Reproduction is the process of formation of new individuals from pre-existing ones. It is the means of multiplication and perpetuation of the species because the older individuals of each species undergo senescene and die. There are two basic types of reproduction : Asexual and Sexual

 Asexual reproduction.                                                                                                                                         

The methods of reproduction which do not involve meiosis and fertilization are known as apomixis or asexual reproduction. Only mitotic divisions are involved in these methods, resulting into the formation of offsprings which are genetically similar to the parent plant.

Asexual reproduction is of following two types :

1. Agamospermy : Agamospermy is a kind of plant apomixis in which the embryos and seeds are formed by asexual reproductive methods without involving meiotic gametogenesis and sexual fusion of gametes. It occurs widely in ferns and the flowering plants. There are three different types of agamospermy :

Agamospermy

Adventive

embryony

Diplospory                               Apospory

Diploid Parthenogenesis

Diploid Apogamy

Generative Apospory

Somatic Apospory

1. Adventive embryony : Formation of embryo directly from the diploid sporophytic cells (nucellus or integument) of ovule is called adventive embryony. Such embryos are formed without involving meiosis and sexual fusion, e.g., Citrus, Opuntia, etc. In Citrus, a seed may possess upto 40 embryos (one normal and rest adventive).
2. Diplospory : In this case, the archesporium differentiates but megaspore mother cell directly gives rise to an unreduced (i.e., without meiosis) embryo sac. It may produce two types of embryos :

1. Diploid parthenogenesis : Embryo develops from unfertilized diploid egg.
2. Diploid apogamy : Embryo develops from any diploid cell of embryo sac except egg.
   3. Apospory : It is the formation of complete embryo sac from the sporophytic cell without meiosis so that the gametophyte remains diploid. Apospory may be of two types :

1. Somatic apospory : Embryo sac is formed from somatic cell.
2. Generative apospory : Embryo sac is formed from archesporium without meiosis.

2. Vegetative propagation : Regeneration or Formation of a new individual from any vegetative part of the body is called vegetative reproduction or vegetative propagation. The lower plants reproduce vegetatively through budding, fission, fragmentation, gemmae, resting buds, spores, etc. Among flowering plants, every part of the body such as roots, stem, leaves and buds take part in vegetative propagation. It is very common mode of reproduction and it may be natural vegetative propagation or artificial vegetative propagation.
   1. Natural methods of vegetative propagation : In natural vegetative propagation, a portion gets deattached from the body of mother plant and it grows into a new individual plant under suitable conditions. Different plant parts are variously modified for vegetative propagation. Some of these are given below.

1. Vegetative propagation by stems : The modified stems like bulbs, runners, rhizomes, corms, tubers, offsets, etc., help the plant to multiply under favourable conditions.

• Bulb : It is a modified shoot that has a very short stem and apical and axillary buds. Some of these grow to form shoots. e.g. Onion, Tulip, Lilies, Garlic, etc.
• Runners : These are creeping modified stems which produce adventitious roots at nodes. Each node gives rise to aerial shoot which becomes a new plant e.g. Doob grass (Cynodon), Wood sorrel (Oxalis), Indian pennywort (Centella), etc.
• Rhizomes : These are underground, horizontally growing stems. They have distinct nodes, internodes and axillary buds. The branches grow from the buds which later separate to form new individuals. e.g. Ginger, Turmeric, Typha, Lotus, Saccharum, Canna, etc.
• Corms : Corms are highly condensed and specialized underground stems which bear many buds. They perennate the unfavourable conditions and produce new plants under favourable conditions. e.g. Saffron (Crocus), Gladiolus, Colocasia, Banana, etc.
• Tubers : Tubers are the modification of underground stem tip having several eyes or buds. Each eye grows into new plants. e.g. Potato.
• Offsets : They are one internode long runners which develop tuft of leaves at the apex e.g. Water lettuce (Pistia), Water hyacinth (Eichhornia), etc.
• Stolons : They are arched runners with cross over small obstacles and develop small plantlets at their nodes. e.g. Strawberry, Vallisneria, etc.

2. Vegetative propagation by roots : The roots of some woody plants produce shoots which grow into new plants; e.g., Murraya, Lebbeck tree (Albizzia), Sisham (Dalbergia sisso), etc. Modified tuberous roots of Sweet potato, Asparagus, Dahlia, Tapioca, Tinospora, etc. develop buds and each of which form a new plant.
3. Vegetative propagation by leaves : The leaves generally do not help in vegetative propagation. However, in Bryophyllum pinnatum and B. daigremontianum, develop along the leaf margins which on deattachment produce independent plants. In elephant ear plant (Begonia) also, leaf buds are produced from petiole and veins throughout the surface of the leaf.
4. Vegetative propagation by reproductive parts : Flowers are primarily associated with sexual reproduction. But in Globba, American aloe (Agave), Onion (Allium cepa), etc. special multicellular structures, called bulbils, occur on the inflorescence. These are the modifications of flowers. Bulbils grow into new plants when shed from the mother plant.
   2. Artificial methods of vegetative propagation : Several methods of vegetative propagation are man made and developed by plant growers and horticulturists for commercial production of crops. They are called artificial methods. In this method a portion is separated from the body of the plant and then it is grown independently. The potato tubers are organs of natural vegetative propagation but are also used artificially. This is useful commercially because the new individuals produced maintain the desirable characters of the parents. A population of these genetically identical plants obtained from an individual is called a clone. Some of the artificial methods are given below :

1. Cuttings : The small piece of any plant organ (stem, root or leaf) use for propagation is called cutting. Leaf cuttings are used to propagate Sansevieria, Begonia, Bryophyllum, Glocinia and Kalanchoe. Root cuttings are used to propagate Citron and Tamarind. Stem cuttings are most commonly used for artificial propagation. When cuttings (about 20-30 cm. long pieces of stem) from such plants are put into the moist soil, they develop

adventitious roots and buds at the base which develops into new plants. Sometimes roots are not easily produced in the cuttings and hence, they are treated with rooting hormone (IBA). Factors such as age of the parent plant, length and diameter of the cutting, season and the type of plants are taken into consideration for the propagation of particular species. Grapes, Sugarcane, Rose, Bougainvillea, Carnation, Coleus, Duranta, etc. are propagated by stem cuttings.

2. Layering : In this method, roots are artificially induced on the stem branches while it is still attached to the parent plant for propagation. There are two common types of layering :

• Mound layering : In this technique a lower branch of stem is bent and covered in such a way that the tip of the branch remains above the ground. After a few days, the covered part of the stem produces adventitious roots. At this stage the branch is cut off from the parent plant and it grows into a new plant. This method is commonly employed for propagating Strawberry, Jasmine, Grape vine, Raspberry, etc.
• Air layering (Gootee) : This is employed in plants with thick branches which can not be easily bent. In this method, part of the stem is girdled (i.e., a ring of bark is removed) or slit at an upward angle. This part is covered with moist moss or cotton and enclosed in a polythene bag to prevent drying. The wrapped portion is called gootee. The roots appear after some time and at that stage the branch is cut and planted. It grows into a new individual. This method is used in vegetative propagation of Litchi, Pomegranate, Orange, Lemon, Guava, Bougainvillia, etc.

3. Grafting : A new variety is produced by joining parts of two different plants is called grafting. The rooted shoot of one plant, called stock, is joined with a piece of shoot of another plant known as scion. The root stock is generally derived from a plant resistant to diseases and efficient in water and mineral absorption. The scion is a stem cutting from a superior quality plant. The grafting ends of both, stock and scion are cut obliquely and then placed over one another in such a way that the cambia of two come in close contact. The two pieces are firmly held together by tape, rubber tubing, etc. This results in fusion of cambia and formation of new vascular tissue. Grafting is generally done between the related varieties or species. This method has been practised for many economically useful plants, such as Rose, Mango, Apple, Pear, Guava, Citrus, Rubber etc. There are various methods of grafting like tongue or whip grafting, wedge grafting and crown grafting. Besides these a technique, called bud grafting, in which only a single bud along with a small portion of bark having intact cambium instead of a scion is employed for

propagation.

4. Propagation by plant tissue culture or Micropropagation : This method includes propagation of plants by culturing the cells, tissues and organs called tissue culture. Small pieces of plant organs or tissues are grown aseptically in a suitable nutrient medium. Initially it results in the formation of undifferentiated mass of cells called callus. Which later differentiates to produce a large number of plantlets. These plantlets are then transferred to separate pots or nursery beds to obtain a large number of plants. Tissue culture technique is useful

Cotton plug

Callus

Nutrient medium solidified with agar

Embryoid Plantlet

in obtaining virus free plants, homozygous diploids and in commercial micropropagation of Orchids, Carnation, Gladiolus, Chrysanthemum and other Ornamental plants. This method is also

A                               B                              C

Fig : Micropropagation : A. Culture tube with tissue,

B. Organised callus, C. Plantlets developed from cellus

employed for quick multiplication of plants.

Important tips

• Grafting is not possible in monocots as they do not bear cambium.
• Slip is a small piece or plantlets which can be separated and used for propagation.
• Tissue culture technique was first thought of by Haberlandt (1902) and Hanning (1908) but successful attempt was made by

White (1932) in case of tomato root.

• Steward (1964) gave the concept of cellular totipotency.
• Guha and Maheshwari (1964) developed haploid culture or pollen grain culture.
• Skoog and Miller (1957) found that morphogenesis or differentiation in callus depends on two hormones–auxin (favours root formation) and cytokinin (favours shoot formation).
• Somatic hybridization or protoplast fusion was first reported by Harrie and Matkins.
• Winkler (1934) introduced the term apomixis.
• Graft hybrid is a chimera shoot formed by an adventitious bud formed at the junction of stock and scion. First reported in 1644 as Bizzaria orange (half orange half cistron) in Italy.
• In angiosperms apospory was first reported by Rosenberg (1907) in Hieracium.
• The ability of mature cells to develop new individual in vitro is called cellular totipotency. Vascular cambium show totipotency which cuts secondary xylem and secondary phloem.
• The formation of sporophyte directly from gametophyte without gamete formation and fusion is called apogamy.
• Walking fern propagates through leaf tip.

 Sexual reproduction.                                                                                                                                           

Sexual reproduction in flowering plants involves transformation of diploid sporophytic cells into haploid gametophytic cells by meiosis and subsequent fusion of haploid gametes of opposite sex to form diploid zygote. The zygote then develops into an embryo which ultimately forms a diploid plant body. In flowering plants, all these steps of sexual reproduction occur within specialized reproductive organs, called the flowers.

1. Structure of the flower : Morphologically flower is a modified shoot meant for sexual reproduction of the plant. Typically, it is

Androecium collection of stamens

a condensed branch in which internodes have become condensed, bringing nodes very close to one another, and the leaves are modified to form floral whorl that directly or indirectly participate in the process of reproduction.

The flower is commonly borne on short or

Filament

long stalk

Corolla

collection of petals,

Anther

dithecous introrse

Style

Stigma bilobed, short

long stalk called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus).

2. Parts of a flower : A typical angiospermic flower consists of four whorls of floral appendages attached on the receptacle :

bright yellow, expanded above, free or polypetalous

Calyx

collection of sepals, green, polysepalous

Pedicel

stalk of flower

Fig : L.S. of a typical flower

Ovary

bicarpellary, syncarpous, bilocular, superior

axile placentation

Receptacle

base of flower on which floral organs are arranged, also called thalamus

calyx, corolla, androecium and gynoecium. Of these, the two lower whorls (i.e., calyx and corolla) are sterile and considered as nonessential, accessory or helping whorls. The two upper whorls (i.e., androecium and gynoecium) are fertile and considered as essential or reproductive whorls.

1. Calyx : It is the outermost whorl of the flower. It is composed of leaf like green sepals. The sepals are essentially green in colour but in some cases they are coloured like petals. Such a condition of calyx is called petaloid. Sepals enclose the bud and protect the delicate part within. They prevent rapid transpiration from the inner parts of the flower.
2. Corolla : This is the second whorl of the flower and consists of a number of petals. Petals are generally brightly coloured and sometimes fragrant which make the flower to become attractive. Petals usually attract the insect pollinators and helps in pollination. The petals and sepals together form the floral envelope (perianth).
3. Androecium : It is the third whorl of flower and is the male reproductive organ consisting of stamens. Each stamen is made of filament and anther. The filament supports anther at its tip. Usually anthers are bilobed and contain four microsporangia (or pollen sacs), but sometimes they have only one lobe and two microsporangia. The portion of stamen which connects the anther and the filament is known as connective.
4. Gynoecium : This is the last and the fourth whorl of flower and is the female reproductive organ of the flower. It occupies the central position on the receptacle and composed of ovary, style and stigma and the component parts are called carpels. Ovary encloses the ovules. Stigma is the receptive spots which lodges the pollen grains. Style is the connection between stigma and ovary.

(3)Functions of a flower

1. Flowers are modifications of shoot to perform the function of sexual reproduction. The fertile leaves become microsporophylls (stamen) and megasporophylls (carpels) which bear anthers and ovules respectively. The anthers produce pollen grains and the ovules possess eggs.
2. Flowers of most of the angiosperms are shaped variously to help diverse modes of pollination.
3. Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and fertilization.
4. The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after fertilization.
5. Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds.

4. Inflorescence : The flowers are arranged in some definite manner on the plant in each species of the flowering plants. The mode of arrangement of flowers on a specialised branch on top of the plant which bears flowers is called inflorescence. The axis of the inflorescence is called peduncle.

Depending upon the arrangement of flowers, inflorescence is classified as follows :

1. Racemose (indeterminate), the tip of axis continues to grow, flowers produced laterally.

Main axis elongated

Main axis shortened

Main axis flattened to form receptacle

1. Raceme : Stalked flowers laterally borne.

e.g. Delphinium, Crotolaria, Radish, Mustard.

2. Panicle : Branched raceme (compound raceme).

e.g. Gold mohur.

3. Spike : Raceme with sessile flowers.

e.g. Achyranthes, Callistemon, Antirrhinum.

4. Compound spike/spikelet : Axis is branched, made of spikelets consisting of one or more flowers.

e.g., Amaranthus, Wheat, Paddy, Sugarcane, Oat.

5. Catkin : Sessile and unisexual flowers, axis pendulous.

e.g. Morus (mulberry), Salix (willow), Populas (poplar), croton.

6. Spadix : Flowers sessile, axis fleshy, bracts coloured (spathe).

e.g., banana, maize, colocasia.

1. Corymb : Floral stalks unequal, flowers lie on the same level.

e.g. Iberis amara (candytuft).

2. Compound corymb : Central axis of corymb branched, each branch bearing corymb.

e.g., Pyrus.

3. Umbel : Floral stalks equal, flowers lie on the same level.

e.g. Hydrocotyle.

4. Compound umbel : Axis of umbel branched, each branch bearing umbel.

e.g., Coriander, Fennel, Carrot.

Capitulum or Head : Florets arising from disc-like receptacle.

e.g. Sunflower, Marigold.

1. Uniparous or Monochasial : Only one lateral branch produced.

2. Cymose (determinate) tip

of axis terminates into a flower, axis growth stops, lateral branches bear flowers; branches also terminate into flower

3. Special types

• Scorpioid : Lateral branches develop on alternate sides.

e.g., Tecoma, Rannenculus.

• Heliocoid : Lateral branches develop on the same side.

e.g., Drosera.

2. Biparous or Dichasial : Two lateral branches produced.

e.g., Ixora, Diathus.

3. Multiparous or Polychasial : More than two lateral branches produced.

e.g. Calotropis, Nerium.

1. Cyathium : Cup-shaped involucre that encloses one female and many male flowers.

e.g., Euphorbia.

2. Verticillaster : Dichasial cyme reduced to scorpioid cymes, one on each side of the node.

e.g., Ocimum, Salvia.

3. Hypanthodium : Cup-shaped receptacle with opening, male flowers near opening of cup, female flowers at base.

e.g., Ficus (Peepal, Banyan).

5. Relative position of floral organs on thalamus : Depending upon the form of thalamus and the position of floral whorls with respect to the ovary, the flowers are of the following three types :

1. Hypogyny : In this case the thalamus is convex-like and ovary occupies the highest position on it. The outer three whorls, viz. sepals, petals and stamens and inserted one above the other but below the ovary. Since the

ovary lies above the other parts, it is described as superior and the rest of the floral whorls as inferior. A flower having hypogyny is called hypogynous. e.g. China rose, Brinjal, Mustard, etc.

2. Perigyny : In some cases, the receptacle or the thalamus forms a swallow or deep cup-shaped structure around the ovary. The pistil is attached at the centre of the concave thalamus. The sepals, petals and stamens are attached at the margins of the thalamus, the flowers are said to

be perigynous and ovary is superior. Different type of flowers show different degrees of perigyny. e.g. Rose, Pea, Bean, Prunus, etc.

1. B                                                  C

Fig : Insertion of Floral parts on thalamus

A. Hypogynous; B. Perigynous; C. Epigynous

3. Epigyny : In this condition the margin of thalamus grows further upward completely enclosing the ovary and getting fused with it and bear the sepals, petals and stamens above the ovary. The ovary in such cases is said to be inferior and the rest of the floral members superior. e.g. Apple, Sunflower, Cucumber, Guava, etc.

6. Placentation : The ovary contains one or more ovules, which later become seeds. The ovule bearing regions of the carpel is called placenta. The arrangement of placentae and ovules within the ovary is called placentation. The placenta is the cushion-like structure to which the ovules are attached inside the cavity of the placenta, placentation is of the following types :

1. Marginal : In this type of placentation, the ovary is simple, unilocular and the ovules are arranged along the margin of the unilocular ovary. The placenta develops along the ventral suture of the ovary. e.g. Pea, Gram,

Goldmohur, etc.

2. Axile : It is found in a compound ovary which is two or more chambered, usually as many as the number of carpels e.g. Petunia and Asphodelus. The placentae bearing the ovules develop from the central column or axis which is formed by the fusion of margins of carpels. In certain cases the number of chambers (loculi) increases due to the false septum formation. e.g. Datura, Tomato, etc.
3. Free central : In this free central placentation, the gynoecium is polycarpellary and

Dorsal suture Ventral suture Placenta

Locule

Ovule

A

Ovary wall

Ovule Locule

Placenta

Central axis

Ovary wall Ovule

Placenta Central

axis Locule septum

2. Ovary wall                   C

Loculoe Ovule

Ovules Septum

syncarpous. The ovary in early stages is multilocular, but soon the septa break down leaving it as a unilocular structure. e.g. Dianthus, Slience, Primula, etc.

D                                                            E                                                               F

Fig : Different types of placentations A. Marginal;

B. Parietal; C. Axile; D. Central; E. Basal; F. Superficial

4. Parietal : In parietal placentation, the ovary is usually one-chambered but in some cases it becomes bilocular due to the formation of false septum, e.g. Brassica compestris (Sarson). The placentae bearing the ovules develop on the inner wall of the ovary at places where the margins of two adjoining carpels meet. The number of placentae corresponds to the number of fused carpels. e.g. Poppy, Mustard, Cactus, etc.

5. Basal : In this type of placentation, ovary is bicarpellary, syncarpous and unilocular and a single ovule is borne at the base of ovary. e.g. Marigold, Sunflower, etc.
6. Superficial : The ovary is multicarpellary, syncarpous, and large number of loculi without specific order

e.g. Waterlily (Nymphea).

7. Symmetry of the flower : The number, shape, size and arrangement of floral parts (i.e. calyx, corolla, androecium and gynoecium) in a flower determines its symmetry. On the basis of symmetry, flowers can be of following three types :
   1. Actinomorphic or Regular flowers : Such flowers can be divided by vertical plane into two equal and similar halves. e.g. Mustard, Hibiscus, Brinjal, etc.
   2. Zygomorphic or Irregular flowers : These flowers can be divided into two equal halves only along one median longitudinal plane. e.g. Pea, Iberis, Ocimum, etc.
   3. Asymmetrical flowers : These flowers cannot be divided into two equal halves along any vertical plane.

e.g. Canna, Maranta.

 Microsporogenesis.                                                                                                                                             

The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis.

Microsporogenesis is well studied under following heads :

1. Structure of anther : The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores.

A typical anther consist of four microsporangia (tetrasporangiate) and such anthers is called dithecous e.g.

Connective

Epidermis

Endothecium

Middle layers Tapetum

Stomium Pollen grains

mostly plants. In members of Malvaceae anthers are reniform or

Fig : T.S. of mature dithecous anther

kidney shaped and consist of two microspoangia (bisporangiate), such anthers is called monothecous. In the smallest parasitic angiosperm, Arceuthobium minutissimum, anthers consist of only one microsporangium (monosporangiate).

The pollen sacs are surrounded by following 4 layers :

1. Epidermis : This is the outermost single layered and protective. In Arceuthobium, cells of epidermis develops a fibrous thickening and the epidermis is designated as exothecium.
2. Endothecium : Inner to epidermis, there is a single layer of radially elongated cells. Cells of endothecium develop fibrous thickening (made up of cellulose with a little pectin and lignin) which help in the dehiscence of anther. In between these cells, a few cells without thickening are also present. These thick walled cells collectively form the stomium.
3. Middle layer : Three to four layers of thin walled cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells.

4. Tapetum : This is the innermost layer of the wall. The cells are multinucleate(undergo endopolyploidy) and polyploid. Tapetal cells are nutritive.

In these cells the Ubisch bodies are present which help in the ornamentation of microspore walls. A compound sporopollenin is secreted in the exine of microspore wall. According to Periasamy and Swamy (1966), developmentally the tapetum has dual nature.

The tapetum is of two types :

1. Amoeboid or Periplasmodial tapetum : In young condition cell wall of tapetal cells breaks, so protoplast of these cells become free between microspore mother cell and form mass of tapetal periplasmodium.

e.g. Alisma, Typha, Tradescantia.

2. Secretory or Glandular tapetum : This is the most common type of tapetum which remains insitu as such throughout. The tapetal cells secretes nourishment that passes into sporogenous cells. This tapetum attains its maximum development at the stage of pollen tetrads and then degenerates.
   2. Development of anther and formation of microspores (Pollen grains) : The young anther consists of homogenous mass of paranchymatous cells surrounded by epidermis. It soon becomes four lobed. In each of the four lobes, some of the hypodermal cells begin to act as archesporial initials. Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell. The primary parietal cell divides to form 3-5 wall layers, i.e., endothecium, middle layers and tapetum. The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes.

Each microspore mother cell divides meiotically to form four haploid microspores or pollen grains and remains arranged in tetrads. The arrangement of pollen grains in a tetrad is

affected by cytokinesis during meiosis. It is of two types :

1. Simultaneous type : The cytokinesis occurs simultaneously at the end of meiosis II to form tetrahedral tetrad. Here wall formation is centripetal. It is common in dicotyledons.
2. Successive type : The cytokinesis occurs twice i.e. each of the two nuclear division is followed by wall formation to form

an isobilateral tetrad. Here the wall formation is centrifugal. It is found in monocotyledons.

Fig : Microsporogenesis and two types of cytokinesis

Besides tetrahedral and isobilateral tetrads, other types of tetrads are linear (e.g. Halophila), T-shaped (e.g.

Aristolochia, Butomopsis) and decussate (e.g. Magnolia).

Tetrahedral         Isobilateral        Decussate         T-Shaped             Linear

Fig : Different types of microspore tetrads

Now the microspores are separated from tetrad. In Drosera, Typha, Elodea, Hydrilla, etc. all the four pollen grains do not separate and thus form compound pollen grains. In the members of the family Cyperaceae (Cyprus), out of 4 pollen in a tetrad, 3 degenerate and one remains alive. So one meiosis produces one pollen. Sometimes

more than four pollens are produced from one microspore mother cell. It is called as polyspory e.g. Cuscuta. In Calotropis (Asclepiadaceae) and some orchids all the pollen grains of an anther lobe form a typical structure called pollinium.

3. Development of male gametophyte (Microgametogenesis) : Microspore or pollen grain is the first cell of male gametophyte (partially developed). It is unicellular and haploid. The shape varies from oval to polyhedral. The wall of the pollen grain is made of two layers.

The outer layer is called exine. It is made up of sporopollenin (derived from carotenoid). It is thick and ornamented. At certain places, exine remains unthickened or missing and these places are known as germ pores. Sporopollenin is resistant to physical and biological decomposition. So pollen wall preserved for long periods in fossil deposits. The inner intine is thin, delicate and is made of cellulose and pectose.

In insect pollinated flowers, the exine of the pollen grain is covered with a yellowish, viscous and sticky substance called pollenkitt. This is perhaps the protective envelope which also sticks to the body of the insects and thus helps in pollination. It is chiefly made up of lipids and carotenoids. In monocots germ pores are absent and there is one germinal furrow. The development of male gametophyte from pollen grain is called microgametogenesis.

4. Pre-pollination development : Microspores starts germinating in situ (i.e. while enclosed inside the microsporangium or pollen sac) and is called precocious. Microspores may be best defined as partially developed male gametophyte. Microspore nucleus divides mitotically to form a smaller generative cell lying next to spore wall and a much larger vegetative cell (or tube cell). A callose layer is deposited around the generative cell. The generative cell loses its contact with the wall of microspore and becomes free in the cytoplasm. The callose layer than dissolves. The pollen grains are shed from the anther at this bicelled stage (rarely three celled).
5. Post-pollination development : The liberated pollen grains are transferred to the receptive surface of the carpel (i.e., stigma) by the process called pollination. On the stigma, the pollen grain absorbs water and swells within a few minutes. It releases the wall-held recognition factors. These factors determine whether the pollen grain will germinate on the stigma or not. Subsequent to mutual recognition, the vegetative (or tube) cell enlarges and comes out through one of the apertures in the form of a pollen tube. The wall of pollen tube is the extension of intine. The tube secretes exogenous pectinases and other hydrolytic enzymes to create a passage for its entry. It absorbs nourishment from the transmitting tissue of the style. Gradually, the vegetative and generative nuclei are carried by the pollen tube, the farmer lying at its tip. The generative cell divides to form two non-motile male gametes. The tube nucleus has no important function and may disintegrate.

Exine        Intine

Vegetative cell

Pollen tube

Male gametes

Vegetative nucleus

A                        B                         C                                                                       D

Nucleus

Vacuole Cytoplasm

Generative

cell               Male gametes

Fig : Different stages of microgametogenesis

 Megasporogenesis.                                                                                                                                             

The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule.

Megasporogenesis can be studied under following heads :

1. Structure of ovule : Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose

Raphe

Funicle

Chalaza

Nucellus

Outer integument Inner integument Antipodals

Polar nuclei Egg apparatus

Nucellus Micropyle

a large parenchymatous tissue known as nucellus.

Fig : Structure of ovule

The residual part of nucellus in the mature seed is called perisperm. In the centre of the nucellus is situated a female gametophyte known as embryo sac.

Following are the conditions seen in ovule in relation to integuments :

1. Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons.
2. Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons.
3. Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc.