post fertilization

POST-FERTILIZATION: Structures and Events

Development of endosperm and embryo, maturation of ovules into seeds and ovary into fruit are collectively termed as post-fertilization events.

A. Endosperm

This is a product of triple fusion and develops from central cell of embryo sac. It is generally a triploid tissue. The cells of this tissue are filled with reserve food materials and are used for the nutrition of the developing embryo.

It is absent in families such as Orchidaceae, Podostemaceae and Trapaceae.

The stony endosperm is present in betel nut (Areca nut) and date palm (Phoenix dactylifera).

Diploid endosperm is found in Oenothera.

Types of endosperm :

(i) Nuclear Endosperm : It is the most common type of endosperm in angiospermic plants. During the development of this endosperm, at first PEN undergoes successive nuclear divisions without wall formation (free nuclear division). In this manner, large number of free nuclei are produced. Finally, wall formation begins and it makes the endosperm a multicellular tissue, e.g., Cotton, Maize, Capsella, Coconut (milk).

(ii) Cellular Endosperm: Primary endosperm nucleus divides many times and each division is followed by wall formation, e.g., Petunia, Utricularia, Coconut (copra).

(iii) Helobial Endosperm: It is an intermediate between nuclear and cellular type, e.g., members of order helobiales (Monocot).

Concept Builder

(i) The seeds with double endosperm is found in coconut (Cocos nucifera), (a) liquid endosperm, (b) cellular endosperm.

(ii) Xenia : The effect of foreign pollen on endosperm character is called xenia. This term was given by Focke. This was first observed in maize endosperm colour.

(iii) Metaxenia : Discovered by Swingle. The effect of foreign pollen on somatic tissue lying outside the endosperm is known as metaxenia, e.g., in date palm size of fruits and maturity time depends upon foreign pollen. .

(iv) Ruminate Endosperm : Endosperm with irregular surface, also known as chewed endosperm, e.g., Passiflora, Annona, Myristica.

(v) Mosaic Endosperm : Endosperm with sugary and starchy parts forming different colour patches of yellow and white, e.g., Maize.

B. Embryo

The development of an embryo from a zygote is called embryogeny. 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.

This is an adaptation to provide assured nutrition to the developing embryo. It means endosperm development precedes embryo development.

Early stages of embryogeny are similar in both monocotyledons and dicotyledons.

(a) Development of Embryo in Dicots

The normal type of dicot embryo development has been studied in Shepherd's purse (Capsella bursa-pastoris) which belongs to family Cruciferae.

This is called as Crucifer or Onagrad type of embryo development. The development of embryo is endoscopic. Zygote (oospore) divides into two unequal cells, larger suspensor cell towards micropyle and a smaller embryonal cell (terminal cell) towards antipodal region.

The suspensor cell undergoes transverse divisions forming 6-10 celled long suspensor. The first cell of the suspensor (towards micropyle) is large and called haustorium or vesicular cell.

The last cell of suspensor (towards embryo cell) is known as hypophysis. It forms radicle tip.

Embryonal cell divides twice vertically and once transversely to produce a two-tiered eight-celled embryo. The epibasal tier forms two cotyledons and a plumule while the hypobasal tier produces only hypocotyl and most of the radicle.

For this the octant embryo undergoes periclinal divisions producing protoderm, procambium and ground meristem. It is initially globular but with the growth of cotyledons it becomes heart-shaped and then assumes the typical shape, e.g., Capsella bursa-pastoris.

 

A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (stem tip).

The cylindrical portion below the level of cotyledons is hypocotyl that terminates, at its lower end in the radicle. In orchids, Orobanche and Utricularia, the embryo does not show distinction of plumule, cotyledons and radicle.

(b) Development of Embryo in Monocot

The normal type of monocot embryo development has been studied in Luzula forsteri and is called Segittaria type. Suspensor is single celled in monocots.

The zygote of oospore divides transversely producing a vesicular suspensor cell towards micropylar end and embryo cell towards the chalazal end.

The embryo cell divides transversely again into a terminal and a middle cell. The terminal cell divides vertically and transversely into globular embryo.

It forms a massive cotyledon and a plumule.

Growth of cotyledon pushes the plumule to one side. Remains of second cotyledon occur in some grasses. It is called epiblast. The single cotyledon of monocots is called scutellum. It is shield shaped and lateral in position but appears terminal.

The middle cell gives rise to hypocotyl and radicle. Radicle is enclosed in an undifferentiated sheath called coleorhiza. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

C. Seed

It is often described as fertilised ovule. A seed typically consists of seed coat(s), cotyledon(s) and an embryonal axis. Mature seeds are of two types:

(a) Non endospermic or exalbuminous seeds:

In seeds like gram, pea, groundnut etc. the endosperm is completely consumed by developing embryo, thus the seeds are called non-endospermic or exalbuminous.

(b) Endospermic or albuminous seed :

In monocots and castor bean (dicots) embryo does not consume all endosperm. So, it persists in the mature seed. Such seeds are called endospermic or albuminous seeds. In these seeds, food is stored in endoseprm, e.g., wheat, maize, barley, sunflower, coconut.

Mostly nucellus is consumed after fertilisation due to absorption of food by the endosperm and embryo, but in some seeds remnants of nucellus are of persistant nature.

This residual, persistent nucellus is called perisperm. Such seeds are called perispermic seeds, e.g., Black pepper (Piper nigrum), beet.

Integuments of ovules harden as tough protective seed coats. The outer seed coat called testa develops from outer integument and inner seed coat called tegmen is derived from inner integument.

The micropyle remains as a small pore in the seed coat. Micropyle facilitates entry of oxygen and water into the seed during germination.

As the seed matures, its water content is reduced and seeds become relatively dry (10-15% moisture by mass).

The embryo may enter a state of inactivity called dormancy or if favourable conditions like adequate moisture, oxygen and suitable temperature are available, they germinate.

The transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The ovary wall develops into the fruit wall, called pericarp.

The fruit may be fleshy (guava, orange, mango) or dry (groundnut, mustard). In some cases thalamus and other floral parts show proliferation alongwith development of ovary wall.

Such fruits are called false fruits, e.g., Apple, Strawberry, Cashew nut. Most fruits develop from the ovary and are called true fruits.

In some plant species, fruits develop without fertilisation. Such fruits are seedless and are called parthenocarpic fruits, e.g., Banana. Parthenocarpy can be induced through the application of growth hormones like Auxins.

The first stimulus for fruit development comes from pollination while second stimulus is received from developing seeds and the third stimulus is provided by the availability of nutrients.

Advantage of seeds to Angiosperms

Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonize in other areas. As they have sufficient food reserves, young seedling are nourished until they are capable of photosynthesizing on their own.

The hard seed-coat provide protection to young embryo. Being product of sexual reproduction, they generate new genetic combinations leading to variations.

Seed forms basis of our agriculture. Dehydration and dormancy of mature seeds are crucial for storage of seeds which can be used as a food throughout the year and also to raise crops in next season. Seed viability varies from species to species. '

Oldest viable seeds of lupine, Lupinus arcticus were excavated from Arctic Tundra. The seed germinated and flowered after an estimated record of 10,000 years of dormancy.

A recent record of 2000 years old viable seed is of date palm (Phoenix dactylifera), discovered during the archeological excavation at King Herod's Palace near Dead Sea.

The ability of a seed to retain the power of germination is called viability. It can be tested by using 0.1 % solution of triphenyl tetrazolium chloride (TTC).

Orchid fruits contain thousands of tiny seeds. Similar in the case in fruits of Ficus and parasitic species such as Orobanche and Striga.