Asexual And Sexual Reproduction In Plants

Class 12 | Pure Science — Botany | Chapter 1

Class 12 Unit 1 Pure_Science

One of the essential features of all living things on the earth is reproduction. Reproduction is a vital process for the existence of a species and it also brings suitable changes through variation in the offsprings for their survival on earth. Plant reproduction is important not only for its own survival but also for the continuation and existence of all other organisms since the latter directly or indirectly depend on plants. Reproduction also plays an important role in evolution. In this unit let us learn in detail about reproduction in plants.

Chapter 1: Asexual and Sexual Reproduction in Plants

Learning Objectives

The learner will be able to:

• Recall various types of reproduction in lower and higher organisms.
• Discuss different methods of vegetative reproduction in plants.
• Recognise modern methods of reproduction.
• Recall the parts of a flower.
• Describe the steps involved in microsporogenesis.
• Recognise the structure of mature anther.
• Describe the structure and types of ovules.
• Explain the stages in megasporogenesis.
• Discuss the structure of embryo sac.
• Recognise different types of pollination.
• Identify the types of endosperms.
• Describe the development of Dicot embryo.
• Differentiate the structure of Dicot and Monocot seed.

Chapter Outline

• 1.1 Asexual reproduction
• 1.2 Vegetative reproduction
• 1.3 Sexual Reproduction
• 1.4 Pre-fertilization structure and events
• 1.5 Fertilization
• 1.6 Post fertilization structure and events
• 1.7 Apomixis
• 1.8 Polyembryony
• 1.9 Parthenocarpy

Panchanan Maheshwari (1904–1966)

Professor P. Maheshwari was an eminent Botanist who specialised in plant embryology, morphology and anatomy. In 1934, he became the Fellow of Indian Academy of Science. He published the book titled "An introduction to the Embryology of Angiosperms" in 1950. He established the International Society for Plant Morphologists, in 1951.

Basically, reproduction occurring in organisms fall under two major categories:

1. Asexual reproduction
2. Sexual reproduction

1.1 Asexual Reproduction

The reproduction method which helps to perpetuate its own species without the involvement of gametes is referred to as asexual reproduction. From Unit one of Class XI we know that reproduction is one of the attributes of living things and the different types of reproduction have also been discussed. Lower plants, fungi and animals show different methods of asexual reproduction. Some of the methods include, formation of Conidia (Aspergillus and Penicillium); Budding (Yeast and Hydra); Fragmentation (Spirogyra); production of Gemma (Marchantia); Regeneration (Planaria) and Binary fission (Bacteria) (Refer chapter one of Unit one of class XI). The individuals formed by this method is morphologically and genetically identical and are called clones. Higher plants also reproduce asexually by different methods which are given below:

1.2 Vegetative Reproduction

1.2.1 Natural Methods

Natural vegetative reproduction is a form of asexual reproduction in which a bud grows and develops into a new plant. The buds may be formed in organs such as root, stem and leaf. At some stage, the new plant gets detached from the parent plant and starts to develop into a new plant. Some of the organs involved in the vegetative reproduction also serve as the organs of storage and perennation. The unit of reproductive structure used in propagation is called reproductive propagules or diaspores. Some of the organs that help in vegetative reproduction are given in Figure 1.1.

A. Vegetative reproduction in root

The roots of some plants develop vegetative or adventitious buds on them. Example: Murraya, Dalbergia and Millingtonia. Some tuberous adventitious roots apart from developing buds also store food. Example: Ipomoea batatus and Dahlia. Roots possessing buds become detached from the parent plant and grow into independent plant under suitable condition.

Do You Know?

Scourge of water bodies / Water hyacinth (Eichhornia crassipes) is an invasive weed on water bodies like ponds, lakes and reservoirs. It is popularly called "Terror of Bengal". It spreads rapidly through offsets all over the water body and depletes the dissolved oxygen and causes death of other aquatic organisms.

B. Vegetative reproduction in stem

From Unit 3 of class XI (Vegetative morphology) you are familiar with the structure of various underground stem and sub aerial stem modifications. These include:

• Rhizome (Musa paradisiaca, Zingiber officinale and Curcuma longa)
• Corm (Amorphophallus and Colocasia)
• Tuber (Solanum tuberosum)
• Bulb (Allium cepa and Lillium)
• Runner (Centella asiatica)
• Stolon (Mentha and Fragaria)
• Offset (Pistia and Eichhornia)
• Sucker (Chrysanthemum)
• Bulbils (Dioscorea and Agave)

The axillary buds from the nodes of rhizome and eyes of tuber give rise to new plants.

C. Vegetative reproduction in leaf

In some plants adventitious buds are developed on their leaves. When they are detached from the parent plant they grow into new individual plants. Examples: Bryophyllum, Scilla, and Begonia.

In Bryophyllum, the leaf is succulent and notched on its margin. Adventitious buds develop at these notches and are called epiphyllous buds. They develop into new plants forming a root system and become independent plants when the leaf gets decayed.

Scilla is a bulbous plant and grows in sandy soils. The foliage leaves are long and narrow and epiphyllous buds develop at their tips. These buds develop into new plants when they touch soil.

Advantages of Natural Vegetative Reproduction

• Only one parent is required for propagation.
• The new individual plants produced are genetically identical.
• In some plants, this enables to spread rapidly. Example: Spinifex
• Horticulturists and farmers utilize these organs of natural vegetative reproduction for cultivation and to harvest plants in large scale.

Disadvantage of Natural Vegetative Reproduction

• New plants produced have no genetic variation.

1.2.2 Artificial Methods

Apart from the above-mentioned natural methods of vegetative reproduction, a number of methods are used in agriculture and horticulture to propagate plants from their parts. Such methods are said to be artificial propagation. Some of the artificial propagation methods have been used by man for a long time and are called conventional methods. Now-a-days, technology is being used for propagation to produce large number of plants in a short period of time. Such methods are said to be modern methods.

I. Conventional Methods

The common methods of conventional propagation are cutting, grafting and layering.

a. Cutting

It is the method of producing a new plant by cutting the plant parts such as root, stem and leaf from the parent plant. The cut part is placed in a suitable medium for growth. It produces root and grows into a new plant. Depending upon the part used it is called as root cutting (Malus), stem cutting (Hibiscus, Bougainvillea and Moringa) and leaf cutting (Begonia, Bryophyllum). Stem cutting is widely used for propagation.

b. Grafting

In this, parts of two different plants are joined so that they continue to grow as one plant. Of the two plants, the plant which is in contact with the soil is called stock and the plant used for grafting is called scion. Examples: Citrus, Mango and Apple. There are different types of grafting based on the method of uniting the scion and stock. They are bud grafting, approach grafting, tongue grafting, crown grafting and wedge grafting.

1. Bud grafting: A T-shaped incision is made in the stock and the bark is lifted. The scion bud with little wood is placed in the incision beneath the bark and properly bandaged with a tape.
2. Approach grafting: In this method both the scion and stock remain rooted. The stock is grown in a pot and it is brought close to the scion. Both of them should have the same thickness. A small slice is cut from both and the cut surfaces are brought near and tied together and held by a tape. After 1–4 weeks the tip of the stock and base of the scion are cut off and detached and grown in a separate pot.
3. Tongue grafting: A scion and stock having the same thickness is cut obliquely and the scion is fit into the stock and bound with a tape.
4. Crown grafting: When the stock is large in size scions are cut into wedge shape and are inserted on the slits or clefts of the stock and fixed in position using graft wax.
5. Wedge grafting: In this method a slit is made in the stock or the bark is cut. A twig of scion is inserted and tightly bound so that the cambium of the two is joined.

c. Layering

In this method, the stem of a parent plant is allowed to develop roots while still intact. When the root develops the rooted part is cut and planted to grow as a new plant. Examples: Ixora and Jasminum. Mound layering and Air layering are few types of layering.

1. Mound layering: This method is applied for the plants having flexible branches. The lower branch with leaves is bent to the ground and part of the stem is buried in the soil and tip of the branch is exposed above the soil. After the roots emerge from the part of the stem buried in the soil, a cut is made in parent plant so that the buried part grows into a new plant.
2. Air layering: In this method the stem is girdled at nodal region and hormones are applied to this region which promotes rooting. This portion is covered with damp or moist soil using a polythene sheet. Roots emerge in these branches after 2–4 months. Such branches are removed from the parent plant and grown in a separate pot or ground.

Advantages of Conventional Methods

• The plants produced are genetically uniform.
• Many plants can be produced quickly by this method.
• Some plants produce little or no seeds; in others, the seeds produced do not germinate. In such cases, plants can be produced in a short period by this method.
• Some plants can be propagated more economically by vegetative propagation. Example: Solanum tuberosum.
• Two different plants with desirable characters such as disease resistance and high yield can be grafted and grown as a new plant with the same desirable characters.

Disadvantages of Conventional Methods

• Use of virus infected plants as parents produces viral infected new plants.
• Vegetative structures used for propagation are bulky and so they are difficult to handle and store.

1.3 Sexual Reproduction

In previous classes reproduction in lower plants like algae and bryophytes was discussed in detail. Sexual reproduction involves the production and fusion of male and female gametes. The former is called gametogenesis and the latter is the process of fertilization. They reproduce by the production of gametes which may be motile or non-motile depending upon the species. The gametic fusion is of three types (Isogamy, Anisogamy and Oogamy). In algae external fertilization takes place whereas in higher plants internal fertilization occurs.

Flower

A flower is viewed in multidimensional perspectives from time immemorial. It is an inspirational tool for the poets. It is a decorative material for all the celebrations. In Tamil literature the five lands are denoted by different flowers. The flags of some countries are embedded with flowers. Flowers are used in the preparation of perfumes. For a Morphologist, a flower is a highly condensed shoot meant for reproduction.

A Flower possesses four whorls — Calyx, Corolla, Androecium and Gynoecium. Androecium and Gynoecium are essential organs. The process or changes involved in sexual reproduction of higher plants include three stages. They are Pre-fertilization, Fertilization and Post fertilization changes.

1.4 Pre-fertilization: Structures and Events

The hormonal and structural changes in plant lead to the differentiation and development of floral primordium. The structures and events involved in pre-fertilization are given below.

1.4.1 Male Reproductive Part – Androecium

Androecium is made up of stamens. Each stamen possesses an anther and a filament. Anther bears pollen grains which represent the male gametophyte.

Development of Anther

A very young anther develops as a homogenous mass of cells surrounded by an epidermis. During its development, the anther assumes a four-lobed structure. In each lobe, a row or a few rows of hypodermal cells becomes enlarged with conspicuous nuclei. This functions as archesporium. The archesporial cells divide by periclinal divisions to form primary parietal cells towards the epidermis and primary sporogenous cells towards the inner side of the anther. The primary parietal cells undergo a series of periclinal and anticlinal division and form 2–5 layers of anther walls composed of endothecium, middle layers and tapetum, from periphery to centre.

Microsporogenesis

The stages involved in the formation of haploid microspores from diploid microspore mother cell through meiosis is called Microsporogenesis. The primary sporogenous cells directly, or may undergo a few mitotic divisions to form sporogenous tissue. The last generation of sporogenous tissue functions as microspore mother cells. Each microspore mother cell divides meiotically to form a tetrad of four haploid microspores (microspore tetrad). The microspore tetrad may be arranged in a tetrahedral, decussate, linear, T shaped or isobilateral manner. Microspores soon separate from one another and remain free in the anther locule and develop into pollen grains.

In some plants, all the microspores in a microsporangium remain held together called pollinium. Example: Calotropis. Compound pollen grains are found in Drosera and Drymis.

T.S. of Mature Anther

Transverse section of mature anther reveals the presence of anther cavity surrounded by an anther wall. It is bilobed, each lobe having 2 thecae (dithecous). A typical anther is tetrasporangiate.

1. Anther Wall

The mature anther wall consists of the following layers:

a. Epidermis: It is single layered and protective in function. The cells undergo repeated anticlinal divisions to cope up with the rapidly enlarging internal tissues.

b. Endothecium: It is generally a single layer of radially elongated cells found below the epidermis. The inner tangential wall develops bands (sometimes radial walls also) of α cellulose (sometimes also slightly lignified). The cells are hygroscopic. In the anthers of aquatic plants, saprophytes, cleistogamous flowers and extreme parasites endothecial differentiation is absent. The cells along the junction of the two sporangia of an anther lobe lack these thickenings. This region is called stomium. This region along with the hygroscopic nature of endothecium helps in the dehiscence of anther at maturity.

c. Middle layers: Two to three layers of cells next to endothecium constitute middle layers. They are generally ephemeral. They disintegrate or get crushed during maturity.

d. Tapetum: It is the innermost layer of anther wall and attains its maximum development at the tetrad stage of microsporogenesis. It is derived partly from the peripheral wall layer and partly from the connective tissue of the anther lining the anther locule. Thus, the tapetum is dual in origin. It nourishes the developing sporogenous tissue, microspore mother cells and microspores. The cells of the tapetum may remain uninucleate or may contain more than one nucleus or the nucleus may become polyploid. It also contributes to the wall materials, sporopollenin, pollen Kitt, trephine and number of proteins that control incompatibility reaction. Tapetum also controls the fertility or sterility of the microspores or pollen grains.

There are two types of tapetum based on its behaviour:

• Secretory tapetum (parietal/glandular/cellular): The tapetum retains the original position and cellular integrity and nourishes the developing microspores.
• Invasive tapetum (periplasmodial): The cells lose their inner tangential and radial walls and the protoplast of all tapetal cells coalesces to form a periplasmodium.

Functions of Tapetum

• It supplies nutrition to the developing microspores.
• It contributes sporopollenin through ubisch bodies thus plays an important role in pollen wall formation.
• The pollen Kitt material is contributed by tapetal cells and is later transferred to the pollen surface.
• Exine proteins responsible for 'rejection reaction' of the stigma are present in the cavities of the exine. These proteins are derived from tapetal cells.

Do You Know?

Many botanists speak of a third type of tapetum called amoeboid, where the cell wall is not lost. The cells protrude into the anther cavity through an amoeboid movement. This type is often associated with male sterility and should not be confused with periplasmodial type.

2. Anther Cavity

The anther cavity is filled with microspores in young stages or with pollen grains at maturity. The meiotic division of microspore mother cells gives rise to microspores which are haploid in nature.

3. Connective Tissue

It is the column of sterile tissue surrounded by the anther lobe. It possesses vascular tissues. It also contributes to the inner tapetum.

Microspores and Pollen Grains

Microspores are the immediate product of meiosis of the microspore mother cell whereas the pollen grain is derived from the microspore. The microspores have protoplast surrounded by a wall which is yet to be fully developed. The pollen protoplast consists of dense cytoplasm with a centrally located nucleus. The wall is differentiated into two layers, namely, inner layer called intine and outer layer called exine.

• Intine is thin, uniform and is made up of pectin, hemicellulose, cellulose and callose together with proteins.
• Exine is thick and is made up of cellulose, sporopollenin and pollen Kitt. The exine is not uniform and is thin at certain areas. When these thin areas are small and round it is called germ pores or when elongated it is called furrows. The sporopollenin is generally absent in germ pores.

The surface of the exine is either smooth or sculptured in various patterns (rod like, grooved, warty, punctuate etc.). The sculpturing pattern is used in the plant identification and classification. Shape of a pollen grain varies from species to species. It may be globose, ellipsoid, fusiform, lobed, angular or crescent shaped. The size of the pollen varies from 10 micrometres in Myosotis to 200 micrometres in members of the family Cucurbitaceae and Nyctaginaceae.

Do You Know?

Palynology is the study of pollen grains. It helps to identify the distribution of coal and to locate oil fields. Pollen grains reflect the vegetation of an area. Liquid nitrogen (−196°C) is used to preserve pollen in viable condition for prolonged duration. This technique is called cryopreservation and is used to store pollen grains (pollen banks) of economically important crops for breeding programmes.

The wall material sporopollenin is contributed by both pollen cytoplasm and tapetum. It is derived from carotenoids. It is resistant to physical and biological decomposition. It helps to withstand high temperature and is resistant to strong acid, alkali and enzyme action. Hence, it preserves the pollen for long periods in fossil deposits, and it also protects pollen during its journey from anther to stigma.

Pollen Kitt is contributed by the tapetum and coloured yellow or orange and is chiefly made of carotenoids or flavonoids. It is an oily layer forming a thick viscous coating over pollen surface. It attracts insects and protects damage from UV radiation.

Do You Know?

Bee pollen is a natural substance and contains high protein, carbohydrate, trace amount of minerals and vitamins. Therefore, it is used as dietary supplement and is sold as pollen tablets and syrups. Further, it increases the performance of athletes, race horses and also heals the wounds caused by burns. The study of honey pollen is called Mellitopalynology.

Development of Male Gametophyte

The microspore is the first cell of the male gametophyte and is haploid. The development of male gametophyte takes place while they are still in the microsporangium. The nucleus of the microspore divides mitotically to form a vegetative and a generative nucleus. A wall is laid around the generative nucleus resulting in the formation of two unequal cells — a large irregular nucleus bearing with abundant food reserve called vegetative cell and a small generative cell.

Generally, at this 2-celled stage, the pollens are liberated from the anther. In some plants the generative cell again undergoes a division to form two male gametes. The pollen is liberated at this 3-celled stage. In 60% of the angiosperms pollen is liberated in 2-celled stage.

The pollen on reaching the stigma absorbs moisture and swells. The intine grows as pollen tube through the germ pore. In case the pollen is liberated at 2-celled stage the generative cell divides in the pollen into 2 male cells (sperms) after reaching the stigma or in the pollen tube before reaching the embryo sac.

1.4.2 Female Reproductive Part – Gynoecium

The gynoecium represents the female reproductive part of the flower. The word gynoecium represents one or more pistils of a flower. The word pistil refers to the ovary, style and stigma. A pistil is derived from a carpel. The word ovary represents the part that contains the ovules. The stigma serves as a landing platform for pollen grains. The style is an elongated slender part beneath the stigma. The basal swollen part of the pistil is the ovary. The ovules are present inside the ovary cavity (locule) on the placenta.

Structure of Ovule (Megasporangium)

Ovule is also called megasporangium and is protected by one or two covering called integuments. A mature ovule consists of a stalk and a body. The stalk or the funiculus (also called funicle) is present at the base and it attaches the ovule to the placenta. The point of attachment of funicle to the body of the ovule is known as hilum. In an inverted ovule, the funicle is adnate to the body of the ovule forming a ridge called raphe.

The body of the ovule is made up of a central mass of parenchymatous tissue called nucellus which has large reserve food materials. The nucellus is enveloped by one or two protective coverings called integuments. Integument encloses the nucellus completely except at the top where it is free and forms a pore called micropyle. The ovule with one or two integuments are said to be unitegmic or bitegmic ovules respectively.

The basal region of the body of the ovule where the nucellus, the integument and the funicle meet or merge is called chalaza. There is a large, oval, sac-like structure in the nucellus toward the micropylar end called embryo sac or female gametophyte.

In some species (unitegmic tenuinucellate) the inner layer of the integument may become specialized to perform the nutritive function for the embryo sac and is called as endothelium or integumentary tapetum (Example: Asteraceae).

• Tenuinucellate type: The sporogenous cell is hypodermal with a single layer of nucellar tissue around it. Normally tenuinucellate ovules have very small nucellus.
• Crassinucellate type: Ovules with sub-hypodermal sporogenous cell. Normally these ovules have fairly large nucellus.

Group of cells found at the base of the ovule between the chalaza and embryo sac is called hypostases and the thick-walled cells found above the micropylar end above the embryo sac is called epistase.

Types of Ovules

The ovules are classified into six main types based on the orientation, form and position of the micropyle with respect to funicle and chalaza.

• Orthotropous: The micropyle is at the distal end and the micropyle, the funicle and the chalaza lie in one straight vertical line. Examples: Piperaceae, Polygonaceae.
• Anatropous: The body of the ovule becomes completely inverted so that the micropyle and funiculus come to lie very close to each other. This is the common type of ovules found in dicots and monocots.
• Hemianatropous: The body of the ovule is placed transversely and at right angles to the funicle. Example: Primulaceae.
• Campylotropous: The body of the ovule at the micropylar end is curved and more or less bean shaped. The embryo sac is slightly curved. Example: Leguminosae.
• Amphitropous: The distance between hilum and chalaza is less. The curvature of the ovule leads to horse-shoe shaped nucellus. Example: some Alismataceae.
• Circinotropous: Funiculus is very long and surrounds the ovule. Example: Cactaceae.

Megasporogenesis

The process of development of a megaspore from a megaspore mother cell is called megasporogenesis. As the ovule develops, a single hypodermal cell in the nucellus becomes enlarged and functions as archesporium. The primary sporogenous cell functions as a megaspore mother cell. The megaspore mother cell (MMC) undergoes meiotic division to form four haploid megaspores.

Based on the number of megaspores that develop into the embryo sac, we have three basic types of development:

• Monosporic: Of the four megaspores formed, usually the chalazal one is functional and other three megaspores degenerate. Example: Polygonum.
• Bisporic: Two megaspores are involved in embryo sac formation. Example: Allium.
• Tetrasporic: All four megaspores are involved in embryo sac formation. Example: Peperomia.

Development of Monosporic Embryo Sac (Polygonum type)

The functional megaspore is the first cell of the embryo sac or female gametophyte. The megaspore elongates along micropylar-chalazal axis. The nucleus undergoes a mitotic division. A large central vacuole appears between the two daughter nuclei. The vacuole expands and pushes the nuclei towards the opposite poles of the embryo sac. Both the nuclei divide twice mitotically, forming four nuclei at each pole. At this stage all the eight nuclei are present in a common cytoplasm (free nuclear division).

After the last nuclear division, cellular organization of the embryo sac occurs:

• Of the four nuclei at the micropylar end, three organize into an egg apparatus; the fourth one is left free as the upper polar nucleus.
• Three nuclei of the chalazal end form three antipodal cells; the fourth one functions as the lower polar nucleus.
• The 2 polar nuclei may remain free or may fuse to form a secondary nucleus (central cell).
• The egg apparatus is made up of a central egg cell and two synergids, one on each side of the egg cell.
• Synergids secrete chemotropic substances that help to attract the pollen tube. The special cellular thickening called filiform apparatus of synergids help in the absorption, conduction of nutrients from the nucellus to embryo sac. It also guides the pollen tube into the egg.

Thus, a 7-celled embryo sac with 8 nuclei is formed.

1.4.3 Pollination

Pollination is a wonderful mechanism which provides food, shelter etc., for the pollinating animals. Many plants are pollinated by a particular animal species and the flowers are modified accordingly and thus there exists a co-evolution between plants and animals.

The process of transfer of pollen grains from the anther to a stigma of a flower is called pollination. Pollination is a characteristic feature of spermatophyte (Gymnosperms and Angiosperms). Pollination in gymnosperms is said to be direct as the pollens are deposited directly on the exposed ovules, whereas in angiosperms it is said to be indirect, as the pollens are deposited on the stigma of the pistil.

In majority of angiosperms, the flower opens and exposes its mature anthers and stigma for pollination. Such flowers are called chasmogamous and the phenomenon is chasmogamy. In other plants, pollination occurs without opening and exposing their sex organs. Such flowers are called cleistogamous and the phenomenon is cleistogamy.

A. Self-pollination or Autogamy

The transfer of pollen on the stigma of the same flower is called self-pollination or Autogamy (Greek: Auto = self, gamos = marriage). Self-pollination is possible only in those plants which bear bisexual flowers. Mechanisms promoting self-pollination:

1. Cleistogamy: Flowers never open and expose the reproductive organs. Examples: Commelina, Viola, Oxalis. In Commelina benghalensis, two types of flowers are produced — aerial (chasmogamous, insect pollinated) and underground (cleistogamous, self-pollinated).
2. Homogamy: When the stamens and stigma of a flower mature at the same time. Example: Mirabilis jalapa, Catharanthus roseus.
3. Incomplete dichogamy: In dichogamous flowers the stamen and stigma of a flower mature at different time; sometimes the time of maturation overlaps favouring self-pollination.

B. Cross-pollination

It refers to the transfer of pollens on the stigma of another flower.

1. Geitonogamy: When the pollen deposits on another flower of the same individual plant. It usually occurs in plants which show monoecious condition. Functionally cross-pollination but genetically similar to autogamy.
2. Xenogamy: When the pollen (genetically different) deposits on another flower of a different plant of the same species.

Contrivances of Cross-pollination (Outbreeding Devices)

1. Dicliny or Unisexuality: When the flowers are unisexual only cross-pollination is possible.

• Monoecious: Male and female flowers on the same plant. Example: Coconut, Bitter gourd. Autogamy is prevented but geitonogamy takes place.
• Dioecious: Male and female flowers on different plants. Example: Borassus, Carica and Phoenix. Both autogamy and geitonogamy are prevented.

2. Monocliny or Bisexuality (Dichogamy): In bisexual flowers anthers and stigmas mature at different times.

• Protandry: The stamens mature earlier than the stigmas. Examples: Helianthus, Clerodendrum.
• Protogyny: The stigmas mature earlier than the stamens. Examples: Scrophularia nodosa and Aristolochia bracteata.

3. Herkogamy: In bisexual flowers the essential organs are arranged so that self-pollination becomes impossible. Example: In Gloriosa superba, the style is reflexed away from the stamens; in Hibiscus the stigmas project far above the stamens.

4. Heterostyly: Some plants produce two or three different forms of flowers that are different in their length of stamens and style.

• Distyly: Two forms — Pin (long style, short stamens, small pollen) and Thrum-eyed (short style, long stamens, large pollen). Example: Primula.
• Tristyly: Three kinds of flowers with respect to length of style and stamens. Pollen from flowers of one type can pollinate only the other two types. Example: Lythrum.

5. Self-sterility / Self-incompatibility: In some plants, when the pollen grain of a flower reaches the stigma of the same, it is unable to germinate on its own stigma. Examples: Abutilon, Passiflora. It is a genetic mechanism.

Agents of Pollination

Pollination is affected by many agents like wind, water, insects etc. On the basis of the agents that bring about pollination, the mode of pollination is divided into abiotic and biotic.

Abiotic Agents

1. Anemophily – Pollination by Wind

The wind pollinated flowers are called anemophilous. Anemophily is a chance event. Common examples: grasses, sugarcane, bamboo, coconut, palm, maize.

Characteristic features of anemophilous plants:

• The flowers are produced in pendulous, catkin-like or spike inflorescence.
• The axis of inflorescence elongates so that the flowers are brought well above the leaves.
• The perianth is absent or highly reduced.
• The flowers are small, inconspicuous, colourless, not scented, do not secrete nectar.
• The stamens are numerous, filaments are long, exerted and versatile.
• Anthers produce enormous quantities of pollen grains; they are minute, light and dry.
• In some plants anthers burst violently and release pollen into the air. Example: Urtica.
• Stigmas are comparatively large, protruding, sometimes branched and feathery.
• Generally single ovule is present.
• Plant produces flowers before the new leaves appear.

Pollination in Maize (Zea mays): The maize is monoecious and unisexual. The male inflorescence (tassel) is borne terminally and female inflorescence (cob) laterally at lower levels. Maize pollens are large and heavy and cannot be carried by light breeze. The mild wind shakes the male inflorescence to release pollen which falls vertically below. The female inflorescence has long stigma (silk) measuring up to 23 cm in length, which projects beyond leaves.

2. Hydrophily – Pollination by Water

Pollination by water is called hydrophily (Example: Vallisneria, Hydrilla). The floral envelop of hydrophilous plants are reduced or absent. In most hydrophilous flowers, the pollen grains possess mucilage covering which protects them from wetting.

• Epihydrophily: Pollination occurs at the water level. Examples: Vallisneria spiralis, Elodea. In Vallisneria spiralis, it is a dioecious submerged and rooted hydrophyte. The female plant bears solitary flowers which rise to the surface using a long-coiled stalk. Male flowers get detached and float on the surface. When a male flower contacts a female flower, pollination takes place, then the stalk of the female flower coils and goes under water where fruits are produced.
• Hypohydrophily: Pollination occurs inside the water. Examples: Zostera marina and Ceratophyllum. In Zostera marina, the pollen grains are long, needle-like. The specific gravity of the pollen is same as that of sea water, so that the pollen floats freely at any depth. The stigma is very large and long.

Biotic Agents – Zoophily

Pollination by the agency of animals is called zoophily. Animals that bring about pollination may be birds, bats, snails and insects.

a. Ornithophily: Pollination by birds. Plants pollinated by birds include Erythrina, Bombax, Syzygium, Bignonia, Strelitzia etc. Humming birds, sun birds, and honey eaters are common pollinators.

Characteristic features:

• The flowers are usually large in size.
• The flowers are tubular, cup shaped or urn-shaped.
• The flowers are brightly coloured (red, scarlet, pink, orange, blue, yellow).
• The flowers are scentless and produce nectar in large quantities.
• The floral parts are tough and leathery.

b. Cheiropterophily: Pollination carried out by bats. Examples: Kigelia africana, Adansonia digitata. Bats are nocturnal and are attracted by the odour of flowers that open at or after dusk. In Adansonia digitata, the ball of stamens and the stigma project beyond the floral envelops. The bat holds the flower by clasping the stamen ball to its breast and while taking nectar its breast becomes laden with pollen grains.

c. Malacophily: Pollination by slugs and snails. Some plants of Araceae are pollinated by snails.

d. Entomophily: Pollination by insects. Pollination by ants is called myrmecophily. Insects well adapted for pollination are bees, moths, butterflies, flies, wasps and beetles.

Characteristic features of entomophilous flowers:

• Flowers are generally large or if small they are aggregated in dense inflorescence.
• Flowers are brightly coloured. Adjacent parts may also be brightly coloured (e.g., Poinsettia, Bougainvillea — bracts become coloured).
• Flowers are scented and produce nectar.
• Pollen and nectar are the floral rewards for the visitors.
• Flowers pollinated by flies and beetles produce foul odour.
• In some flowers juicy cells are present which are sucked by insects.

Pollination in Salvia (Lever Mechanism)

The flower is protandrous and the corolla is bilabiate with 2 stamens. A lever mechanism helps in pollination. Each anther has an upper fertile lobe and lower sterile lobe separated by a long connective which helps the anthers to swing freely. When a bee visits the flower, it sits on the lower lip which acts as a platform. It enters the flower to suck nectar by pushing its head into the corolla. During entry the body strikes against the sterile end of the connective. This makes the fertile part of the stamen to descend and strike at the back of the bee. The pollen gets deposited on the back of the bee. When it visits another flower, the pollen gets rubbed against the stigma and completes pollination.

Pollination in Calotropis (Translator Mechanism)

This mechanism is found in members of Asclepiadaceae. The flowers are bisexual with 5 stamens forming gynostegium (union of stigma with the androecium). The pollen in each anther lobe of a stamen unites into a mass, forming a pollinium. Pollinia are attached to a clamp or clip-like sticky structure called corpusculum. The filamentous part arising from each pollinium is called retinaculum. The whole structure looks like inverted letter 'Y' and is called translator. When the insect visits the flower for nectar, the translator gets attached to the proboscis or leg of the visitor. During the visit to the next flower the pollinia come in contact with the receptive stigma.

Pollination in Aristolochia (Trap Mechanism)

The flowers are axillary and perianth is tubular with a hood at the top. The basal region is swollen and possesses gynostegium with six anthers. The inner wall of tubular middle part of the perianth is slippery and lined with stiff hairs pointed downwards. Small flies enter and cannot escape because they are trapped by the hairs. As soon as the anthers of the flower ripe, the hairs wither and flower bends down. The flies escape with pollen and enter another flower where they dust pollen on the stigma.

Do You Know?

Pollination – A Composite Event: Pollination provides information about evolution, ecology, animal learning and foraging behaviour. The relationship between Yucca and moth (Tegeticula yuccasella) is an example for obligate mutualism. The moth bores a hole in the ovary of the flower and lays eggs in it. Then it collects pollen and pushes it in the form of balls down the hollow end of the stigma. Fertilization takes place and seeds develop. Larvae feed on developing seeds. The moth cannot survive without Yucca flowers and the plant fails to reproduce sexually without the moth.

In Bee orchid (Ophrys) the morphology of the flower mimics that of female wasp (Colpa). The male wasp mistakes the flowers for a female wasp and tries to copulate. This act of pseudocopulation helps in pollination. The pollination in Fig (Ficus carica) by the Wasp (Blastophaga psenes) is also an example for similar plant–insect interaction.

Significance of Pollination

• Pollination is a pre-requisite for the process of fertilisation. Fertilisation helps in the formation of fruits and seeds.
• It brings the male and female gametes closer for the process of fertilisation.
• Cross-pollination introduces variations in plants due to the mixing up of different genes. These variations help the plants to adapt to the environment and results in speciation.

1.5 Fertilization

The fusion of male and female gamete is called fertilization. Double fertilization is seen in angiosperms.

Events of Fertilization

The stages involved in double fertilization are: germination of pollen to form pollen tube in the stigma; growth of pollen tube in the style; direction of pollen tube towards the micropyle of the ovule; entry of the pollen tube into one of the synergids of the embryo sac; discharge of male gametes; syngamy and triple fusion. The events from pollen deposition on the stigma to the entry of pollen tube into the ovule is called pollen-pistil interaction.

Pollen on the Stigma

If the pollen is compatible with the stigma it germinates to form a tube. This is facilitated by the stigmatic fluid in wet stigma and pellicle in dry stigma. These two also decide the incompatibility and compatibility of the pollen through recognition rejection protein reaction. The first visible change in the pollen after it lands on stigma is hydration. The pollen wall proteins are released from the surface. During germination its entire content moves into the pollen tube. The remaining part of the pollen tube is occupied by a vacuole which is cut off from the tip by callose plug. The extreme tip of pollen tube appears hemispherical and transparent when viewed through the microscope — this is called cap block. As soon as the cap block disappears the growth of the pollen tube stops.

Types of Style

• Hollow style (Open style): Common among monocots. A hollow canal runs from the stigma to the base of the style, lined by a single layer of glandular canal cells (Transmitting tissue). The pollen tube grows on the surface of the cells lining the stylar canal.
• Solid style (Closed type): Common among dicots. Characterized by the presence of central core of elongated, highly specialised cells called transmitting tissue. The pollen tube grows through the intercellular spaces of the transmitting tissue.
• Semi-solid style (Half closed type): Intermediate between solid and open type.

Entry of Pollen Tube into the Ovule

• Porogamy: When the pollen tube enters through the micropyle.
• Chalazogamy: When the pollen tube enters through the chalaza.
• Mesogamy: When the pollen tube enters through the integument.

Irrespective of the place of entry into the ovule, the pollen tube enters the embryo sac at the micropylar end, directly into one of the synergids. The growth of pollen tube towards the ovary, ovule and embryo sac is due to the presence of chemotropic substances. After reaching the embryo sac, a pore is formed in pollen tube wall at its apex or just behind the apex. The content of the pollen tube (two male gametes, vegetative nucleus and cytoplasm) are discharged into the synergid. The tube nucleus disorganizes.

Do You Know?

Obturator may originate from different regions of the ovule (Placenta – Euphorbiaceae; Funiculus – Anacardiaceae; Style – Thymelaeaceae; Ovary wall – Ottelia alismoides).

1.5.1 Double Fertilization and Triple Fusion

S.G. Nawaschin and L.Guignard in 1898 and 1899, observed in Lilium and Fritillaria that both the male gametes released from a male gametophyte are involved in the fertilization. They fertilize two different components of the embryo sac. Since both the male gametes are involved in fertilization, the phenomenon is called double fertilization and is unique to angiosperms.

• One of the male gametes fuses with the egg nucleus (syngamy) to form Zygote.
• The second gamete migrates to the central cell where it fuses with the polar nuclei or their fusion product, the secondary nucleus, and forms the primary endosperm nucleus (PEN). Since this involves the fusion of three nuclei, this phenomenon is called triple fusion. This act results in endosperm formation which forms the nutritive tissue for the embryo.

1.6 Post-Fertilization: Structures and Events

After fertilization, several changes take place in the floral parts up to the formation of the seed. The events after fertilization (endosperm, embryo development, formation of seed, fruits) are called post fertilization changes.

Post-Fertilization Transformations

Parts Before Fertilization	Transformation After Fertilization
Sepals, petals, stamens, style and stigma	Usually wither and fall off
Ovary	Fruit
Ovule	Seed
Egg	Zygote
Funicle	Stalk of the seed
Micropyle (ovule)	Micropyle of the seed (facilitates O₂ and water uptake)
Nucellus	Perisperm
Outer integument of ovule	Testa (outer seed coat)
Inner integument	Tegmen (inner seed coat)
Synergid cells	Degenerate
Secondary nucleus	Endosperm
Antipodal cells	Degenerate

Endosperm

The primary endosperm nucleus (PEN) divides immediately after fertilization but before the zygote starts to divide, to form the endosperm. The PEN is the result of triple fusion (two polar nuclei and one sperm nucleus) and thus has 3n number of chromosomes. It is a nutritive tissue and regulatory structure that nourishes the developing embryo.

Depending upon the mode of development, three types of endosperms are recognized in angiosperms:

1. Nuclear endosperm: The PEN divides into two without any wall formation. The subsequent divisions are free nuclear so that the endosperm consists of only free nuclei and cytoplasm. The nuclei may either remain free or may become separate by walls in later stages. Examples: Coccinia, Capsella and Arachis.
2. Cellular endosperm: The PEN divides into 2 nuclei which are immediately followed by a wall formation. Subsequent divisions are also followed by walls. Examples: Adoxa, Helianthus and Scoparia.
3. Helobial endosperm: The PEN moves towards the base of the embryo sac where it divides into two nuclei, separated by a wall to form a large micropylar chamber and a small chalazal chamber. The nucleus of the micropylar chamber undergoes several free nuclear divisions whereas that of the chalazal chamber may or may not divide. Examples: Hydrilla and Vallisneria.

Non-endospermous (ex-albuminous) seeds: Endosperm completely consumed by the developing embryo. Examples: Pea, Groundnut and Beans.

Endospermous (albuminous) seeds: Endosperm persists in the mature seeds. Examples: Paddy, Coconut and Castor.

Ruminate endosperm: The endosperm with irregularity and unevenness in its surface. Example: Areca catechu. The unequal radial elongation of the layer of seed coat results in rumination of endosperm in Passiflora. In Annonaceae and Aristolochiaceae, definite ingrowth or infolding of the seed coat produces ruminate endosperm. The irregular surface of the seed coat makes endosperm ruminate in Myristica.

Functions of Endosperm

• It is the nutritive tissue for the developing embryo.
• In majority of angiosperms, the zygote divides only after the development of endosperm.
• Endosperm regulates the precise mode of embryo development.

Do You Know?

Aleurone tissue consists of highly specialised cells of one or few layers which are found around the endosperm of cereals (barley and maize). Aleurone grain contains sphaerosomes. During seed germination cells secrete certain hydrolytic enzymes like amylases, proteases which digest reserved food material present in the endosperm cells.

Do You Know?

Coconut milk is a basic nutrient medium which induces the differentiation of embryo (embryoids) and plantlets from various plant tissues. Coconut water from tender coconut is free-nuclear endosperm and white kernel part is cellular.

Embryogenesis – Development of Dicot Embryo

The development of Dicot embryo (Capsella bursa-pastoris) is of Onagrad or crucifer type. The embryo develops at the micropylar end of embryo sac.

1. The Zygote divides by a transverse division forming upper or terminal cell and lower or basal cell.
2. The basal cell divides transversely and the terminal cell divides vertically to form a four-celled proembryo.
3. A second vertical division right angle to the first one takes place in terminal cell forming a four-celled stage called quadrant.
4. A transverse division in the quadrant results in eight cells arranged in two tiers of four each called octant stage.
5. The upper tier of four cells is called epibasal or anterior octant and the lower tier is called hypobasal or posterior octants.
6. A periclinal division in the octants results in the formation of 16-celled stage — outer eight cells represent the dermatogen which undergoes anticlinal division to produce epidermis; inner eight cells divide to form outer layer of periblem (gives rise to cortex) and central region of pleurome (forms stele).
7. The basal cells undergo several transverse divisions to form a six to ten-celled suspensor. The uppermost cell of the suspensor enlarges to form a haustorium. The lowermost cell of the suspensor is called hypophysis.
8. A transverse division and two vertical divisions of hypophysis results in the formation of eight cells arranged in two tiers — the upper tier gives rise to root cap and epidermis.
9. At this stage embryo proper appears heart-shaped, cell divisions in the hypocotyl and cotyledon regions result in elongation.
10. Further development results in curved horse-shoe shaped embryo in the embryo sac.
11. The mature embryo has a radicle, two cotyledons and a plumule.

Seed

The fertilized ovule is called seed and possesses an embryo, endosperm and a protective coat. Seeds may be endospermous (wheat, maize, barley and sunflower) or non-endospermous (Bean, Mango, Orchids and cucurbits).

Cicer seed (Example for Dicot seed)

The mature seeds are attached to the fruit wall by a stalk called funiculus. The funiculus disappears leaving a scar called hilum. Below the hilum a small pore called micropyle is present. Each seed has a thick outer covering called seed coat developed from integuments of the ovule. The outer coat is called testa (hard) and the inner coat is thin, membranous and called tegmen. In Pea plant the tegmen and testa are fused. Two cotyledons laterally attached to the embryonic axis store the food materials. The portion of embryonal axis projecting beyond the cotyledons is called radicle or embryonic root. The other end is the plumule. Embryonal axis above the level of cotyledon is called epicotyl whereas the cylindrical region between the level of cotyledon is called hypocotyl.

Oryza seed (Example for Monocot seed)

The seed of paddy is one-seeded and is called Caryopsis. Each seed remains enclosed by a brownish husk which consists of glumes arranged in two rows. The seed coat is a brownish, membranous layer closely adhered to the grain. Endosperm forms the bulk of the grain and is the storage tissue. It is separated from embryo by a definite layer called epithelium. The embryo is small and consists of one shield-shaped cotyledon known as scutellum. A short axis with plumule and radicle protected by the root cap is present. The plumule is surrounded by a protective sheath called coleoptile. The radicle including root cap is covered by a protective sheath called coleorhiza. The scutellum supplies the growing embryo with food material absorbed from the endosperm with the help of the epithelium.

Do You Know?

Fresh weight of an orchid seed may be 20.33 microgram and that of double coconut (Lodoicea maldivica) is about 6 kg.

1.7 Apomixis

Reproduction involving fertilization in flowering plants is called amphimixis and wherever reproduction does not involve union of male and female gametes is called apomixis. The term Apomixis was introduced by Winkler in the year 1908. It is defined as the substitution of the usual sexual system (Amphimixis) by a form of reproduction which does not involve meiosis and syngamy.

Maheswari (1950) classified Apomixis into two types:

• Recurrent apomixis: It includes vegetative reproduction and agamospermy.
• Non-recurrent apomixis: Haploid embryo sac developed after meiosis, develops into an embryo without fertilization.

Recurrent Apomixis

1. Vegetative reproduction: Plants propagate by any part other than seeds.

• Bulbils – Fritillaria imperialis
• Bulbs – Allium
• Runner – Mentha arvensis
• Sucker – Chrysanthemum

2. Agamospermy: Embryos are formed by eliminating meiosis and syngamy.

• Adventive embryony: An embryo arises directly from the diploid sporophytic cells either from nucellus or integument. Also called sporophytic budding because gametophytic phase is completely absent. Examples: Citrus and Mangifera.
• Diplospory: A diploid embryo sac is formed from megaspore mother cell without a regular meiotic division. Examples: Eupatorium and Aerva.
• Apospory: Megaspore mother cell (MMC) undergoes normal meiosis and four megaspores formed gradually disappear. A nucellar cell becomes activated and develops into a diploid embryo sac. Also called somatic apospory. Examples: Hieracium and Parthenium.

1.8 Polyembryony

Occurrence of more than one embryo in a seed is called polyembryony. The first case of polyembryony was reported in certain oranges by Anton von Leeuwenhoek in the year 1719.

Categories of Polyembryony

1. Cleavage polyembryony (Example: Orchids)
2. Formation of embryo by cells of the Embryo sac other than egg (Synergids – Aristolochia; antipodals – Ulmus; endosperm – Balanophora)
3. Development of more than one Embryo sac within the same ovule (Derivatives of same MMC, derivatives of two or more MMC – Casuarina)
4. Activation of some sporophytic cells of the ovule (Nucellus/integuments – Citrus and Syzygium)

Practical Applications

• The seedlings formed from the nucellar tissue in Citrus are found better clones for orchards.
• Embryos derived through polyembryony are found virus free.

1.9 Parthenocarpy

The ovary becomes the fruit and the ovule becomes the seed after fertilization. However, in a number of cases, fruit-like structures may develop from the ovary without the act of fertilization. Such fruits are called parthenocarpic fruits. Invariably they will not have true seeds. Many commercial fruits are made seedless. Examples: Banana, Grapes and Papaya.

Significance of Parthenocarpy

• The seedless fruits have great significance in horticulture.
• The seedless fruits have great commercial importance.
• Seedless fruits are useful for the preparation of jams, jellies, sauces, fruit drinks etc.
• High proportion of edible part is available in parthenocarpic fruits due to the absence of seeds.

Summary

• Reproduction is one of the attributes of living things. Lower plants, microbes and animals reproduce by different methods (fragmentation, gemma, binary fission, budding, regeneration).
• Organisms reproduce through asexual and sexual methods. Asexual methods in angiosperms occur through natural or artificial methods.
• Artificial method of reproduction involves cutting, layering and grafting.
• Sexual reproduction includes gametogenesis and fertilization. External fertilization occurs in lower plants like algae but in higher plants internal fertilization takes place.
• A flower is a modified shoot meant for reproduction. Stamen is the male reproductive part and produces pollen grains.
• The development of microspore is called microsporogenesis. The microspore mother cell undergoes meiotic division to produce four haploid microspores.
• In majority of Angiosperms the anther is dithecous and are tetrasporangiate. It possesses epidermis, endothecium, middle layers and tapetum.
• The hygroscopic nature of endothecial cell along with thin walled stomium helps in the dehiscence of anther.
• Tapetum nourishes the microspores and also contributes to the wall materials of the pollen grain.
• Pollen grain is derived from the microspore and possesses thin inner intine and thick outer exine. Sporopollenin is present in exine and is resistant to physiological and biological decomposition.
• Microspore is the first cell of male gametophyte. The nucleus of the microspore divides to form a vegetative nucleus and a generative nucleus. The generative nucleus divides to form two male nuclei.
• Gynoecium is the female reproductive part of a flower. The ovary bears ovules attached to the placenta. There are six major types of ovules.
• The development of megaspore from megaspore mother cell is called megasporogenesis.
• A monosporic embryo sac (Polygonum type) possesses three antipodals in chalazal end, three cells in the micropylar end (1 egg and 2 Synergids) and two polar nuclei fused to form secondary nucleus — thus a 7-celled, 8-nucleated embryo sac.
• The transfer of pollen grains to the stigma of a flower is called pollination. Self-pollination and cross-pollination are two types.
• Double fertilization and triple fusion are characteristic features of angiosperms.
• After fertilization the ovary transforms into a fruit and the ovule becomes a seed.
• Endosperm is triploid in angiosperms and is of three types – Nuclear, cellular, helobial.
• Reproduction which doesn't involve meiosis and syngamy is called apomixis.
• Occurrence of more than one embryo in a seed is called polyembryony.
• Formation of fruit without the act of fertilization is called parthenocarpy.

Evaluation

Exercise MCQ – Choose the Correct Answer

1. Choose the correct statement from the following:

a. Gametes are involved in asexual reproduction b. Bacteria reproduce asexually by budding c. Conidia formation is a method of sexual reproduction d. Yeast reproduces by budding

2. An eminent Indian embryologist is:

a. S.R.Kashyap b. P.Maheswari c. M.S. Swaminathan d. K.C.Mehta

3. Identify the correctly matched pair:

a. Tuber – Allium cepa b. Sucker – Pistia c. Rhizome – Musa d. Stolon – Zingiber

4. Size of pollen grain in Myosotis:

a. 10 micrometres b. 20 micrometres c. 200 micrometres d. 2000 micrometres

5. First cell of male gametophyte in angiosperm is:

a. Microspore b. Megaspore c. Nucleus d. Primary Endosperm Nucleus

6. Match the following:
1) External fertilization     i) pollen grain
2) Androecium             ii) anther wall
3) Male gametophyte       iii) algae
4) Primary parietal layer     iv) stamens

a. 1-4, 2-1, 3-2, 4-3 b. 1-3, 2-4, 3-1, 4-2 c. 1-3, 2-4, 3-2, 4-1 d. 1-3, 2-1, 3-4, 4-2

7. Arrange the layers of anther wall from locus to periphery:

a. Epidermis, middle layers, tapetum, endothecium b. Tapetum, middle layers, epidermis, endothecium c. Endothecium, epidermis, middle layers, tapetum d. Tapetum, middle layers, endothecium, epidermis

8. Identify the incorrect pair:

a. Sporopollenin – exine of pollen grain b. Tapetum – nutritive tissue for developing microspores c. Nucellus – nutritive tissue for developing embryo d. Obturator – directs the pollen tube into micropyle

9. Assertion: Sporopollenin preserves pollen in fossil deposits.
Reason: Sporopollenin is resistant to physical and biological decomposition.

a. Assertion is true; reason is false b. Assertion is false; reason is true c. Both Assertion and reason are not true d. Both Assertion and reason are true

10. Choose the correct statement(s) about tenuinucellate ovule:

a. Sporogenous cell is hypodermal b. Ovules have fairly large nucellus c. Sporogenous cell is epidermal d. Ovules have single layer of nucellus tissue

11. The correct order of haploid, diploid and triploid structure in fertilized embryo sac is:

a. Synergid, zygote and primary endosperm nucleus (PEN) b. Synergid, antipodal and polar nuclei c. Antipodal, synergid and primary endosperm nucleus (PEN) d. Synergid, polar nuclei and zygote

12. Which of the following represent megagametophyte?

a. Ovule b. Embryo sac c. Nucellus d. Endosperm

13. In Haplopappus gracilis, number of chromosomes in cells of nucellus is 4. What will be the chromosome number in Primary endosperm cell?

a. 8 b. 12 c. 6 d. 2

14. Transmitting tissue is found in:

a. Micropylar region of ovule b. Pollen tube wall c. Stylar region of gynoecium d. Integument

15. The scar left by funiculus in the seed is:

a. Tegmen b. Radicle c. Epicotyl d. Hilum

16. A plant called X possesses small flower with reduced perianth and versatile anther. The probable agent for pollination would be:

a. Water b. Air c. Butterflies d. Beetles

17. Consider the following statements:
i) In Protandrous flowers pistil matures earlier
ii) In Protogynous flowers pistil matures earlier
iii) Herkogamy is noticed in unisexual flowers
iv) Distyly is present in Primula

a. i and ii are correct b. ii and iv are correct c. ii and iii are correct d. i and iv are correct

18. Ruminate endosperm is found in:

a. Cocos b. Areca c. Vallisneria d. Arachis

19. Coleorhiza is found in:

a. Paddy b. Bean c. Pea d. Tridax

20. Caruncle develops from:

a. Funicle b. Nucellus c. Integument d. Embryo sac

21. Parthenocarpic fruits lack:

a. Endocarp b. Epicarp c. Mesocarp d. Seed

22. In majority of plants pollen is liberated at:

a. 1-celled stage b. 2-celled stage c. 3-celled stage d. 4-celled stage Exercise Short Answer Questions

23. What is reproduction?

24. List out two sub-aerial stem modifications with example.

25. What is layering?

26. What are clones?

27. How do Dioscorea reproduce vegetatively?

28. A detached leaf of Bryophyllum produces new plants. How?

29. Differentiate Grafting and Layering.

30. Write short notes on approach grafting.

32. Distinguish mound layering and air layering.

33. List down the advantages of conventional methods.

35. Differentiate Secretory and invasive tapetum.

36. What is Cantharophily?

37. List any two strategies adopted by bisexual flowers to prevent self-pollination.

38. What is endothelium?

39. Name the cell which divides to form male nuclei.

41. Define the term Diplospory.

42. What is polyembryony? How can it be commercially exploited?

45. What is Mellitophily?

46. Give examples for Helobial endosperm.

47. 'Endothecium is associated with dehiscence of anther' – Justify the statement.

48. List out the functions of tapetum.

49. Write short note on Pollen Kitt.

50. Distinguish tenuinucellate and crassinucellate ovules.

51. Give short notes on types of ovules.

53. Write short note on Heterostyly.

Exercise Long Answer Questions

31. "Tissue culture is the best method for propagating rare and endangered plant species" – Discuss.

34. Explain the conventional methods adopted in vegetative propagation of higher plants.

40. 'The endosperm of angiosperm is different from gymnosperm'. Do you agree? Justify your answer.

43. Do you think parthenocarpy and apomixis are different processes? Justify.

44. Why does the zygote divide only after the division of Primary endosperm cell?

52. 'Pollination in Gymnosperms is different from Angiosperms' – Give reasons.

54. Enumerate the characteristic features of Entomophilous flowers.

55. Explain the pollination mechanism in Salvia.

56. Discuss the steps involved in Microsporogenesis.

57. With a suitable diagram explain the structure of an ovule.

58. Give a concise account on steps involved in fertilization of an angiosperm plant.

59. What is endosperm? Explain the types.

60. Explain the development of a Dicot embryo.

61. Differentiate the structure of Dicot and Monocot seed.

62. Give a detailed account on parthenocarpy. Add a note on its significance.

Glossary

Apospory (n) The process of embryo sac formation from diploid cells of nucellus as a result of mitosis. Budding (n) A method of asexual reproduction where small outgrowth (Bud) from a parent cell is produced. Callus (n) Undifferentiated mass of cells obtained through tissue culture. Clone (n) Genetically identical individuals. Endothecium (n) A single layer of hygroscopic, radially elongated cells found below the epidermis of anther which helps in dehiscence of anther. Fertilization (n) The act of fusion of male and female gamete. Grafting (n) Conventional method of reproduction where stock and scion are joined to produce new plant. Horticulture (n) Branch of plant science that deals with the art of growing fruits, vegetables, flowers and ornamental plants. Nucellus (n) The diploid tissue found on the inner part of ovule next to the integuments. Pollen Kitt (n) A sticky covering found on the surface of the pollen that helps to attract insects. Regeneration (n) Ability of organisms to replace or restore the lost parts. Sporopollenin (n) Pollen wall material derived from carotenoids and is resistant to physical and biological decomposition. Tapetum (n) Nutritive tissue for the developing sporogenous tissue. Transmitting tissue (n) A single layer of glandular canal cells lining the inner part of style. Do You Know?

Pollen calendar shows the production of pollen by plants during different seasons. This benefits the allergic persons. Pollen grains cause allergic reactions like asthma, bronchitis, hay fever, allergic rhinitis etc. Parthenium hysterophorus L. (Family – Asteraceae) is commonly called Carrot grass and is a native of tropical America. It was introduced into India as a contaminant along with cereal wheat. The pollen of this plant causes allergy.

More to Know

• The receptacle becomes fleshy and edible around the fruit enclosing the seeds as in Pyrus malus (apple).
• The calyx may persist and enlarge (Solanum melongena) or may cover the fruit (Physalis minima).
• The flower stalk or axis below the gynoecium enlarges into a juicy pear shaped body which is edible (Anacardium occidentale). The Perianth becomes fleshy as in Jack fruit.
• The cells present at the tip of the outer integument around the micropyle develop into a fleshy structure called caruncle (Ricinus communis).
• The funiculus develops into a fleshy structure which is often very colourful and called aril (Myristica and Pithecellobium).
• The nucellar tissue is either absorbed completely by the developing embryo sac and embryo or small portion may remain as storage tissue. The remnant of nucellar tissue in the seed is called perisperm. Example: Black pepper and beet root.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. Choose the correct statement from the following

a) Gametes are involved in asexual reproduction b) Bacteria reproduce asexually by budding c) Conidia formation is a method of sexual reproduction d) Yeast reproduces by budding

Answer: d) Yeast reproduces by budding

Explanation: According to the chapter, Budding is a method of asexual reproduction found in Yeast and Hydra. Bacteria reproduce by binary fission and conidia formation is a method of asexual reproduction found in Aspergillus and Penicillium.

2. An eminent Indian embryologist is

a) S.R. Kashyap b) P. Maheswari c) M.S. Swaminathan d) K.C. Mehta

Answer: b) P. Maheswari

Explanation: Professor P. Maheshwari was an eminent Botanist who specialised in plant embryology, morphology and anatomy. He published the book "An introduction to the Embryology of Angiosperms" in 1950.

3. Identify the correctly matched pair

a) Tuber - Allium cepa b) Sucker - Pistia c) Rhizome - Musa d) Stolon – Zingiber

Answer: c) Rhizome - Musa

Explanation: The chapter states that rhizome examples include Musa paradisiaca, Zingiber officinale and Curcuma longa. Allium cepa is a bulb, Pistia is an offset, and Zingiber is a rhizome (not stolon).

4. Size of pollen grain in Myosotis

a) 10 micrometer b) 20 micrometer c) 200 micrometer d) 2000 micrometer

Answer: a) 10 micrometer

Explanation: The chapter states that the size of pollen varies from 10 micrometres in Myosotis to 200 micrometres in members of the family Cucurbitaceae and Nyctaginaceae.

5. First cell of male gametophyte in angiosperm is

a) Microspore b) megaspore c) Nucleus d) Primary Endosperm Nucleus

Answer: a) Microspore

Explanation: The chapter clearly states that "The microspore is the first cell of the male gametophyte and is haploid."

6. Match the following

Column A	Column B
1) External fertilization	1) pollen grain
2) Androecium	2) anther wall
3) Male gametophyte	3) algae
4) Primary parietal layer	4) stamens

a) 2-4, 2-1, 3-2, 4-3 b) 2-3, 2-4, 3-1, 4-2 c) 1-3, 2-4, 3-2, 4-1 d) 1-3, 2-1, 3-4, 4-2

Answer: c) 1-3, 2-4, 3-2, 4-1 (External fertilization – algae; Androecium – stamens; Male gametophyte – anther wall derived; Primary parietal layer – pollen grain)

Explanation: External fertilization occurs in algae; Androecium is made up of stamens; Male gametophyte is the pollen grain; Primary parietal cells form the anther wall layers including endothecium, middle layers and tapetum.

7. Arrange the layers of anther wall from locus to periphery

a) Epidermis, middle layers, tapetum, endothecium b) Tapetum, middle layers, epidermis, endothecium c) Endothecium, epidermis, middle layers, tapetum d) Tapetum, middle layers, endothecium, epidermis

Answer: d) Tapetum, middle layers, endothecium, epidermis

Explanation: The chapter states that primary parietal cells form 2–5 layers of anther walls composed of endothecium, middle layers and tapetum, from periphery to centre. So from locule (centre) to periphery: Tapetum → Middle layers → Endothecium → Epidermis.

8. Identify the incorrect pair

a) sporopollenin - exine of pollen grain b) tapetum – nutritive tissue for developing microspores c) Nucellus – nutritive tissue for developing embryo d) obturator – directs the pollen tube into micropyle

Answer: c) Nucellus – nutritive tissue for developing embryo

Explanation: The chapter states the nucellus is the central mass of parenchymatous tissue with large reserve food materials and the endosperm is the nutritive tissue for the developing embryo, not the nucellus directly.

9. Assertion: Sporopollenin preserves pollen in fossil deposits. Reason: Sporopollenin is resistant to physical and biological decomposition

a) assertion is true; reason is false b) assertion is false; reason is true c) Both Assertion and reason are not true d) Both Assertion and reason are true

Answer: d) Both Assertion and reason are true

Explanation: The chapter states that sporopollenin "is resistant to physical and biological decomposition" and "preserves the pollen for long periods in fossil deposits." Both statements are correct and the reason correctly explains the assertion.

10. Choose the correct statement(s) about tenuinucellate ovule

a) Sporogenous cell is hypodermal b) Ovules have fairly large nucellus c) sporogenous cell is epidermal d) ovules have single layer of nucellus tissue

Answer: a) Sporogenous cell is hypodermal

Explanation: The chapter states: "If the sporogenous cell is hypodermal with a single layer of nucellar tissue around it is called tenuinucellate type. Normally tenuinucellate ovules have very small nucellus."

11. The correct order of haploid, diploid and triploid structure in fertilized embryo sac is

a) synergid, zygote and primary endosperm nucleus (PEN) b) synergid, antipodal and polar nuclei c) antipodal, synergid and primary endosperm nucleus (PEN) d) synergid, polar nuclei and zygote

Answer: a) synergid, zygote and primary endosperm nucleus (PEN)

Explanation: Synergids are haploid (n); the zygote is formed by fusion of egg (n) and male gamete (n) making it diploid (2n); the Primary Endosperm Nucleus (PEN) is formed by triple fusion (2 polar nuclei + 1 sperm) making it triploid (3n).

12. Which of the following represent megagametophyte

a) Ovule b) Embryo sac c) Nucellus d) Endosperm

Answer: b) Embryo sac

Explanation: The chapter states that the functional megaspore forms the female gametophyte or embryo sac. The embryo sac is the megagametophyte, developing from the functional megaspore within the nucellus.

13. In Haplopappus gracilis, number of chromosomes in cells of nucellus is 4. What will be the chromosome number in Primary endosperm cell?

a) 8 b) 12 c) 6 d) 2

Answer: c) 6

Explanation: Nucellus is diploid (2n = 4), so n = 2. The Primary Endosperm Nucleus (PEN) is formed by triple fusion and has 3n chromosomes. Therefore, PEN = 3 × 2 = 6 chromosomes.

14. Transmitting tissue is found in

a) Micropylar region of ovule b) Pollen tube wall c) Stylar region of gynoecium d) Integument

Answer: c) Stylar region of gynoecium

Explanation: The chapter states that in solid (closed) style, there is a central core of elongated highly specialised cells called transmitting tissue. In hollow style, the canal is lined by a single layer of glandular canal cells (Transmitting tissue) lining the stylar canal.

15. The scar left by funiculus in the seed is

a) tegmen b) radicle c) epicotyl d) hilum

Answer: d) hilum

Explanation: The chapter states: "The mature seeds are attached to the fruit wall by a stalk called funiculus. The funiculus disappears leaving a scar called hilum."

16. A Plant called X possesses small flower with reduced perianth and versatile anther. The probable agent for pollination would be

a) water b) air c) butterflies d) beetles

Answer: b) air

Explanation: The chapter states that anemophilous (wind-pollinated) flowers have characteristic features including: flowers are small, inconspicuous; perianth is absent or highly reduced; stamens are numerous with long, exerted and versatile filaments/anthers.

17. Consider the following statement(s): i) In Protandrous flowers pistil matures earlier ii) In Protogynous flowers pistil matures earlier iii) Herkogamy is noticed in unisexual flowers iv) Distyly is present in Primula

a) i and ii are correct b) ii and iv are correct c) ii and iii are correct d) i and iv are correct

Answer: b) ii and iv are correct

Explanation: The chapter states: "Protogyny: The stigmas mature earlier than the stamens." (Statement ii is correct.) Distyly in Primula is explicitly mentioned in the chapter. In Protandry, stamens (not pistil) mature earlier. Herkogamy is in bisexual flowers, not unisexual.

18. Ruminate endosperm is found in

a) Cocos b) Areca c) Vallisneria d) Arachis

Answer: b) Areca

Explanation: The chapter states: "The endosperm with irregularity and unevenness in its surface forms ruminate endosperm (Example: Areca catechu)."

19. Coleorrhiza is found in

a) Paddy b) Bean c) Pea d) Tridax

Answer: a) Paddy

Explanation: The chapter states regarding the Oryza (paddy) seed: "The radicle including root cap is also covered by a protective sheath called coleorhiza." This structure is found in monocot seeds like paddy.

20. Caruncle develops from

a) funicle b) nucellus c) integument d) embryo sac

Answer: c) integument

Explanation: The chapter states: "The cells present at the tip of the outer integument around the micropyle develop into a fleshy structure called caruncle (Ricinus communis)."

21. Parthenocarpic fruits lack

a) Endocarp b) Epicarp c) Mesocarp d) seed

Answer: d) seed

Explanation: The chapter states that parthenocarpic fruits develop from the ovary without fertilization and "Invariably they will not have true seeds." Examples include Banana, Grapes and Papaya.

22. In majority of plants pollen is liberated at

a) 1 celled stage b) 2 celled stage c) 3 celled stage d) 4 celled stage

Answer: b) 2 celled stage

Explanation: The chapter states: "Generally, at these 2 celled stage, the pollens are liberated from the anther" and "In 60% of the angiosperms pollen is liberated in 2 celled stage."

II. Answer Briefly

23. What is reproduction?

Answer: Reproduction is a vital process for the existence of a species. It brings suitable changes through variation in the offspring for their survival on earth. Basically, reproduction in organisms falls under two major categories: asexual reproduction and sexual reproduction.

24. List out two sub-aerial stem modifications with example.

Answer: Two sub-aerial stem modifications are: (1) Runner – a stem that creeps along the ground surface; Example: Centella asiatica. (2) Stolon – a lateral branch that bends towards the ground and produces roots; Examples: Mentha and Fragaria.

25. What is layering?

Answer: In layering, the stem of a parent plant is allowed to develop roots while still intact with the parent plant. When the roots develop, the rooted part is cut and planted to grow as a new plant. Examples include Ixora and Jasminum.

26. What are clones?

Answer: The individuals formed by asexual reproduction are morphologically and genetically identical to each other and to the parent plant. Such genetically identical individuals are called clones.

27. How do Dioscorea reproduce vegetatively?

Answer: Dioscorea reproduces vegetatively through bulbils. Bulbils are a type of vegetative propagule found in plants like Dioscorea and Agave. These bulbils develop into new plants when they detach from the parent and grow under suitable conditions.

28. A detached leaf of Bryophyllum produces new plants. How?

Answer: In Bryophyllum, the leaf is succulent and notched on its margin. Adventitious buds develop at these notches and are called epiphyllous buds. They develop into new plants forming a root system and become independent plants when the leaf gets decayed.

29. Differentiate Grafting and Layering.

Answer: In grafting, parts of two different plants (stock and scion) are joined so that they continue to grow as one plant; examples are Citrus and Mango. In layering, the stem of a parent plant is allowed to develop roots while still intact with the parent, then the rooted part is detached and grown as a new plant; examples are Ixora and Jasminum.

30. Write short notes on approach grafting.

Answer: In approach grafting, both the scion and stock remain rooted. The stock is grown in a pot and brought close to the scion. Both must have the same thickness. A small slice is cut from both, and the cut surfaces are brought near, tied together and held by a tape. After 1–4 weeks, the tip of the stock and base of the scion are cut off and grown in a separate pot.

31. "Tissue culture is the best method for propagating rare and endangered plant species" - Discuss.

Answer: Tissue culture (micropropagation) is a modern method used to raise new plants. It is used to produce a large number of plants in a short period of time. Since it can produce many genetically uniform plants from very small tissue samples, it is particularly useful for propagating rare and endangered plant species where conventional methods would require large quantities of plant material that may not be available.

32. Distinguish mound layering and air layering.

Answer: In mound layering, the lower flexible branch with leaves is bent to the ground, part of the stem is buried in soil with the tip exposed; after roots emerge, it is cut from the parent. In air layering, the stem is girdled at a nodal region, hormones are applied, the region is covered with damp soil using a polythene sheet; roots emerge after 2–4 months and the branch is removed and grown separately.

📝 Summary

1. Reproduction is a vital process for the existence of a species, bringing variations in offspring for survival, and plants play a central role as other organisms directly or indirectly depend on them. It also plays an important role in evolution.
2. Asexual reproduction occurs without the involvement of gametes, producing individuals that are morphologically and genetically identical (clones), and includes methods such as budding, fragmentation, binary fission, and formation of conidia seen in lower plants, fungi, and animals.
3. Higher plants reproduce asexually through vegetative reproduction, which can be natural or artificial, using reproductive structures called propagules or diaspores formed from organs such as roots, stems, and leaves.
4. In natural vegetative reproduction, buds develop from roots (e.g., Murraya, Dahlia), stems, and leaves to form new plants; some of these organs also serve dual roles as storage and perennation structures.
5. Professor Panchanan Maheshwari (1904–1966) was an eminent botanist who specialised in plant embryology, morphology, and anatomy, and made landmark contributions including publishing "An Introduction to the Embryology of Angiosperms" in 1950 and establishing the International Society for Plant Morphologists in 1951.