Class 12 CBSE Chapter 2 Sexual Reproduction in flowering plants (PART-4) -

Class 12 CBSE Chapter 2 Sexual Reproduction in flowering plants (PART-4)

FERTILIZATION

The fusion of male gamete with female gamete is called fertilization.
Fertilization was discovered by Strasburger (1884) in Monotrapa plant. This process of fertilization is completed in the 4 steps :
- Germination of pollen grain and pollen tube.
- Entry of pollen tube into the ovule.
- Entry of pollen tube into embryo sac.
- Fusion of gametes.

GERMINATION OF POLLEN GRAIN

After pollination, the pollen grains land on the stigma of the carpel and absorb moisture and nutrients, causing them to swell up. The intine layer of the pollen grain grows out through the germinal pore of the exine layer, forming a tube-like outgrowth called the pollen tube.

In most angiosperms, including Capsella, only one pollen tube develops, which is called the monosiphonous condition. However, in families such as Malvaceae and Cucurbitaceae, more than one pollen tube may develop, known as the polysiphonous condition.

As the pollen tube grows down from the stigma into the style, the vegetative nucleus of the pollen grain enters the tube first, followed by the generative cell. The tube nucleus always occupies the terminal position in the pollen tube. The vegetative nucleus controls the growth of the pollen tube while the generative cell divides mitotically to form two non-motile male gametes.

The growth of the pollen tube requires the presence of certain nutrients, such as boron and calcium ions (mainly boron), and the optimal temperature for growth is between 20-30°C. The pollen tube exhibits apical growth and chemotropic movement, meaning it grows towards the ovules in response to chemical signals released by the female reproductive organs.

Overall, the germination of pollen grains is a complex process that involves the growth of a pollen tube and the formation of male gametes, which are necessary for successful fertilization and reproduction in angiosperms.

ENTRY OF POLLEN TUBE INTO THE OVULE

The pollen tube enters the ovary when the ovule becomes mature. Within the ovary, obturators help guide the pollen tube towards the micropyle, which is the opening through which the pollen tube must enter the ovule.

A mature ovule that contains a mature embryo sac has three possible paths for the entry of the pollen tube:

Porogamy: In this method, the pollen tube enters the ovule through the micropyle. Porogamy is the most common method and is found in most angiosperms, including Capsella.
Chalazogamy: In this method, the pollen tube enters the ovule through the chalaza, which is the base of the ovule. Chalazogamy was first discovered in Casuarina by Treub in 1891 and is found in some species such as Betula and Juglans (walnut).
Mesogamy: In this method, the pollen tube enters the ovule either through the integuments, which are the layers of protective tissue that surround the ovule (as in Cucurbita), or through the funiculus, which is the stalk that attaches the ovule to the ovary wall (as in Pistacia and Populus).

ENTRY OF POLLEN TUBE INTO EMBRYO SAC

Pollen tubes can enter the ovule through various passages, but to reach the embryo sac, they must enter through the micropyle or chalaza, depending on the species.

Once inside the ovule, the pollen tube grows towards the embryo sac, guided by chemical signals from the synergids. This is known as chemotropic movement.

As the pollen tube approaches the egg apparatus, one of the synergids starts to degenerate, allowing the pollen tube to enter the embryo sac.

When the pollen tube enters the embryo sac, the vegetative nucleus degenerates, and the tube tip bursts, releasing the contents, including the two male gametes, into the central cell of the embryo sac.

Degenerating nuclei of the synergids and the tube cell form two dark structures known as P-bodies or Pollen Pockets. These structures play a role in regulating the degeneration of the synergids and the fertilization process.

After fertilization, the primary endosperm nucleus undergoes multiple rounds of mitotic division to form the endosperm, which stores nutrients that are utilized by the developing embryo during its growth and seed germination. The primary nutrients stored in endosperm are in the form of starch.
Endosperm is a specialized tissue in seed plants that provides nourishment to the developing embryo. It is typically triploid, resulting from the fusion of one sperm cell with two polar nuclei during double fertilization.
In gymnosperms, the endosperm is formed before fertilization, and it is always haploid, containing a single set of chromosomes. It provides nutrients for the developing embryo before fertilization occurs.

TYPES OF ENDOSPERMS

Endosperm can be classified into three types based on its development: nuclear, cellular, and helobial.

NUCLEAR ENDOSPERM

Nuclear endosperm is commonly found in dicotyledonous plants (Polypetalae) and also present in Capsella.
This type of endosperm develops through free nuclear divisions of the primary endosperm nucleus, resulting in a multinucleated endosperm. Later on, cytokinesis takes place to form a multicellular endosperm.
Nuclear endosperm is the most common type of endosperm in angiosperms.
The milky fluid found in green coconut is an example of nuclear endosperm, which is also known as a liquid syncytium.

CELLULAR ENDOSPERM

Cellular endosperm is found in plants of the gamopetalae group, such as Petunia and Datura.
During its development, each division of the primary endosperm nucleus is followed by cytokinesis, resulting in a cellular endosperm from the beginning.

HELOBIAL ENDOSPERM

Helobial endosperm is found in monocots.
During its development, the first division of the primary endosperm nucleus is followed by unequal cytokinesis, resulting in the formation of two unequal-sized cells (the cell towards the micropyle is large). Free nuclear divisions then take place in each cell, making it multinucleated. Eventually, cytokinesis takes place at once, leading to the formation of a cellular endosperm. Helobial endosperm is an intermediate type.

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DEVELOPMENT OF EMBRYO

• Embryogeny refers to the process of developing a mature embryo from a zygote or oospore. • During embryonic development, the zygote begins to divide, ultimately leading to the development of the embryo along with the endosperm.

DEVELOPMENT OF EMBRYO IN DICOTYLEDONS •

In angiosperms, the zygote undergoes a resting phase before development begins. Once the endosperm is formed, the zygote begins to absorb nutrients from it and increases in size, eventually forming an oospore. • The first division of the oospore is transverse, resulting in two cells: a basal (suspensor) cell located towards the micropyle and an apical (embryonal) cell located towards the chalaza. • The basal and embryonal cells divide simultaneously, with the basal cell dividing transversely and the apical cell dividing vertically to produce two suspensor cells and two embryonal cells arranged in a ‘T’ shape (the quadrant stage). • The two suspensor cells divide by transverse division, forming a long filament-like structure called the suspensor, which pushes the developing embryo into the nutrient-rich endosperm. • The micropylar cell of the suspensor swells up to form a haustorial cell, and the cell of the suspensor lying near the embryonal cells is called the hypophysis. This cell combines with the radicle to form the root cap. • The four celled quadrant embryo further divides transversely to produce the eight celled octant stage of the embryo, with four hypobasal cells giving rise to the radicle and hypocotyl and four epibasal cells giving rise to the two cotyledons and plumule. • All the cells of the octant embryo divide by periclinal division to form a 16-celled globular embryo (proembryo). • Due to fast division of the embryonal cells, a heart-shaped embryo is formed with all its cells being meristematic. • The two lobes of the heart-shaped embryo grow rapidly and develop into two cotyledons that turn downwards due to the curved position of the ovule of Capsella. The tissues present below the cotyledon junction give rise to the plumule and behind it, the epicotyl is formed. • The radicle is formed from the tissues present opposite to the plumule near the hypophysis. The curved position of the embryo is called the torpedo or chordate stage, with the embryonal axis or tigellum present between the plumule and radicle. • The cotyledons are present laterally to the embryonal axis, and the plumule is formed terminally in the Dicotyledon embryo. • This type of embryo development is known as Crucifer or Onagrad type, which is the most common type of development in dicots. • Capsella is considered a typical angiosperm for the study of embryonic development of angiosperms because it exhibits Crucifer type development. • Once the embryo is formed, the suspensor dries up and degenerates, resulting in meroblastic development. • The ovule is modified into a seed, with the testa formed by the outer integument and the tegmen formed by the inner integument. The micropyle of the ovule remains unchanged and is also present in the seed. • The entire ovary is modified into a fruit, which is formed by a fertilized ovary and is therefore called a true fruit. • Some angiosperms form fruit without fertilization, which is known as parthenocarpic.

EMBRYONIC DEVELOPMENT IN MONOCOTYLEDONS

Monocotyledons exhibit the Lilium type of embryonic development. The first division is a transverse division in the oospore, which results in the formation of two cells – the embryonal cell, located towards the chalazal end, and the basal cell, located towards the micropylar end. The basal cell does not divide further and eventually grows in size to form a single-celled, vesicular suspensor.

The embryonal cell divides transversely, giving rise to two cells: the cotyledon cell at the terminal end and the embryonal axis cell in the middle. The cotyledon cell continuously divides to form the apical cotyledon or scutellum. The embryonal axis cell undergoes a transverse division to give rise to two cells: one that forms the plumule initial and the other that forms the radicle initial. Both these initials are responsible for the lateral formation of the embryo.

The plumule initial divides to form the plumule of the embryo, while the radicle initial divides to form the radicle. The plumule and radicle are covered by cap-like, hard protective structures. The covering of the plumule is called the coleoptile, while the covering of the radicle is known as the coleorhiza. These structures are broken during germination. In monocotyledons, the plumule lies in a lateral position and the cotyledon in a terminal position.

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STRUCTURE OF SEED

The whole plant exists in a seed. In angiosperm, after fertilization, egg cell changes into embryo, ovary changes into fruit and ovule changes into seed.
Morphologically, ripened ovule is known as In other words, seed is a mature, integumented megasporangium.
Seeds are characteristic of spermatophytes (gymnosperms and angiosperms).
Seed is a dormant structure containing protective coverings, reserve foods and embryo (2n).

SEED COAT

Outer protective covering of the seed is called seed coat, which develops from integuments of ovule.

In seeds, developing from bitegmic ovules, there are two distinct layers in seed coat-the outer layer is thick, hard and leathery (developing from outer integument), called testa, whereas inner layer is thin and papery or membranous (developing from inner integument), called tegmen. In seeds developing from unitegmic ovules, there is single layered seed coat. The seed coat performs usual protective function.

The seed is attached to the fruit wall or pericarp by means of a stalk called funicle or funiculus.

The point of attachment of funiculus to the body of mature seed is called hilum.

A small opening or pore called micropyle is present just near the hilum, which is the way of entry of water into the seed. As most of the ovules in angiosperms are inverted or anatropous, so a ridge called raphe is formed.

EMBRYO

Embryo is the most important part of the seed, which represents tiny future plant. The origin of embryo takes place from fertilized egg (zygote).
The embryo is having an embryonal axis or main axis called tigellum, to which one or two cotyledons (seed leaves) are attached, depending upon whether the seed is monocot or dicot.
The portion of embryonal axis or tigellum below the point of attachment of cotyledons, is called hypocotyl, which bears radicle or future root at its tip. Similarly, portion of embryonal axis or tigellum above the point of attachment of cotyledons is called epicotyl, which bears plumule (future shoot) at its tip.

COTYLEDONS

The cotyledons of the embryo are simple structures, generally thick & swollen due to storage of food reserves (as in legumes). In some seeds (e.g., legumes), reserve food is stored in cotyledons, whereas in others (e.g., cereals), there is special nutritive tissue called

The reserve food materials in seeds may be carbohydrate (e.g., wheat, rice) or proteins (legumes) or fats (e.g., castor, peanut, sunflower, etc.)

Sometime, some part of nucellus remains unused which is present in the form of thin layer around the endosperm is called E.g., Betel nut, Black pepper, Castor, etc.
In Monocot embryo, there is a single cotyledon called scutellum. In some grasses – wheat, on opposite side of scutellum is a tongue shaped outgrowth remains of second cotyledon is present called
In Dicot albuminous seed-castor (Ricinus communis), there is a specific outgrowth called caruncle or strophiole, present over micropyle. It is formed by proliferation of cells of outer integument at tip. Caruncle is somewhat spongy and helps in absorption of water during germination of seed.

TYPES OF SEEDS

Seeds can be classified based on various criteria. One way is by the presence or absence of endosperm in the seed.

ENDOSPERMIC OR ALBUMINOUS SEED :

Endospermic or albuminous seeds are seeds in which the food is stored in the endosperm. During germination, the endosperm provides nutrients to the growing embryo. Examples of endospermic seeds include most monocots such as wheat, rice, and maize, as well as some dicots like castor.

NON-ENDOSPERMIC OR EX-ALBUMINOUS SEED Non-endospermic or ex-albuminous seeds are seeds that do not have endosperm. Instead, the cotyledons store the food for the growing embryo. Examples of ex-albuminous seeds include most dicots such as beans, peas, and peanuts, as well as some monocots like orchids.

Endospermic dicot seeds: E.g., Castor, Papaya, Cotton.

Non-endospermic dicot seeds: E.g., Gram, Bean, Pea, Cucumber ,Tamarind.

Endospermic monocot seeds: E.g., Maize, Rice, Wheat and Coconut.

Non-endospermic monocot seeds: E.g., Pothos (money plant), Vallisneria, Alisma, Amorphophallus.

GERMINATION OF SEED

A seed is a fertilized and ripened ovule containing a dormant embryo, reserve food for its future development, and a protective covering. Germination is the process of a seed developing into a seedling by resuming its active growth.

There are two main types of germination: epigeal and hypogeal.

Epigeal germination occurs when the cotyledons are pushed out of the soil due to hypocotyl growth or elongation. This type of germination is found in plants such as cotton, papaya, castor, onion, cucurbits, tamarind, and mustard. In some cases, the cotyledons become green leaf-like structures (cotyledonary leaves) and perform photosynthesis until the seedling becomes independent, while in other plants, the cotyledons do not assume leaf-like shapes and fall off.

Hypogeal germination occurs when the plumule (shoot) emerges from the ground first, followed by the cotyledons remaining underground. This type of germination is found in most monocots and some dicots such as maize, rice, wheat, coconut, and pea.

Vivipary is a special type of germination found in mangrove vegetation in muddy, saline conditions. In vivipary, there is no resting period for the embryo, and germination occurs inside the fruit while it is still attached to the parent plant. This is known as “in situ germination.” The seedling is separated in the mud with the help of lateral roots developing from the basal end of the radicle. Examples of plants exhibiting vivipary include Rhizophora, Avicennia, Ceriops, Breguira, and Sonneratia.

DORMANCY OF SEED

Seed dormancy is an essential characteristic feature, which allows seeds to remain viable for extended periods. Dispersed through water, air, or insects, many seeds are incapable of immediate germination. The period between maturation and germination is known as the “dormancy period,” during which the embryo remains inactive, and growth processes are temporarily suspended for reasons of seed dormancy.

THREE MAIN BASIC REASONS FOR SEED DORMANCY IMPERMEABILITY OF SEED COAT

Seed coats of many species in Leguminosae and Convolulaceae families are completely impermeable to moisture and oxygen at maturity, due to their thick and hard cell wall covered with a water-proof lignin layer. Such seeds take more time for germination, and seed dormancy is overcome naturally by the action of microbes in soil or the digestive tract of fruit-eating birds or by high temperature or cold.

To overcome seed dormancy, various artificial methods can be used, such as making minute holes or pores with a sharp apparatus, rubbing seeds on a hard surface, or partially degenerating the seed coat by the action of sulfuric acid.

DORMANT EMBRYO

In many species, the embryo is immature at seed ripening, and even in favorable environmental conditions or with the removal of the seed coat, it fails to germinate. This is known as “embryo dormancy.” Such seeds must complete their enzymatic and chemical reactions before germination. Keeping such seeds at low temperatures and desirable moisture can break seed dormancy. The low temperature is the main reason for the germination of seeds of apple, peaches, pears, maple, and pine.

GERMINATION INHIBITORS

The germination of some seeds is prevented by the presence of chemical compounds called “germination inhibitors,” such as ferulic acid present in tomato juice, coumarin, abscisic acid, dormin, and para-ascorbic acid. This condition is commonly found in xerophytic plants, and the washing away of these germination inhibitors with water is known as “leaching.”

FRUIT DEVELOPMENT A fruit is defined as a fertilized ovary, either with or without seeds, along with any accessory structures closely associated with it.

The study of fruits is known as pomology.

Fruits can be classified into two main categories based on their development: true fruits and false fruits.

True fruits are those that develop solely from the ovary and do not involve any other floral parts in their formation.

False fruits, on the other hand, involve other floral parts in addition to the ovary in their development. Examples of false fruits include aggregate fruits. In some cases, fruits can develop without fertilization, resulting in seedless fruits or parthenocarpic fruits. Common examples of such fruits include bananas, guavas, and apples.

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APOMIXIS, PARTHENOCARPY, POLYEMBRYONY

Apomixis is a form of reproduction in which seeds are formed without fertilization. It can occur through various processes, such as the development of an embryo from a diploid cell of the nucellus, resulting in a diploid seed with the same genetic makeup as the parent. Apomixis is often associated with polyploidy and is observed in certain plants such as dandelions, citrus, and some grasses.

Parthenocarpy refers to the development of fruit without fertilization, leading to a seedless fruit. Some varieties of pineapple, grapes, apple, pear, and banana may occur naturally through parthenocarpy.

Polyembryony is the presence of multiple embryos in a seed. It may result from the presence of multiple egg cells in the embryo sac or multiple embryo sacs in the ovule, leading to the fertilization of all egg cells. Polyembryony is commonly found in gymnosperms and certain angiosperms, such as orange, lemon, and Nicotiana.

There are two types of polyembryony: true and false. In true polyembryony, multiple embryos are formed inside a single embryo sac of the seed through cleavage of the zygote or fertilization of synergids or antipodal cells. In false polyembryony, multiple embryos are formed from separate embryo sacs inside the ovule.

Spontaneous polyembryony occurs naturally, while induced polyembryony is artificially induced. Adventive embryony is also a form of polyembryony in which additional embryos are formed from the nucellus or integuments.

Lilium is an example of a plant that exhibits all three types of polyembryony.

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