Chapter 23 Sexual Reproduction in Flowering Plants by Teaching Care online coaching classes
Chapter 23 Sexual Reproduction in Flowering Plants by Teaching Care online coaching classes
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
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 :
- 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 There are three different types of agamospermy :
- 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, g., Citrus, Opuntia, etc. In Citrus, a seed may possess upto 40 embryos (one normal and rest adventive).
- Diplospory : In this case, the archesporium differentiates but megaspore mother cell directly gives rise to an unreduced (e., without meiosis) embryo sac. It may produce two types of embryos :
- Diploid parthenogenesis : Embryo develops from unfertilized diploid
- Diploid apogamy : Embryo develops from any diploid cell of embryo sac except
- Apospory : It is the formation of complete embryo sac from the sporophytic cell without meiosis so that the gametophyte remains Apospory may be of two types :
- Somatic apospory : Embryo sac is formed from somatic
- Generative apospory : Embryo sac is formed from archesporium without
- 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
- 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 Some of these are given below.
- Vegetative propagation by stems : The modified stems like bulbs, runners, rhizomes, corms, tubers, offsets, , help the plant to multiply under favourable conditions.
- Bulb : It is a modified shoot that has a very short stem and apical and axillary 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 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. 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 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 e.g. Potato.
- Offsets : They are one internode long runners which develop tuft of leaves at the apex g. Water lettuce (Pistia), Water hyacinth (Eichhornia), etc.
- Stolons : They are arched runners with cross over small obstacles and develop small plantlets at their e.g. Strawberry, Vallisneria, etc.
- Vegetative propagation by roots : The roots of some woody plants produce shoots which grow into new plants; 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.
- Vegetative propagation by leaves : The leaves generally do not help in vegetative propagation. However, in Bryophyllum pinnatum and 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.
- Vegetative propagation by reproductive parts : Flowers are primarily associated with sexual 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.
- 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 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 :
- 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 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.
- Layering : In this method, roots are artificially induced on the stem branches while it is still attached to the parent plant for 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,
- 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.
- Grafting : A new variety is produced by joining parts of two different plants is called 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 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 Tissue culture technique is useful
Nutrient medium solidified with agar
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,
- Organised callus, C. Plantlets developed from cellus
employed for quick multiplication of plants.
- Grafting is not possible in monocots as they do not bear
- Slip is a small piece or plantlets which can be separated and used for
- 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
- 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
- Graft hybrid is a chimera shoot formed by an adventitious bud formed at the junction of stock and First reported in 1644 as Bizzaria orange (half orange half cistron) in Italy.
- In angiosperms apospory was first reported by Rosenberg (1907) in
- 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
- The formation of sporophyte directly from gametophyte without gamete formation and fusion is called apogamy.
- Walking fern propagates through leaf
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.
- Structure of the flower : Morphologically flower is a modified shoot meant for sexual reproduction of the 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
collection of petals,
Stigma bilobed, short
long stalk called the pedicel. It has an upper swollen region known as receptacle (thalamus or torus).
- 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
collection of sepals, green, polysepalous
stalk of flower
Fig : L.S. of a typical flower
bicarpellary, syncarpous, bilocular, superior
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.
- 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 They prevent rapid transpiration from the inner parts of the flower.
- Corolla : This is the second whorl of the flower and consists of a number of 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).
- 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.
- Gynoecium : This is the last and the fourth whorl of flower and is the female reproductive organ of the 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
- 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
- Flowers of most of the angiosperms are shaped variously to help diverse modes of
- Flowers provide seat for germination of pollen, development of pollen tube, formation of gametes and
- The ovary part of the carpel gets transformed into fruit and the ovules are transformed into seeds after
- Some floral parts like calyx and various modifications in ovaries help in the dispersal of fruits and
- 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 :
- Racemose (indeterminate), the tip of axis continues to grow, flowers produced
Main axis elongated
Main axis shortened
Main axis flattened to form receptacle
- Raceme : Stalked flowers laterally
e.g. Delphinium, Crotolaria, Radish, Mustard.
- Panicle : Branched raceme (compound raceme).
e.g. Gold mohur.
- Spike : Raceme with sessile
e.g. Achyranthes, Callistemon, Antirrhinum.
- Compound spike/spikelet : Axis is branched, made of spikelets consisting of one or more
e.g., Amaranthus, Wheat, Paddy, Sugarcane, Oat.
- Catkin : Sessile and unisexual flowers, axis
e.g. Morus (mulberry), Salix (willow), Populas (poplar), croton.
- Spadix : Flowers sessile, axis fleshy, bracts coloured (spathe).
e.g., banana, maize, colocasia.
- Corymb : Floral stalks unequal, flowers lie on the same
e.g. Iberis amara (candytuft).
- Compound corymb : Central axis of corymb branched, each branch bearing
- Umbel : Floral stalks equal, flowers lie on the same
- Compound umbel : Axis of umbel branched, each branch bearing
e.g., Coriander, Fennel, Carrot.
Capitulum or Head : Florets arising from disc-like receptacle.
e.g. Sunflower, Marigold.
- Uniparous or Monochasial : Only one lateral branch
- Cymose (determinate) tip
of axis terminates into a flower, axis growth stops, lateral branches bear flowers; branches also terminate into flower
- Special types
- Scorpioid : Lateral branches develop on alternate
e.g., Tecoma, Rannenculus.
- Heliocoid : Lateral branches develop on the same
- Biparous or Dichasial : Two lateral branches
e.g., Ixora, Diathus.
- Multiparous or Polychasial : More than two lateral branches
e.g. Calotropis, Nerium.
- Cyathium : Cup-shaped involucre that encloses one female and many male
- Verticillaster : Dichasial cyme reduced to scorpioid cymes, one on each side of the
e.g., Ocimum, Salvia.
- Hypanthodium : Cup-shaped receptacle with opening, male flowers near opening of cup, female flowers at
e.g., Ficus (Peepal, Banyan).
- 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 :
- Hypogyny : In this case the thalamus is convex-like and ovary occupies the highest position on it. The outer three whorls, 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.
- 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 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.
- B C
Fig : Insertion of Floral parts on thalamus
- Hypogynous; B. Perigynous; C. Epigynous
- 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 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.
- 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 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 :
- Marginal : In this type of placentation, the ovary is simple, unilocular and the ovules are arranged along the margin of the unilocular The placenta develops along the ventral suture of the ovary. e.g. Pea, Gram,
- Axile : It is found in a compound ovary which is two or more chambered, usually as many as the number of carpels 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.
- Free central : In this free central placentation, the gynoecium is polycarpellary and
Dorsal suture Ventral suture Placenta
Ovary wall Ovule
axis Locule septum
- Ovary wall C
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;
- Parietal; C. Axile; D. Central; E. Basal; F. Superficial
- Parietal : In parietal placentation, the ovary is usually one-chambered but in some cases it becomes bilocular due to the formation of false septum, 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.
- Basal : In this type of placentation, ovary is bicarpellary, syncarpous and unilocular and a single ovule is borne at the base of e.g. Marigold, Sunflower, etc.
- Superficial : The ovary is multicarpellary, syncarpous, and large number of loculi without specific order
e.g. Waterlily (Nymphea).
- Symmetry of the flower : The number, shape, size and arrangement of floral parts (e. calyx, corolla, androecium and gynoecium) in a flower determines its symmetry. On the basis of symmetry, flowers can be of following three types :
- Actinomorphic or Regular flowers : Such flowers can be divided by vertical plane into two equal and similar e.g. Mustard, Hibiscus, Brinjal, etc.
- Zygomorphic or Irregular flowers : These flowers can be divided into two equal halves only along one median longitudinal e.g. Pea, Iberis, Ocimum, etc.
- Asymmetrical flowers : These flowers cannot be divided into two equal halves along any vertical
e.g. Canna, Maranta.
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 :
- 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.
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 :
- Epidermis : This is the outermost single layered and protective. In Arceuthobium, cells of epidermis develops a fibrous thickening and the epidermis is designated as
- 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 In between these cells, a few cells without thickening are also present. These thick walled cells collectively form the stomium.
- 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
- Tapetum : This is the innermost layer of the 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 :
- 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
e.g. Alisma, Typha, Tradescantia.
- 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
- 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, 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 :
- Simultaneous type : The cytokinesis occurs simultaneously at the end of meiosis II to form tetrahedral tetrad. Here wall formation is It is common in dicotyledons.
- Successive type : The cytokinesis occurs twice 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.
- 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 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.
- Pre-pollination development : Microspores starts germinating in situ (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).
- Post-pollination development : The liberated pollen grains are transferred to the receptive surface of the carpel (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.
A B C D
cell Male gametes
Fig : Different stages of microgametogenesis
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 :
- 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 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
Outer integument Inner integument Antipodals
Polar nuclei Egg apparatus
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 :
- Unitegmic : Ovule with a single integument, g., sympetalous or gamopetalous dicotyledons.
- Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and
- Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg,
- Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of e.g., Castor (Ricinus).
- Ategmic : In some parasites like Loranthus, Viscum, Santalum , there is no integument. Such an ovule is called ategmic.
- Kinds of ovules : Depending upon the shape and orientation, the ovules of angiosperms are classified into following types :
- Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight This is most primitive type of ovules. e.g. Betel, Piper, Polygonum.
- Anatropous : The body of the ovule is completely inverted (turn at 180o angle ) so that micropyle and hilum come to lie very close to each e.g. 82% of angiosperm families.
- Hemianatropous : Ovule turns at 90o angle upon the funicle or body of ovule is at right angle to the funicle g. Ranunculus.
- Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down
slightly. e.g. in members of Leguminaceae and
- Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe g. Lemna, Poppy, Alisma.
- Circinotropous : The ovule is initially
Integuments Embryo sac
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).
Nucellus Embryo sac
- 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
Integument Nucellus Embryo sac
Embryo sac Chalaza
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
Fig : Different forms of the ovule in longitudinal section. A, orthotropous; B, anatropous; C, hemianatropous;
D, campylotropous; E, amphitropous; F, circinotropous
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.
- Development of female gametophyte (Megagametogenesis) : The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.
- Monosporic type (Polygonum) : In this type, only one megaspore situated towards chalazal end takes part in the development of embryo 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 nucleus :
- Egg apparatus : This is a group of 3 cells situated at the micropylar end. The centrally located cell is called egg 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.
- Polar nuclei : These are situated in the centre of the embryo sac representing a large binucleate central Generally, both the polar nuclei fuse before fertilization and form a single diploid nucleus called secondary nucleus or definitive nucleus.
- Antipodals : The three cells situated at the chalazal end are called antipodals. These cells generally degenerate soon after
- Polygonum type occurs in about 70% of angiosperms and is the common
- Bisporic type : In this type two megaspore nuclei take part in embryo sac
- Tetrasporic type : This type of embryo sac develops from four megaspore
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 :
- Self pollination (2) Cross pollination
Autogamy (Same flower)
Geitonogamy (Different flowers of the same plant)
(Xenogamy or Allogamy) Flowers from different plants of the same species
- Anemophily (Wind)
- Hydrophily (Water)
- Entomophily (Insect)
- Ornithophily (Birds)
- Chiropterophily (Bats)
- Malacophily (Snails)
- 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 It is of two types :
- 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
- Geitonogamy : It is an 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
- Pollen grains are not
- The purity of the generation is
- New and healthier varieties are not
- It results in weaker progeny, producing weaker seeds and
Contrivances for self pollination : The major contrivances or adaptations which favours self pollination are :
- Bisexuality : Flowers should be bisexual or
- Homogamy : Anthers and stigma of the bisexual flowers of some plants mature at the same 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.
- Cleistogamy : Some plants never open to ensure complete self-pollination. This condition is called cleistogamy, g., Commelina bengalensis, Oxalis, Viola, etc. The cleistogamous flowers are bisexual small, inconspicious, colourless and do not secrete nectar.
- Cross pollination : Cross pollination involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another It is also called xenogamy.
- Seeds are more and
- Progenies are
- Adaptability is
- New varieties can be
- The process is not definite because plants depend on
- Large amount of pollen grains are
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 :
- Diclincy or Unisexuality : In unisexual flowers stamens and carpels are found in different Unisexuality can be of two types :
- Monoecious plant : When male and female flowers are borne on the same e.g., Maize, Cucurbits, Castor.
- Dioecious plant : When male and female flowers are borne on different e.g., Carica papaya, Cannabis.
- 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 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.
- 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 These are known as thrum-eyed. e.g., Oxalis.
- Herkogamy : In some bisexual flowers where the stigma and anthers mature at the same time, self pollination is avoided by some sort of The flowers show following contrivances :
- The male and female sex organs lie at some distance from each
- In some flowers corolla has peculiar forms which act as barrier in self e.g. Aristolochia.
- In some other flowers, the pollens are held together to form pollinia which can only be carried away by e.g. Orchids and Calotropis.
- 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 incompatibility and this condition of flower is called self sterility, intraspecific incompatibility or self 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 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
- Anemophily : When flowers are pollinated by wind agency, the phenomenon is known as Wind pollinated flowers produce very large amount of pollen grains to compensate the wastage. Pollen grains of such plants are small, light, dry, and smooth. The female flowers have large feathery or brush like stigmas to catch the pollen grains. 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.
- Hydrophily : When the pollination takes place through the agency of water, it is known as 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 e.g. Vallisneria (Ribbon weed).
(ii) Biotic agents
- 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, bettles, butterflies, wasp, etc. All the flowers pollinated by insects are brightly coloured, have a sweet smell and produce 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. The insects visit the flower for nectar, edible pollen grain and shelter. Bees obtain both nectar and pollen grains from the flowers and have basket for collecting pollen. 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.
- 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 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.
- 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 nector than ornithophilous flowers and open at night emit a good fragrance. g. Kigelia pinnata (Sausage tree), Adansonia (Baobab tree), Bauhinia megalandra, Anthocephalus (Kadam tree), etc.
- Malacophily : Pollination by slugs and snails is called Land plants like Chrysanthemum
and water plant like lemna shows malacophily. Arisaema (aroid; snake plant) is often visited by snails.
The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884).
- Germination of pollen grain on stigma and growth of pollen tube : Pollen grains reach the receptive stigma of the carpel by the act of 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 in 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.
- Entry of pollen tube into ovule : After reaching ovary, the pollen tube enters the Pollen tube may enter the ovule by any one of the following routes :
- Porogamy : When the pollen tube enters the ovule through micropyle, it is called It is the most common type. e.g. Lily.
- 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.
- Mesogamy : The pollen tube enters the ovule through its
A B C
Fig : Entry of pollen tube in to the ovule
- Porogamy; B. Chalazogamy; C. Mesogamy
middle part i.e. through integument (e.g. Cucurbita, Populus) or through funicle (e.g. Pistacia).
- 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 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.
- 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 G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms only.
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.
- Types of endosperm : On the basis of development, endosperm are of three types :
- Nuclear endosperm : In the nuclear type of endosperm development, the primary endosperm nucleus divides by repeated mitotic free nuclear divisions without the formation of 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
Fig : Different stages in the development of nuclear type of endosperm
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.
- Cellular endosperm : In the cellular type of endosperm development, the first nuclear division of the primary endosperm nucleus is immediately followed by the wall 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.
Fig : Different stages in the development of cellular type of endosperm
- Helobial endosperm : In the helobial type of endosperm development, the endosperm is intermediate between cellular and nuclear 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.
(2) Some terms related to endosperm
A B C D
17 Fig : Different stages in the development of helobial type of endosperm (cc= chalazal chamber, mc= micropylar chamber)
- Ruminate endosperm : Mature endosperm with irregularity and unevenness in its surface is called ruminate 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.
- 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
- Xenia : The effect of pollen on endosperm is called xenia. This term was given by Focke (1881). g. Maize.
- Metaxenia : The effect of pollen of somatic tissue lying outside the endosperm is known as Metaxenia term given by the swingle (1928). e.g. Datepalm.
- Development of embryo (Embryogeny) : The zygote after a period of rest develops into The process of development of mature embryo from diploid zygote is called embryogenesis.
- In dicotyledons : The normal type of dicot embryo development has been studied in Shephered purse (Capsella bursapastoris) family 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.
B C D E F G
Fig : Successive stages of the development of dicotyledonous embryo up to globular stage
Fig : Development of globular embryo into mature dicotyledonous embryo
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.
- 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.
Fig : Different stages in the development of monocotyledonous embryo
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.
- Polyembryony : Occurrence of more than two embryo in the seed is known as polyembryony. It was discovered by V. Leeuwenhock (1719) in Citrus. It may be :
- Cleavage polyembryony : Due to cleavage of zygote or proembryo into two or more embryos and each split part develops into an 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.
- Simple polyembryony : Due to presence of more than one embryo sac and so oospore or egg. g. Brassica.
- Mixed polyembryony : More than one pollen tube entering an ovule and fertilizing synergids (as in
Argemone maxicana) and antipodal cell (as in Ulmus americana).
- Adventive polyembryony : Diploid nucellus or integument cells form embryos 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.
- Development of seed : The fertilized ovule forms seed. The ovule increases greatly in size. The integuments dry up. The outer one becomes hard or leathery and forms the outer seed coat or testa while the inner one, if persist, forms the
The nucellus is generally used up during the development of embryo but in some cases it remains outside the endosperm in the form of a thin layer, called perisperm. The endosperm may persist or completely digested during embryogenesis.
A scar is usually visible on one side of the outer seed coat. It is known as hilum and marks the point of attachment to the stalk. With these changes, the ovule changes into seed and enters a period of dormancy while the ovary ripens into a fruit.
Exalbuminous : Gram, Pea, Bean, Mustard, Mango, Groundnut, etc. Albuminous : Castor, Poppy, Artabotrys, Custard apple (Ananas) etc. Monocotyledonous seeds
Exalbuminous : Orchids, Alisma, Najas, Pothos, Amorphophallus, Vallisneria, etc.
Albuminous : Cereals, Millets, Palms, Lilies, etc. The seeds are of following types :
- Non-endospermic or Exalbuminous seeds : In exalbuminous seeds endosperm is completely consumed by the developing embryo, and the mature seeds are without
The food is stored in cotyledons.
Testa (outer layer) Tegmen (inner layer)
Embryonal axis (tigellum)
Fig : Structure of gram seed
- Endospermic or Albuminous seed : In albuminous seeds, embryo not consumed all endosperm. So it persists in the mature seed. In these seeds food stored in endosperm. In monocot seed the membranous covering of:
- Radicle is called
- Plumule is called
Pericarp + Seed coat
Cotyledonary node Radicle
Remnants of style
Embryo Base of style
Seed and fruit coverings
Epithelium Coleoptile Plumule
Coleorrhiza Kernel stalk
Fig : Structure of maize grain (A) Entire seed; (B) Grain in L.S.
- Germination of seeds : The process by which the dormant embryo of the seed resumes active growth and grows into a new plant is known as
(3) Types of seed germination
- Epigeal germination : In this type of germination, the cotyledons come above the surface of the soil into the air and light due to the rapid growth and elongation of the The cotyledons turn green and make food for a plant. The food in them is utilized by the growing stem. They finally dry up and fall off and seedling becomes an independent plant. Germination of seeds of Bean, Gourd, Castor, Cotton, etc. is of epigeal nature.
Fig : Successive stages of epigeal germination of dicotyledonous and albuminous seed of castor
- Hypogeal germination : In this type of germination, the cotyledons remain in the soil or just above the In this case epicotyl elongates pushing the plumule upwards. The cotyledons do not turn green and gradually dry up and fall off. Common examples of hypogeal germination are the seeds of Pea, Mango, Groundnut, etc.
- Viviparous germination : This is a special type of germination found in mangrove These plants are found in marshy lands and on sea coasts. In viviparous germination seeds inside the fruit germinate while still attached to the parent plant and nourished by it. The embryo grows not only out of the seed but also out of the fruit and projects from it in the form of a green seedling displaying root and hypocotyl. Due to its increasing weight the seedlings separate from the parent tree and falls into the mud or water and soon develops lateral roots. Vivipary is seen in Rhizophora and Sonneratia.
Fig : Stages of germination of maize (hypogeal germination)
(4) Factors for seed germination
Fig : Vivipary (A) Twig of Rhizophora showing viviparous germination; (B) Seedling growing in mud
- External factors : Water, oxygen, suitable temperature.
- Internal factors : Foods and growth regulators, completion of rest period,
- Seed dormancy : In several plants seeds germinate as soon as they have undergone maturation and provided proper conditions for e.g. seeds of Bean, Pea, Maize etc. In some plants seeds are incapable of germination because of some inhibitory factors. Such seeds are unable to germinate even under suitable conditions. This is called seed dormancy.
- Causes of seed dormancy : The seed dormancy may be due to many causes some of which are as follows :
- Impermeability of seed coats to e.g. Xanthium.
- Impermeability of seed coats to e.g., Chenopodium and many leguminous seeds.
- Seed coat is mechanically hard, thus resisting the growth of e.g. Mustard, Capsella, Amaranthus.
- Presence of rudimantary or immature e.g., Ginkgo biloba (a gymnosperm).
- Some plants produce such chemical compounds that inhibit the germination of their own e.g. Tomato, (possesses inhibitor ferulic acid).
- Overcoming of seed dormancy : Seed dormancy is overcomed naturally by several means. It can also be broken down
- Mechanical scarification : Seed coat is weakened by use of abrasives like and by making
- Chemical scarification : Seed coat weakening is done by acid
- Alternating high and low temperature
- Low temperature
- Exposure to
- Providing high
- Alcohol washing of seeds so as to remove inhibitors in seed coat
- Soaking of seeds in some chemicals like thiourea and KNO3
- Formation of fruit : Fruit is defined as fertilized The ovary develops into fruit. The ovary wall at maturity forms the wall of the fruit, which is known as pericarp. Sometimes, other parts of flower such as tepals, (e.g., Morus), bracts (e.g., Ananas) or thalamus (e.g., Pyrus) are also involved in the formation of fruit and such fruits are called false fruits or pseudocarps.
The fate of various parts of the ovary during the formation of fruits is summarized below :
Fruits Pericarp Seed
Stalk of the seed Hilum
Perisperm (when present) Micropyle
(2) Types of fruits
Dehiscent Legume or Pod Follicle
Siliqua Silicula Capsule
Indehiscent Caryopsis Achene Cypsela
Schizocarpic Lomentum Cremocarp Regma Carcerulus
Drupe Berry Pepo
Etario of follicles Etario of achenes Etario of drupes Etario of berries
Compound Sorosis Syconus
They are classified into three groups : Simple, aggregate and multiple or compound fruit.
Simple fruits : They are formed from mono-or polycarpellary but syncarpous ovary. They may be dry or fleshy.
- Simple dry fruits have thin, hard and dry, They are of three kinds :
(a) Ehiscent or Capsular (b) Achenial or Indehiscent (c) Schizocarpic
- Dehiscent fruit : These fruits are dry, many seeded and split open at They are of following types :
- Legume or Pod : It is characteristic of the family leguminosae; developed from monocarpellary unilocular superior ovary with marginal placentation. It can open or dehisces by both ventral and dorsal sutures. g., in Cicer arietinum (Gram); Pisum sativum (Pea) and Phaseolus mungo (Black gram).
- Follicle : It is very much resembles the legume but on ripening it opens generally along the ventral e.g. Calotropis, Larkspur, etc.
- Siliqua : The fruit is developed from bicarpellary, syncarpous and superior ovary which bears ovules on two parietal placenta. The ovary is unilocular but later becomes bilocular due to the development of a false partition wall called replum. It dehisces from the base towards the apex by both the sutures g. in Brassica (Mustard) and is characteristic of the family Cruciferae.
- Silicula : It is flattened and short in length from siliqua type, found in Iberis (Candytuft) and Capsella bursa (Shepherd’s purse).
- Capsule : It is mono or polycarpellary, dry dehiscent, many seeded fruit which developes from a superior or inferior ovary. It dehisces in almost all the ways e., longitudinal and transverse, along both the sutures. Majority of capsules show longitudinal-dehiscence which again are of different types :
Loculicidal : Lines of dehiscence appear along the dorsal sutures, e.g. Gossypium herbacium (Cotton) and
Abelmoschus esculentus (Lady’s finger).
Septicidal : Lines of dehiscence appear along the ventral sutures or septations of the ovaries e.g. in Viola
(Pansy), Linseed (Linum).
Septifragel : Lines of dehiscence along irregular lines, but the seeds remain attached to the placenta, as in
Datura stramonium (Thorn apple).
- Achenial or Indehiscent fruits : These fruits do not burst at maturity but the seeds are liberated only by the decaying of the These are of following types :
- Achene : It is small, dry one seeded fruit which develops from a superior or inferior monocarpellary In this type, the pericarp is tough but thin and free from the seed coat, e.g. in Mirabilis (four o’clock plant) and Clematis. Some times achenes occur in a group from apocarpus ovary where carpels are many
e.g. in Nelumbium (Lotus).
- Caryopsis : It is very small, dry and one seeded fruit which develops from a superior monocarpellary Here the pericarp is closely fused with seed coat. It is the characteristic of family graminae, e.g., in Oryza sativa (Paddy), Triticum aestivum (Wheat) and Zea mays (Maize).
- Cypsela : It is dry, one seeded fruit which develops from an inferior , bicarpellary ovary. Here the pericarp is free from seed coat but the thalamus is fused with The fruit is provided with a crown of hairs at the top called pappus e.g. in Helianthus annuus (Sun flower), Tridax, Cosmos, Sonchus, etc.
- Nut or Glans : It is dry, one seeded fruit which develops from a superior, bi or polycarpellary ovary having a hard pericarp, free from seed coat g. in Areca catechu (Betalnut), Anacardium occidentale (Cashewnut) and Trapa natans (Water chestnut). Here the thalamus and sometimes the cotyledons of true fruit are also edible.
- Samara : It is dry, one or two seeded fruit, develops from a single mono-or bicarpellary ovary. The pericarp is free from test a and produces a wing like outgrowth which helps in the dispersal of seeds g. in Hiptage and Elm.
- Schizocarpic or Splitting fruits : These resemble both (achenial) indehiscent fruits as well as capsular fruits having many However, they break into one seeded segments known as mericarps. By splitting usually the mericarps are indehiscent but in Ricinus (Castor) they are dehiscent. The important schizocarpic fruits are :
- Lomentum : It is a dry, many seeded fruits which develops from a monocarpellary, superior, unilocular ovary with marginal placentation. The fruit arises just like a legume but when ripened it becomes partitioned between seeds into single seeded mericarps g. in Acacia arabica (gum tree), Mimosa (touch me not) and Dalbergia sisoo (India red wood tree).
- Cremocarp : It is a dry fruit, develops from bicarpellary, syncarpous, bilocular ovary. The fruit when mature breaks into single seeded mericarps which remain attached to the top of the central axis called carpophore, g. in Daucus carota (Carrot); Foeniculum vulgare (fennel).
- Regma : It develops from tri-or penta-carpellary superior syncarpous The locules are many as the carpels known as Cocci (sing. Coccus), attached to carpophore and separate by splitting e.g. Euphorbia, Geranium and Ricinus.
- Carcerulus : It is a dry fruit, develops from bi or polycarpellary syncarpous, multilocular superior ovary with axile Many single seeded, mericarps are formed by splitting and formation of false septa.
e.g. Ocimum sanctum (Sacred basil), Althaea rosea.
- Simple fleshy fruit : The fruits are simple, but the pericarp is fleshy and edible. It is differentited into three layers epicarp, mesocarp and Fleshy fruits are of following types :
- Drupe : It is a fleshy fruit formed from mono-or poly carpellary superior ovary, where one or more ovules may developes into Here the epicarp is thin and leathery. The mesocarp is thick, fleshy, juicy and edible in Mangifera (Mango) and fibrous in Cocos (Coconut). The endocarp is hard and stony in both the cases. In Cocos, pericarp is not edible. The portion inner to endocarp is the liquid endosperm which is edible.
- Berry : It is usually many seeded fleshy fruit develops from polycarpellary, syncarpous, superior Rarely it is single seeded as in Borassus (Palm). Here the epicarp remains as the skin of the fruit. The mesocarp and endocarp are fused together to form the pulp of the fruit. e.g. Brinjal, Tomato, Banana, etc.
- Pepo : It is a special type of berry. Here the epicarp and thalamus form the outer ring of the fruit. The mesocarp, endocarp and placentae are fused to form pulp which is edible; seeds are many. Common examples are Cucurbita maxima (Sweet gourd); Cucumis sativa (Cucurbit).
- Pome : The fruit develops from inferior, pentacarpel The fruit is covered by the fleshy thalamus, which is fused with the pericarp and edible. The outer part again encloses the inner stiff and membranous portion enclosing the seeds; common example is Pyrus indica (Apple).
- Hesperidium : It is another type of berry; it develops from a polycarpellary, syncarpous, superior ovary with many Here the outer skin is thick and leathery that represents the epicarp, which contains oil glands. The fibrous portion fused with epicarp is the mesocarp. The endocarp consists of many chambers with juicy glands. Common examples are Citrus medica (Lemon) and Citrus sinensis (Sweet orange).
- Balausta : This is many chambered, many seeded fruit developing from a multicarpellary, syncarpous but inferior ovary. The pericarp of balausta is leathery or tough. The carpels are arranged in two rows. Carlyx is The seeds have succulent seed coat (testa) which form the edible part; e.g. Punica granatum (Pomegranate).
Aggregate fruits : The aggregate fruits are formed from polycarpellary, apocarpous ovary. Each ripened is called fruitlet or etaerio e.g. the lotus, rose fruit and strawberry are a collection of achenes; raspberry, a collection of drupes and custard apple is a collection of berries.
- Composite or compound fruits : Multiple fruit develops from entire inflorescence called sorosis or
- Sorosis : Develops from spike or spadix inflorescence g. Pineapple, Jackfruit, Mulberry, etc.
- Syconus : Develops from hypanthodium inflorescence g. Ficus carica. (banyan).
Dispersal of fruits and seeds.
- Dispersal by wind (Anemochory) : The wind is probably the most important agency of seed dispersal in The fruits and seeds show following devices which help in dispersal by wind.
- Light weight and minute seeds : Seeds of some plants (g. Orchids) are sufficiently light and minute in size to be easily carried away to great distances by air currents.
- Winged seeds and fruits : Some seeds (g. Oroxylon, Cinchona, Moringa) or fruits (Acer, Hiptage, Terminalia, Dipterocarpus) develop one or more thin membranous wings to ensure their dispersal by wind.
- Parachute mechanism : In members of the family Asteraceae (Compositae) g., Taraxacum, Sonchus, sepals are modified into tufts of hairs called pappus. The pappus is persistent and hence found attached to even small, single seeded fruits. It acts like a parachute that allows the wind to carry them to great distances. Seeds of many nasty weeds are also dispersed by this method.
- Censer mechanism : In Antirrhinum (dog flower), Aristolochia, Papaver (poppy), Argemone mexicana (Prickly poppy), Nigella (love-in-a-mist), etc. the fruit is a capsule. At maturity it ruptures but the seeds do not come However, when the capsule is shaken violently by the wind, the seeds are scattered in all directions. In this process all the seeds do not escape together.
- Rolling mechanism : In some species, like Amaranthus albus, Chenopodium album, etc., plants dry out after bearing fruits and seeds. Eventually the entire plant breaks off at the base of the stem due to the force of wind and rolls over the ground, shedding the seeds all along the way. Such rolling plants are collectively known as tumble weeds.
- Hairs : In cotton, hairs are the outgrowth from the seed coat and occur all along its
- Persistant styles : Clematis, Naravilia, Geranium have persistent and feathery styles which help the fruit to be easily carried by wind.
- Balloon like appendages : In plants like Cordiospermum and Nicandra fruits develop balloon like appendages which make the fruits light to be easily carried by
- Dispersal by water (Hydrochory) : Fruits and seeds, specialized for dispersal by water, generally develop some kind of floating devices and a protective covering which makes them water resistant. g. fibrous mesocarp in Coconut, spongy thalamus in Lotus.
- Dispersal by animals (Zoochory) : Fruit and seeds dispersed by animals can be divided into following three categories on the basis of their adaptive features :
- Hooked fruits and seeds : The surface of many fruits is covered with hooks (g., Xanthium, Urena), barbs (e.g., Andropogon), spines (e.g., Tribulus), bristles (e.g., Pupalia), or stiff hairs (e.g., Aristida), by means of
which they adhere to the body of animals or clothes of human beings and they are carried unwarily from one place to another.
- Sticky fruits and seeds : Some fruits like those of Boerhaavia, Cleome, and Plumbago have sticky glands by which they adhere to the fur of grazing animals and are thus dispersed. Seeds of Viscum (mistletoe), Loranthus, have a viscid layer which adhere to the beak of the bird which eat them.
- Edible fruits : Human beings, birds, squirrels, bats, are of great help in the dispersal of edible fruits from one place to another.
- Dispersal by explosive or Spring like mechanism (Autochory) : A less common method of seed dispersal is by means of explosive fruits. Such fruits open with force and scatter the seeds in all directions. g. Balsam fruit (Impatiers), Oxalis, night jasmine (Nyctanthus), castor (Ricinus), camel’s foot climber (Bauhinia vahlii).
- Parthenocarpy : The formation of fruits without fertilization is called parthenocarpy. Such fruits are either seedless or non-viable Parthenocarpy is of two types :
- 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. g., Grapes, Banana, Pineapple and Noval oranges.
- 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. g., Tomato, Black berry, Fig, Lemon, Apple, Orange, Pear. etc.
- Maheshwari is the greatest embryologist of India.
- Rudolf Camerarius (1694) first describe sexual reproduction in
- The study of pollen grain is called palynology.
- Origin of pollen sac is eusporangiate while that of megaspore mother cell (embryo sac or megagametophyte) is
- The male gamete are non-motile and
- Pollen grain of Zoostera is filamentous and without
- The exine is made up of sporopollenin which is derived from
- Adansonia Flowers bears 1500-2000
- In Aristolochia elagans all types of pollen tetrads (tetrahedral, isobilateral, T-shaped, ^ shaped and decussate) are
- The pollen tube was first observed by B. Amici (1824) in Portulaca.
- Edible pollens are produced in
- Best temperature for growth of pollen tube is 20-30o
- Pollen tube secretes IAA, cytokinins and hydrolysing enzymes for separation of cells in case of solid
- Ubisch discovered the role of tapetum in anthers of
- Size of pollen (i) Smallest-Myosotis, 5-3.5 mm.(ii) Biggest Mirabilis, diamter 250 mm (iii) Longest Zoostera – 2500 mm.
- Anthesis is development or opening of flower
- A fully developed normal type embryo sac is 8-nucleate and 7-celled
- 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
- Most common type of ovule is anatropous (82% of total).
- Embryo sac (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 Plants commonly causing hay fever are Amaranthus, Chenopodium, Sorghum and Castor.
- Double fertilization first studied by Nawaschin (1898) in Fritillaria and It was confirmed by Guignard (1899).
- Erythrina is pollinated by crows as well as
- The study of seed is called spermology.
- The seed with double endosperm is found in Coconut (Cocus nucifera) (i) Liquid endosperm (ii) Cellular
- Stony endosperm is present in Betel nut (Areca nut) and Date palm (Phoenix dactylifera).
- Largest fruit and seed is found in Lodoicea maldivica (double Coconut). The fresh weight of seed is about 6
- Smallest seed Orchid (light and dry).
- Analytical study of flower and its floral parts is called as Anthology.
- Angiosperms differ from gymnosperms in having
- Largest flower in the world is Rafflesia arnoldi and smallest flower is Wolffia microscopia.
- The term parthenocarpy was introduced by Noll (1902).
- A flower is said to be complete when all the four whorls are present and incomplete when any one of them is
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