Post-Fertilisation of Flower: 4 Events | Biology

Soon after the double fertilization, the flower begins to lose its shine. The major events in post-fertilisation of a flower include development of endosperm and embryo, maturation of ovules into seeds and ovary into fruit.

1. Development of an Endosperm:

Endosperm development precedes embryo development. The primary endosperm cell divides repeatedly to form a triploid endosperm tissue.

In most of the cases, the PEN undergoes successive nuclear divisions without cytokinesis, to give rise to free nuclear endosperm.

Subsequently, cell wall formation starts from the periphery and the endosperm becomes completely cellular, e.g., coconut, rice, maize, sunflower, etc..

Functions of an Endosperm:

i. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.

ii. The endosperm may be completely utilised by the developing embryo before the maturation of seeds as in pea, bean and mustard, etc.

2. Development of an Embryo:

Embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after certain amount of endosperm is formed. The nutrition for the development is provided by the endosperm. The zygote gives rise to pro-embryo, the globular, heart-shaped and mature embryo.

Dicot Embryo:

A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates in the radicle or root tip. The root tip is covered with a root cap (colypira), e.g., mango, apple, raddish, rose, etc.

Monocot Embryo:

Embryo of monocots possess only one cotyledon at one end and this cotyledon is called scutellum. At the lower end, the embryonal axis has radical and root cap enclosed in an undifferentiated sheath called a coleorhiza.

The portion of the embryonal axis above the scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure called coleoptile. e.g., grass, banana, bamboo, palm, etc.

3. Development of a Seed:

Double fertilisation in angiosperms triggers the transformation of ovule into a seed. Seeds are formed inside the fruits.

Structure of a Seed:

A typical seed consists of a seed coat, cotyledon and an embryo axis. The seed coat is often double-layered, formed by the integuments. The outer integument forms the outer layer of seed coat called testa, which is hard and inner integument forms the inner layer of seed coat called tegmen.

The cotyledons are generally thick and swollen with food materials. They are 1-2 in number and rich in reserve food material. Micropyle is the small opening found on the seed coat which facilitates the entry of water and oxygen into the seed.

Changes in Seeds during Development:

The changes usually occur in the ovule during the development of seeds.

i. The zygote develops into an embryo. The triploid primary endosperm nucleus gives rise to a nutritive tissue called endosperm.

ii. This endosperm may persist or become completely digested during embryogenesis the seeds having copious amounts of endosperm tissue are called albuminous seeds, (wheat, corn and onion) and the seeds in which the endosperm is used up are non-albuminous (beans, pea and grams).

iii. The nucellus is generally used up during the development of an embryo. In some cases, the endosperm is present in the form of a layer. This layer is called as perisperm.

iv. The embryonal axis has hypocotyl and radicle on its lower end and epicotyl and plumule on its lower end.

v. The outer integument becomes hard and leathery.

vi. The micropyle remains in the form of a fine pore on the surface of seed. Funicle is transformed into stalk of the seed.

vii. As the seed matures, its water content is reduced and seeds become relatively dry (10-15% mixture by mass). The hilimi marks the point of attachment to the stalk.

viii. The general metabolic activity of the embryo slows down. This state is called dormancy. On return of favourable conditions, the seeds again germinate into plants.

Advantages of Seeds:

Following are the advantages of seed:

(a) The stored food in seeds support the growth of seeds till they become nutritionally independent.

(b) Seeds are adaptive for seed dispersal to from new plants colony.

(c) These can be stored for longer time as food source.

(d) Seeds are the products of sexual reproduction, so they provide genetic recombination.

4. Formation of a Fruit:

A fruit is formed as a result of cell division and differentiation in the ovary, which is transformed into fruit as a result of stimuli received from pollination as well as developing seed. The wall of the ovary develops into the wall of a fruit called pericarp which may be fleshy (e.g., guava, tomato, cucumber) or may be leathery and dry (e.g., pea, bean, mustard). Pericarp protects the seeds and help in dispersal of seed.

Fruits are of following two types:

(i) True Fruits:

The fruit derived from an ovary of a flower not associated with any non-capillary part is termed as true fruit, e.g., mango, tomato. The other floral parts in these fruits degenerate and fall-off.

(ii) False Fruits:

The fruit derived from an ovary along with other accessory floral part is called false fruit, e.g., apple, cashew nut, strawberry, etc. Thalamus also contributes in fruit formation.

Parthenocarpy:

Some fruits develops without undergoing fertilisation, these are called parthenocarpic fruits, e.g., banana. Parthenocarpy can be induced through the application of growth hormones and such fruits are seedless.