REPRODUCTION IN FLOWERING PLANTSFlame Institute

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 :

(i) 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).

(ii) 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:

(a) Diploid parthenogenesis : Embryo develops from unfertilized diploid egg.

(b) Diploid apogamy : Embryo develops from any diploid cell of embryo sac except egg.

(iii) 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 :

(a) Somatic apospory : Embryo sac is formed from somatic cell.

(b) 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. It is very common mode of reproduction and it may be natural vegetative propagation or artificial vegetative propagation.

(i) 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.

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

(b) 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.

(c) 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.

(d) 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 stem. These are the modifications of axillary buds. Bulbils grow into new plants when shed from the mother plant.

(ii) 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. Some of the artificial methods are given below :

(a) 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. Grapes, Sugarcane, Rose, Bougainvillea, Carnation, Coleus, Duranta, etc. are propagated by stem cuttings.

(b) 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 (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.

(c) 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.

(d) 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 in obtaining virus free plants, homozygous diploids and in commercial micropropagation of Orchids, Carnation, Gladiolus, Chrysanthemum and other Ornamental plants. This method is also employed for quick multiplication of plants.

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 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 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 : 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.

(3) Functions of a flower

(i) 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.

(ii) Flowers of most of the angiosperms are shaped variously to help diverse modes of pollination.

(iii) Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and fertilization.

(iv) The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after fertilization.

(v) Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and seeds.

(4) 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 :

(i) Hypogyny : In this case the thalamus is convex and ovary occupies the highest position on it. The outer three whorls, viz. sepals, petals and stamens 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.

(ii) 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 half inferior or half superior. Different type of flowers show different degrees of perigyny. e.g., Rose, Pea, Bean, Prunus, etc.

(iii) 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.

(5) 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 :

(i) 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.

(ii) 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.

(iii) Free central : In this free central placentation, the gynoecium is polycarpellary and 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.

(iv) Parietal : In parietal placentation, the ovary is usually one-chambered but in some cases it becomes bilocular due to the formation of false septum called replum, 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.

(v) 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.

(vi) Superficial : The ovary is multicarpellary, syncarpous, and large number of loculi without specific order e.g., Waterlily (Nymphea).

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.

The pollen sacs are surrounded by following 4 layers :

(i) 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.

(ii) 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 thin walled cells collectively form the stomium.

(iii) 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.

(iv) 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

(a) Amoeboid or Periplasmodial tapetum.

(b) Secretory or Glandular tapetum.

(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 in the tetrads can be tetrahedral, isobilateral, linear, T-shaped and decussate.

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 start 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.

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 (Megasporangium) : 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 a large parenchymatous tissue known as nucellus.

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 :

(i) Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons.

(ii) Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons.

(iii) Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc.

(iv) Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of Euphorbiaceae. e.g., Castor (Ricinus).

(v) Ategmic : In some parasites like Loranthus, Viscum, Santalum etc., there is no integument. Such an ovule is called ategmic.

(2) Kinds of ovules : Depending upon the shape and orientation, the ovules of angiosperms are classified into following types :

(i) Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight line. This is most primitive type of ovules. e.g., Betel, Piper, Polygonum.

(ii) Anatropous : The body of the ovule is completely inverted (turn at 180^o angle ) so that micropyle and hilum come to lie very close to each other. e.g., 82% of angiosperm families.

(iii) Hemianatropous : Ovule turns at 90^o angle upon the funicle or body of ovule is at right angle to the funicle e.g., Ranunculus.

(iv) Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down slightly. e.g., in members of Leguminosae and Cruciferae.

(v) Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe e.g. Lemna, Poppy, Alisma.

(vi) Circinotropous : The ovule is initially orthotropous but becomes anatropous due to unilateral growth of funicle. The growth continues till the ovule once again becomes orthotropous. As a result funicle completely surrounds the body of the ovule e.g., Opuntia (prickly pear).

(3) Formation of megaspore : The ovule or the megasporangium develops as a small protuberance of the placental tissue. In the very young ovule a single hypodermal cell is differentiated as archesporium cell. The archesporial cell may directly function as megaspore mother cell (tenuinucellate ovule) or may divide periclinally to form an outer parietal cell and an inner sporogenous cell (crassinucellate ovule). The sporogenous cell directly behaves as megaspore mother cell (or megasporocyte). The diploid megaspore mother cell enlarges in size and divides by meiosis to form a linear tetrad of four haploid megaspores. Occasionally T-shaped or inverted T-shaped (^) tetrads are also formed. Megaspore is the first cell of female gametophyte.

Of the linear tetrad, three megaspores towards the micropyle degenerate. The lowermost, i.e., the chalazal megaspore enlarges and remains functional. It later produces an embryo sac.

(4) Development of female gametophyte (Megagametogenesis) : The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.

(i) Monosporic type (Polygonum) : In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac. The functional haploid megaspore enlarges in size and by means of three successive mitotic divisions, gives rise to an 8-nucleate embryo sac. Of these, four nuclei occur at micropylar end and the other four at the chalazal end. Three nuclei at the micropylar end form egg apparatus and the fourth migrates from the both pole to the centre and form polar nucleus.

A fully developed typical or polygonum type of embryo sac is large and oval structure consisting of seven cells and eight nuclei.

(a) Egg apparatus : This is a group of 3 cells situated at the micropylar end. The centrally located cell is called egg cell. On its sides are present two synergids. Egg cell has a large vacuole at its upper end and a prominent nucleus near its lower end. Synergids show a filiform apparatus attached to their upper wall. It is known to attract and guide the pollen tube. Each of the synergids has a vacuole at its lower end and the nucleus at its upper end.

(b) Polar nuclei : These are situated in the centre of the embryo sac representing a large binucleate central cell. Generally, both the polar nuclei fuse before fertilization and form a single diploid nucleus called secondary nucleus or definitive nucleus.

(c) Antipodals : The three cells situated at the chalazal end are called antipodals. These cells generally degenerate soon after fertilization.

Polygonum type occurs in about 70% of angiosperms and is the common type.

(ii) Bisporic type : In this type two megaspore nuclei take part in embryo sac formation.

(iii) Tetrasporic type : This type of embryo sac develops from four megaspore nuclei.

Pollination

The process of transfer of pollen grains, from an anther to the stigma of the same flower or of different flower. It is of two types :

(1) Self pollination : This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types :

(i) Autogamy : It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower.

(ii) Geitonogamy : It is a kind of pollination in which the pollen from the anthers of one flower are transferred to the stigma of another flower borne on the same plant. It usually occurs in plants which show monoecious condition (unisexual, male and female flowers are borne on the same plant). Geitonogamy involves two flowers but these belong to the same parent plant.

Merits

Pollen grains are not wasted.
The purity of the generation is maintained.

Demerits

New and healthier varieties are not formed
It results in weaker progeny, producing weaker seeds and plants.

Contrivances for self pollination : The major contrivances or adaptations which favours self pollination are :

(a) Bisexuality : Flowers should be bisexual or hermophrodite.

(b) Homogamy : Anthers and stigma of the bisexual flowers of some plants mature at the same time. They are brought close to each other by growth, bending or folding to ensure self pollination. This condition is called homogamy. e.g., Mirabilis (Four O, clock), Catharanthus (= Vinca), Potato, Sunflower, Wheat, Rice, etc.

(c) Cleistogamy : Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, e.g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicious, colourless and do not secrete nectar.

(2) Cross pollination : Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. It is also called xenogamy.

Merits

Seeds are more and viable.
Progenies are healthier.
Adaptability is better.
New varieties can be produced.

Demerits

The process is not definite because plants depend on agencies.
Large amount of pollen grains are wasted.

Contrivances for cross pollination : Nature favours cross pollination. All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated.

The main contrivances ensuring cross pollination are as follows :

(i) Diclincy or Unisexuality : In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types :

Monoecious plant : When male and female flowers are borne on the same plant. g., Maize, Cucurbits, Castor.
Dioecious plant : When male and female flowers are borne on different plants. g., Carica papaya, Cannabis.

(ii) Dichogamy : In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. When stamens mature earlier than the stigma, it is known as protandry and the flowers are called protandrous e.g., Coriander, Jasmine, Sunflower, Lady’s finger, etc. When stigma matures earlier than the stamens, it is known as protogyny and the flowers are called protogynous. e.g., Rose, Tobacco, Crucifers, etc.

(iii) Heterostyly : The plants of some species in which flowers are dimorphic. Thus facilitate cross pollination. Some of them possess a long style but short stamens and are known as pin-eyed while others have short style and long stamens. These are known as thrum-eyed. e.g., Oxalis.

(iv) Herkogamy : In some bisexual flowers where the stigma and anthers mature at the same time, self pollination is avoided by some sort of barrier. The flowers show following contrivances :

The male and female sex organs lie at some distance from each other.
In some flowers corolla has peculiar forms which act as barrier in self pollination. g., Aristolochia.
In some other flowers, the pollens are held together to form pollinia which can only be carried away by insects. g., Orchids and Calotropis.

(v) Self sterility or Incompatibility : When pollen grain of an anther do not germinate on the stigma of the same flower, then such flower is called self sterile or incompatible and this condition of flower is called self sterility, intraspecific incompatibility or self incompatibility. In these flowers cross pollination is the only means for fertilization and production of seeds.

Agents for cross pollination : Cross pollination involves external agents for the transfer of pollen grains of one flower to the stigma of another flower. There are two main groups of agents : (i) Abiotic agents like wind and water (ii) Biotic agents which include animals of different types such as insects, birds, bats, snails, etc.

(i) Abiotic agents

(a) Anemophily : When flowers are pollinated by wind agency, the phenomenon is known as anemophily. Anemophilous flowers are small and inconspicuous with long and versatile stamens. e.g., Sugarcane, Maize, Wheat, Bamboo, Pinus, Papaya, Grasses, Typha, Datepalm, Coconut, Mulberry, Chenopodium, etc. This type of pollination mainly observed in Graminae.

(b) Hydrophily : When the pollination takes place through the agency of water, it is known as hydrophily. All aquatic plants are not hydrophilous some are anemophilous e.g., Potamogeton, Myriophyllum or Entomophilous e.g., Alisma, Lotus. Hydrophily is of two types :

Hypohydrophily : Plants which are pollinated inside the water g., Zostera, Ceratophyllum, Najas, etc.
Epihydrophily : Plants which are pollinated outside the water. g., Vallisneria (Ribbon weed).

(ii) Biotic agents

(a) Entomophily : When pollination is brought about by the agency of insects, it is known as entomophily or insect pollination. About 80% pollination occurs by insects like moths, beetles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which has a spinous and sticky exine due to presence of pollenkitt. The stigmas of such flowers are long rough and sticky. Salvia is excellent example of insect pollination is which pollination occurs by lever or turn pipe mechanism. Other examples of insect plants are Yucca (by Tageticula moth), Orchid Ophrys speculum (by Colpa aurea a hairy wasp), Ficus (by Blastophega), etc. Yucca is pollinated by Pronuba (= Tegaticula) yuccasella which passes its larval stage inside the ripening ovary. The flower of orchid ophrys resemble in shape colour and odour to female wasp of colpa aurea (mimicry). The male wasps pollinate the flowers mistaking them as female (pseudocopulation).

(b) Ornithophily : When flowers are pollinated by birds, the phenomenon is known as ornithophily. The most common bird pollinators are Sun bird, Humming bird, Crow, Bulbul, Parrot, Mynah, etc. The birds visit a large variety of flowers such as Bombax (red silk cotton), Erythrina (Coral tree), Callistemon (Bottle brush), Bignonia, Agave, etc. Flowers are brightly coloured and produce plenty of nectar and large quantities of pollen. Humming bird pollinates while hovering over the flowers and sucking nectar. The bird can derive about half of its body weight of nectar in a single day. The nectar is chiefly made of sugars and provides a sweet drink to the bird.

(c) Chiropterophily : It is a mode of pollination performed by bats. The flowers they visit are large, dull-coloured and have a strong scent. Chiropterophilous flowers produce abundant pollen grains. These flowers secrete more nectar than ornithophilous flowers and open at night emit a good fragrance. e.g., Kigelia pinnata (Sausage tree), Adansonia (Baobab tree), Bauhinia megalandra, Anthocephalus (Kadam tree), etc.

(d) Malacophily : Pollination by slugs and snails is called malacophily. Land plants like Chrysanthemum and water plant like lemna shows malacophily. Arisaema (aroid; snake plant) is often visited by snails.

(e) Myrmecophily : Pollination by ants. e.g., Anemone nemarosa (fruit).

Fertilization

The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).

(1) Germination of pollen grain on stigma and growth of pollen tube : Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity.

G.B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two male gametes. Later, the vegetative cell degenerates. The pollen tube now reaches the ovule after passing through the style.

(2) Entry of pollen tube into ovule : After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes :

(i) Porogamy : When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily.

(ii) Chalazogamy : The entry of pollen tube into the ovule from chalazal region is known as chalazogamy. Chalazogamy is less common. e.g., Casuarina, Juglans, Betula, etc. It was first observed by Treub (1981) in Casuarina.

(iii) Mesogamy : The pollen tube enters the ovule through its middle part i.e., through integument (e.g., Cucurbita, Populus) or through funicle (e.g., Pistacia).

(3) Entry of pollen tube into embryo sac : The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus. The synergids direct the growth of pollen tube by secreting some chemical substances (chemotropic secretion). The tip of pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of pollen tube enlarge and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.

(4) Double fertilization : The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger (1884).

The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S.G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms only.

Endosperm

Endosperm is the nutritive tissue for the developing embryo and also the seedling. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion or fusion of a male gamete with secondary nucleus of the central cell.

(1) Types of endosperm : On the basis of development, endosperm are of three types :

(i) Nuclear endosperm : In the nuclear type of endosperm development, the primary endosperm nucleus divides by repeated mitotic free nuclear divisions without the formation of walls. It results in the formation of a large number of free nuclei in the central cell of the embryo sac. A big central vacuole develops in the embryo sac pushing all the nuclei to the peripheral cytoplasm. Finally cell wall formation takes place from the periphery of the embryo sac towards the centre leading to the formation of cellular endosperm tissue. In Coconut, the endosperm is multicellular in the outer part and free nuclear in the centre. Nuclear endosperm is the most common type of endosperm and mostly found in polypetalae. e.g., Cotton, Zeamays, Capsella etc.

(ii) Cellular endosperm : In the cellular type of endosperm development, the first nuclear division of the primary endosperm nucleus is immediately followed by the wall formation. The first division results in the formation of two equal sized chambers : chalazal and micropylar chambers. The subsequent divisions are followed by regular cell wall formation. This type of endosperm formation is common in gamopetalae. e.g., Petunia, Datura.

(iii) Helobial endosperm : In the helobial type of endosperm development, the endosperm is intermediate between cellular and nuclear types. The division of primary endosperm nucleus is followed by wall formation and as a result two chambers : micropylar and chalazal chambers, are formed. Generally the chalazal cell does not divide further and function as haustorium. Nucleus of the large micropylar cell divides by repeated free nuclear divisions and further development takes place in the same way as the nuclear endosperm. Helobial type of endosperm development is prevalent in monocotyledons. e.g., Erumurus.

(2) Some terms related to endosperm

(i) Ruminate endosperm : Mature endosperm with irregularity and unevenness in its surface is called ruminate endosperm. Rumination is caused by the activity of seed coat or by the endosperm itself. It is found in about 32 families of angiosperm. e.g., Annonaceae, Palmae, Myristicaceae, etc.

(ii) Mosaic endosperm : In some cases, the tissue of endosperm is not homogeneous but there are patches of different colours. Such type of endosperm is called mosaic endosperm and was observed by Webber (1990) in Zea mays. In maize endosperm, red and white patches appear irregularly distributed. In Petunia and Tomato, endosperm shows two types of tissues – some consisting of diploid cells and some triploid cells. These two types of cells intermix to form mosiac.

(iii) Xenia : The effect of pollen on endosperm is called xenia. This term was given by Focke (1881). e.g., Maize.

(iv) Metaxenia : The effect of pollen on somatic tissue lying outside the endosperm is known as metaxenia. Metaxenia term given by the swingle (1928). e.g., Datepalm.

Embryo

(1) Development of embryo (Embryogeny) : The zygote after a period of rest develops into embryo. The process of development of mature embryo from diploid zygote is called embryogenesis.

(i) In dicotyledons : The normal type of dicot embryo development has been studied in Shepherd’s purse (Capsella bursapastoris) family Cruciferae. This is called as crucifer or onagrad type of embryo development. This development of embryo is endoscopic i.e., apex is downward or towards inside. The first division of zygote is transverse which produces a basal cell (cb) towards the micropyle and a terminal cell (ca) towards chalaza. The basal cell divides by transverse division and the terminal cell by a longitudinal division, so 4 celled T-shaped proembryo is produced. The two basal cells divide by transverse division and form 6-10 celled suspensor. The upper most cell of the suspensor is vasicular cell and lowest cell is called hypophysis which forms radicle and root cap.

The two apical cells first divide by longitudinal division (at right angle to first one) and then by transverse and periclinal division. So sixteen celled globular embryo is produced. Due to differentiation of cotyledons globular embryo becomes heart shaped.

Mature embryo in dicots consists of two lateral cotyledons, terminal plumule or stem tip and radicle or root tip.

(ii) In monocotyledons : The normal type of monocot embryo development has been studied in Sagittaria sagittaefolia. The early development of dicot and monocot embryos is similar upto globular stage. Later on differentiation starts. Suspensor is single celled and vascular. There is only one terminal cotyledon called scutellum (shield shaped). In grasses the second cotyledon is reduced called epiblast.

The basal cell (cb) divides by a transverse wall into two cells – ci and m. The cell ci divides once again to form n and n’ cells. Of these n’ is the outermost which develops into suspensor. The cell n forms parts of root cap the cell m contributes to the remaining part of root cap and a part of the radicle.

The terminal cell (ca) divides by two vertical walls, at right angles to one another. This results in the formation of a quadrant (q). Cells of the quadrant divide periclinally differentiating into the peripheral cells and the inner group of cells. The repeated divisions in both peripheral and central group of cells results in the formation of two regions –l and l’. Region l produces the lower part of cotyledon while upper part of cotyledon, hypocotyl and plumule are formed by l’ region.

(2) Polyembryony : Occurrence of more than two embryo in the seed is known as polyembryony. It was discovered by A.V. Leeuwenhock (1719) in Citrus. It may be :

(i) Cleavage polyembryony : Due to cleavage of zygote or proembryo into two or more embryos and each split part develops into an embryo. This type of polyembryony is common in gymnosperms than in angiosperms. Erythronium americanum, Nymphaea advena, Crotalaria, etc., are some of the angiosperms showing cleavage polyembryony.

(ii) Simple polyembryony : Due to presence of more than one embryo sac and so oospore or egg. e.g., Brassica.

(iii) Mixed polyembryony : More than one pollen tube entering an ovule and fertilizing synergids (as in Argemone maxicana) and antipodal cell (as in Ulmus americana).

(iv) Adventive polyembryony : Diploid nucellus or integument cells form embryos e.g., Citrus, Opuntia, Mangifera.

If extra embryos develop from same embryo sac, it is called true polyembryony and if embryos develop elsewhere it is called false polyembryony. In Balanophora, an extra embryo develops from endosperm.

Parthenocarpy

The formation of fruits without fertilization is called parthenocarpy. Such fruits are either seedless or non-viable seeds. Parthenocarpy is of two types :

(1) Natural parthenocarpy : When seedless fruits are produced without any special treatment from the ovaries in the absence of pollination and fertilization, the phenomenon is called natural parthenocarpy. e.g., Grapes, Banana, Pineapple and Noval oranges.

(2) Induced parthenocarpy : When seedless fruits are produced by spraying the flowers with either water extract of pollen grains or growth promoting hormones such as Indole acetic acid (IAA), Naphthalene acetic acid (NAA), Gibberellic acid (GA), etc. the phenomenon is called induced parthenocarpy. e.g., Tomato, Black berry, Fig, Lemon, Apple, Orange, Pear. etc.

? Tissue culture technique was first thought by Haberlandt (1902) and Hanning (1908) but successful attempt was made by White (1932) in case of tomato root.

? In angiosperms apospory was first reported by Rosenberg (1907) in Hieracium.

? Rudolf Camerarius (1694) first describe sexual reproduction in plants.

? Origin of pollen sac is eusporangiate while that of megaspore mother cell (embryo sac or megagametophyte) is leptosporangiate.

? Pollen grain of Zoostera is filamentous and without exine.

? Adansonia Flowers bears 1500-2000 stamens.

? In Aristolochia elagans all types of pollen tetrads (tetrahedral, isobilateral, T-shaped, ^ shaped and decussate) are found.

? Edible pollens are produced in rose.

? Best temperature for growth of pollen tube is 20-30^oC.

? Pollen tube secretes IAA, cytokinins and hydrolysing enzymes for separation of cells in case of solid styles.

? Ubisch discovered the role of tapetum in anthers of angiosperms.

? Size of pollen (i) Smallest-Myosotis, 2.5-3.5mm. (ii) Biggest Mirabilis, diamter 250mm (iii) Longest Zoostera – 2500mm.

? Onagard or Crucifer type of embryo development is endoscopic (i.e., apex is downward or towards inside) in tracheophytes and exoscopic (towards outside or tip of archegonium) in bryophytes.

? Embryosac (polygonum type) was first studied by Strasburger.

? The pollination mechanism of Calotropis is referred as translator mechanism.

? Hay fever is allergic reaction to the presence of pollen in the air. Plants commonly causing hay fever are Amaranthus, Chenopodium, Sorghum and Castor.

? Erythrina is pollinated by crows as well as squirrels.

? The seed with double endosperm is found in Coconut (Cocus nucifera) (i) Liquid endosperm (ii) Cellular endosperm.

? Stony endosperm is present in Betel nut (Areca nut) and Date palm (Phoenix dactylifera).

? Largest embryo in monocots is found in coconut. Loranthus is a monocot with 2-6 cotyledons.

Asexual reproduction/Vegetative propagation

Apomixis in plant means development of a plant

(a) From root cuttings

(b) Without fusion of gametes

(d) From stem of cuttings

Micropropagation is

(a) Germination of seed with cotyledons above the soil

(b) A technique to obtain new plants by cultivating the cells or tissues in culture medium

(d) The mature stage of endosperm

Nitsch was able to get strawbarries of curious shapes by

(a) Removing all the perianth

(b) Splitting the ovary

(d) Inserting a needle into the ovary

Juvenile and adult plants are

(a) Morphologically different (b) Physiologically same

Which type of propagation is better for the plant

(a) By seeds (b) By tubers

Plants identical to mother plants can be had obtained from

[CPMT 1975]

(a) Seeds (b) Stem cutting

A clone is a group of individuals obtained through

[DPMT 1986]

(a) Self pollination (b) Hybridization

Among the following which one is not a method of vegetative propagation [AFMC 1990]

(a) Budding (b) Layering

After culturing the anther of a plant, a few diploid plants were found along with haploid plants. The diploid plants could have arisen from [AIIMS 1993]

(a) Generative cell of pollen (b) Cells of anther wall