryogenesis

Fertilization

The process of fusion of male and female gametes to form the zygote is called fertilization. In angiosperms, the male gametes are carried to the egg cell by pollen tube, this phenomenon is called siphonogamy.

The pollen grains reach the surface of stigma by pollination. Pollen grains absorbs moisture from the surface of stigma. As a result, pollen grains swells and the intine comes out in the form of a tube called pollen tube. Pollen tube is driven forward by dissolving the tissue of style with the help of enzymes present at its tip. The direction of the pollen tube is controlled by the chemicals secreted by synergids. Such movement by chemicals is known as achemotactic movement.

The pollen tube enters the ovary by three routes -

• Through chalaza
• Through integuments OR
• Through micropyle

The entry of pollen tube through the micropyle is the most common and is called as aporogamy.

The entry of pollen tube through the chalaza is called as chalazogamy.

The entry of pollen tube through the integuments is called as mesogamy.

Chalazogamy and mesogamy are collectively called aporogamy.

The pollen tube enters through synergids and breaks up to release its two male gametes.Out of these two male gametes, one fuses with the egg cell to form a zygote. This process is called syngamy. Another male gamete travels little farther and fuses with the secondary nucleus to form endosperm mother cell. This process is called triple fusion. Since two types of fusion i:e syngamy and triple fusion, takes place in the embryo sac the phenomenon is called double fertilization.

Syngamy and triple fusion together are known as double fertilization which is the characteristics of angiosperms.

Syngamy results in the formation of the zygote which later divides to give rise to an embryo. Triple fusion results in the formation of endosperm mother cell which later divides to form endosperm tissue. Endosperm tissue is the nutritive tissue for the growing embryo.

Double fertilization is the fusion of two male gametes brought by a pollen tube to two different cells of the same female gametophyte in order to produce two different structures.

Embryogenesis

The process of formation of an embryo from a zygote is known as embryogenesis. It can be studied under two headings:

• Dicot embryogenesis
• Monocot embryogenesis

Dicot Embryogenesis

The zygote elongates and then divides by a transverse wall into two unequal cells. The larger nasal cell is called suspensor cell and the other towards the chalaza is called terminal or embryo cell. The suspensor cell divides transversely a few times to produce a filamentous suspensor of 6-10 cells. The suspensor helps in pushing the embryo in the endosperm. The first cell of suspensor towards the micropylar end becomes swollen and functions as haustorium. The haustorium helps to draw nutrition. The last cell of the suspensor towards chalaza is called hypophysis which later gives rise to the radicle and root cap.

The embryo cell undergoes two vertical divisions and one transverse division to form eight-celled octant structure. Four cells towards the chalaza are known as epibasal cells and four towards micropyle are called hypobasal cells. The epibasal cells later give rise to two cotyledons and plumule. The hypobasal cells produce the hypocotyl.

The octant divides periclinally to produce an outer layer of protoderm. The inner cells differentiate into procambium and ground meristem. Protoderm forms epidermis, procambium gives rise to vascular strand and ground meristem produces cortex and pith,

Initially, the embryo is globular and undifferentiated. Early embryo with radial symmetry is called proembryo. It later becomes heart-shaped. The growth rate of cotyledons is very high and plumule remains at the place from where cotyledons are separated.

Dicot embryo

• A typical dicotyledonous embryo consists of an embryonal axis and cotyledons.
• Embryonal axis above the cotyledon is the epicotyls.
• The terminal part of the epicotyls is the plumule (gives rise to the shoot).
• The embryonal axis below the cotyledon is the hypocotyl.
• The terminal part of the hypocotyl is called the radicle (root tip).
• The root tip is covered by the root cap called calyptras.

Monocot Embryogenesis

The zygote elongates and then divides transversely to form basal and terminal cells. The basal cell towards the micropylar end becomes large and called as suspensor cell. It may function as haustorium. The terminal cell divides by transverse wall to form two cells. The top cell after a series of divisions forms plumule and a single cotyledon. Cotyledon called scutellum, grows rapidly and pushes the terminal plumule to one side. The plumule comes to lie in a depression. The middle cell, after many divisions forms hypocotyl and radical. It also adds a few cells to the suspensor. In some cereals, both plumule and radicle get covered by sheaths developed from scutellum called coleoptiles and coleorhiza respectively.

Monocot embryo

• A monocot embryo possesses only one cotyledon.
• In grass family, the cotyledon is called scutellum.
• The scutellum is situated towards one side of the embryonal axis.
• Radicle and the root cap is enclosed by a sheath called coleorhiza.
• The portion of the embryonal axis above the level of attachment of scutellum is called epicotyls.
• The epicotyl has the shoot apex and plumule enclosed by a hollow foliar structure called coleoptile.
• The seed is the final product of the sexual reproduction.
• The seed consists of a seed coat, cotyledon, and an embryo axis.
• Cotyledon stores the reserve food material for the development and germination.
• Matured seed without endosperm is called non-aluminous.
• A part of the endosperm retained in matured seed is calleda luminous.
• Remainants of nucellus in the matured seed is called perisperm.
• The wall of the ovary develops into the wall of fruit called pericarp.
• The fruit developed from the ovary is called true fruit.
• In apple, strawberry, cashew, the thalamus contributes to the fruit formation, such type of fruit is called false fruit.
• The fruit developed without fertilization is called parthenocarpic fruits.

Apomixis and polyembryony

• Apomixis is very common in Asteraceae and grasses.
• Seeds are produced without fertilization.
• Apomixis is a type of asexual reproduction which mimics the sexual reproduction.
• The diploid egg cell is formed without meiosis and develops into seed without fertilization.
• In Citrus and Mango, the nucellar cells start dividing, protrude into the embryo sac and develop into an embryo.
• Ovule having more than one embryo is termed as polyembryony.
• Hybrid plants are developed by apomixis to maintain the genetic identity.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.