Samacheer Class 12 Bio Botany - Asexual and Sexual Reproduction in Plants

Reproduction is the biological process by which organisms produce young ones of their own kind. It is a vital process essential for the existence and continuation of a species across generations. Reproduction also brings about suitable variations and genetic diversity in the offspring, which enables them to adapt to changing environmental conditions and improves their chances of survival on Earth. Through reproduction, organisms pass on their genetic material to the next generation, ensuring the perpetuation of the species while simultaneously introducing variations that contribute to evolution and natural selection. Both asexual and sexual reproduction serve this fundamental purpose, though they differ in the mechanisms by which genetic material is transmitted and variation is generated.

Subaerial stem modifications.
The stem is partly aerial and partly underground.
a) Runner. (Ex. oxalis, Centella Asiatica)
* It is running horizontally on the soil surface.
* Nodes have axillary buds, scale leaves, and adventitious roots.
* Runner arises from the axillary bud.
* Mother plant produces many runners in all directions.
* They break off and grow into individual plants.
b) Sucker. (Ex. Musa (banana), chrysanthemum)
Grows horizontally for a distance under the soil. Then it emerges obliquely upwards.
c) Stolon (Ex. Strawberry, Vallisneria)
Develop from underground stems.
They grow horizontally outwards.
d) Offset (condensed runners)
Unlike runners, they produce tilt of leaves above and duster of roots below Ex. Pistia, Eichhornia.

• It is an artificial method of vegetative propagation.
• The stem of the parent plant is allowed to develop roots while still intact.
• The root develops. The rooted part is cut. It is planted to grow as a new plant.
• Ex. Ixora, Jasminum.

• Bryophyllum undergoes vegetative reproduction in the leaf.
• The succulent leaf is notched in its margin.
• Adventitious buds develop at these notches. They are called epiphyllous buds.
• These buds develop a root system. When the leaf decays, they become independent plants.

Grafting and layering are both vegetative propagation techniques but differ significantly in their methods and outcomes. In grafting, two different plants are used: the stock (rootstock) and the scion (shoot to be grafted). The scion is joined to the stock, and the new plant that develops may possess characteristics of both parents or exhibit new variations due to the combination of different genetic materials. Grafting is useful for propagating plants with desirable traits and overcoming incompatibility issues. In contrast, layering involves only one parent plant from which a new plant is developed. A branch or stem of the parent plant is induced to develop roots while still attached to the parent, and then it is separated to form an independent plant. Since layering uses only one parent plant, no variation is expected, and the new individual is genetically identical to the parent plant, making it a true clone. Layering is simpler and does not require the skill needed for grafting but is slower and produces fewer plants.

Tissue culture, also known as micropropagation, is the growth of plant tissue in a special culture medium under suitable controlled environmental conditions. It involves the regeneration of a whole plant from a single cell or a small piece of tissue, making it an extremely valuable technique for plant propagation. This statement is justified by numerous advantages that tissue culture offers. Rare and endangered plant species can be propagated rapidly and in large numbers, helping to conserve biodiversity and prevent extinction. In a short duration, plants with desirable characteristics can be multiplied many times over, which is far more efficient than conventional propagation methods. Tissue culture produces genetically identical plants, ensuring that desirable traits are preserved exactly in all propagated individuals. The technique can be performed in any season, independent of environmental conditions, providing year-round propagation capability. Plants that do not produce viable seeds or have seeds that are difficult to germinate can be successfully propagated through tissue culture. Meristem culture, which uses the apical meristem tissue, produces disease-free plants by eliminating viruses and other pathogens that may be present in the parent plant. Additionally, cells can be genetically modified or transformed during tissue culture, allowing for the creation of plants with enhanced traits or resistance to diseases and pests. These multiple advantages make tissue culture the best method for propagating rare and endangered plant species.

Mound Layering: In mound layering, a lower flexible branch with leaves is bent to the ground and a part of the stem is buried in the soil while the tip of the branch remains exposed above the soil. After roots emerge from the buried stem portion, a cut is made to separate the rooted portion from the parent plant, and the buried plant grows into a new independent plant. Air Layering: In air layering, a branch of the parent plant is girdled or ringed by removing a ring of bark from the stem. The exposed wood is then wrapped with moist moss or soil and covered with a waterproof material like plastic to maintain moisture. Roots develop from the cambium region of the girdled portion while the branch is still attached to the parent plant. Once sufficient roots have developed, the rooted branch is cut below the root system and planted separately to grow as a new plant. The main difference is that mound layering involves burying the stem in soil on the ground, while air layering develops roots in the air above the ground on an intact branch.

Conventional methods of vegetative propagation.
a) Cutting (Ex. Hibiscus)
* Plant parts like stem, leaf are cut from the parent plant.
* Cut part is placed in suitable medium,
* It produces root and grows into a new plant.
b) Grafting (Ex. Citrus, Mango)
* Two different plants are joined.
* They grow as one plant.
* Plant in soil is called stock.
* Plant used for grafting is the scion.
* It is of 5 types.
i) Bud grafting – scion is placed in the incision of stock.
ii) Approach grafting – Cut surfaces of stock scion are tied together.
iii) Crown Grafting – Wedge-shaped scion is inserted into the cleft of stock.
iv) Tongue grafting – Stock and scion are cut obliquely scion is fit into stock and bound with tape.
v) Wedge grafting – Twig of the scion is inserted into slot in the stock.
c) Layering
Stem of parent plant is allowed to develop roots while still intact. The root develops. The rooted part is cut and planted to grow as a new plant.
I) Mound Layering
* Flexible branch is buried in soil.
* Roots emerge from buried stem. It grows into a new plant.
ii) Air Layering
* Nodal region is girdled.
* Hormones are applied.
* Rooting is promoted.
* This area is covered by moist soil.
* Roots emerge in 2-4 months.
* Roots branches are removed from parent. They are grown separately.

• 1682 – Nehemiah Grew mentioned stamens as the male organ of a flower.
• 1694 – R.J. Camerarius described the structure of a flower, anther, pollen, and ovule
• 1761 – J.G. Kolreuter gave a detailed account of the importance of insects in pollination.
• 1824 – G.B. Amici discovered the pollen tube.
• 1848 – Hofmeister described the structure of pollen tetrad.
• 1870 – Hanstein described the development of embryos in Capsella and Alisma.
• 1878 – E. Strasburger reported polyembryony.
• 1884 – E. Strasburger discovered the process of Syngamy.
• 1899 – S.G. Nawaschin and L. Guignard independently discovered Double fertilization.
• 1904- E. Hanning initiated embryo culture.
• 1950 – D.A. Johansen proposed classification for embryo development.
• 1964 – S. Guha and S.C. Maheswari raised haploids from Datura pollen grains
• 1991 – E.S. Coen and E.M. Meyerowitz proposed the ABC model to describe the genetics of initiation and development of floral parts
• 2015 – K.V. Krishnamurthy summarized the molecular aspects of pre and post-fertilization reproductive development in flowering plants.

The genetic ability of a plant cell to produce the entire plant under suitable condition is said to be totipotency.
* This characteristic feature of a cell is utilized in horticulture, forestry and industries to propagate plants.
* The mature phloem parenchyma cells removed from the carrot were placed in a suitable medium under controlled conditions.
* It stimulate to start dividing again to produce a new carrot plant.
Importance of modern methods of reproduction in plants.
* Rapid Multiplication of desired plants in short duration.
* Genetically identical plants are produced.
* Tissue culture can be done at any season
* Plants without viable seeds (or) difficult to germinate can be propagated.
* Rare, Endangered plants are propagated.
* Meristem culture produces disease-free plants.
* Cells are genetically modified or transformed.

• It is the cross-pollination of flowers by beetles. They feed on pollen or juicy tissues of their flower.
• The plants using this mode of pollination
• Er. Nymphaea species of plants – Rhinoceros beetle.
• Giant Water lily – Scarab beetle
• Illicium plant – Diptera files.

Bisexual flowers have evolved several strategies to prevent self-pollination and promote cross-pollination, ensuring genetic diversity. Two major strategies are dichogamy and herkogamy. Dichogamy is a temporal mechanism in which the anthers and stigmas of a bisexual flower mature at different times, making simultaneous pollination of the stigma by pollen from the same flower impossible. Protandry occurs when the stamens mature earlier than the stigma, so pollen is shed before the stigma becomes receptive. Protogyny is the opposite condition, where the stigma matures and becomes receptive before the anthers release their pollen. Herkogamy is a spatial or structural mechanism in which self-pollination is made physically impossible by the arrangement and positioning of the stamens and stigmas within the flower. For example, in Hibiscus, the stigmas project above the stamens, preventing the flower's own pollen from reaching the stigma. In some plants, even if pollen from the same flower reaches the stigma, it is unable to germinate or is prevented from germinating through a genetic mechanism called self-incompatibility. This is a genetic barrier that prevents the pollen tube from growing through the style. Examples of plants exhibiting self-incompatibility include Abutilon and Passiflora. These mechanisms work together to ensure that plants reproduce through cross-pollination, which increases genetic variation and produces healthier, more vigorous offspring.

Yes, the endosperm of angiosperms is significantly different from that of gymnosperms in both origin and composition. In angiosperms, the endosperm is triploid, meaning it contains three sets of chromosomes. It is formed by triple fusion, a unique process in which two polar nuclei from the central cell of the female gametophyte fuse with one sperm nucleus from the pollen grain. This triple fusion occurs during double fertilization and results in the formation of the primary endosperm nucleus, which then undergoes mitotic divisions to form the endosperm tissue. The endosperm surrounds the embryo and serves as the primary source of nutrition for the developing embryo and the germinating seedling. In contrast, the endosperm of gymnosperms is haploid, containing only one set of chromosomes. The endosperm in gymnosperms is formed before fertilization occurs, developing from the female gametophyte tissue. Gymnosperms such as pine produce embryos with many cotyledons, and the primary endosperm tissue provides nutrition in the form of starch and other stored reserves. The fundamental difference lies in the timing of endosperm formation, the ploidy level, and the mechanism of formation. The triploid endosperm of angiosperms is a result of double fertilization and represents an advanced reproductive strategy that ensures efficient nutrition for the developing seed, whereas the haploid endosperm of gymnosperms is a simpler structure formed from the female gametophyte tissue before fertilization.

Polyembryony is the occurrence of more than one embryo within a single seed. This phenomenon can arise through various mechanisms, including the development of adventitious embryos from nucellar tissue, integumentary tissue, or other somatic tissues of the ovule, in addition to the embryo formed from the fertilized egg cell. Polyembryony has significant practical applications in agriculture and horticulture. Seedlings obtained from nucellar tissue of citrus plants are superior clones that are genetically identical to the parent plant and are therefore preferred for establishing commercial orchards. These nucellar seedlings maintain the desirable characteristics of the parent variety without any genetic variation. Additionally, embryos derived from polyembryonic seeds, particularly those from nucellar origin, are typically virus-free because viruses are generally not transmitted through the nucellar tissue. This makes polyembryony an excellent natural mechanism for obtaining disease-free planting material. The commercial exploitation of polyembryony allows horticulturists to rapidly propagate elite varieties of citrus and other polyembryonic plants while ensuring genetic uniformity and disease-free status, making it economically valuable for large-scale cultivation.

The endothecium is the innermost layer of the anther wall and plays a crucial role in anther dehiscence. The inner tangential walls of endothecial cells develop bands of thickened cellulose deposits, which are sometimes slightly lignified, making these cells hygroscopic in nature. However, the cells located along the junction between the two sporangia of an anther lobe lack these characteristic thickenings. This specialized region is called the stomium. During anther maturation, the hygroscopic nature of the endothecial cells causes them to lose water and shrink, creating mechanical stress. Combined with the weakness of the stomium region, which lacks reinforcing thickenings, this differential shrinkage leads to the rupture and dehiscence of the anther at maturity, allowing pollen grains to be released and dispersed.

• Supplies nutrition to developing microspores.
• Contributes sporopollenin through ubisch bodies. They play role in pollen wall formation.
• Pollenkitt material is contributed by tapetal cells. It is layer transferred to pollen surface.
• Exine proteins for rejection reaction are derived from tapetal cells.

Pollenkitt is a sticky, oily layer that forms a thick viscous coating over the pollen grain surface. It is contributed by the tapetum during pollen development and is typically colored yellow or orange due to its composition of carotenoids or flavonoids. Pollenkitt serves multiple important functions in pollen ecology and survival. It acts as an attractant to insects, drawing pollinators to flowers through visual and chemical cues. Additionally, it provides protection to pollen grains from damage caused by ultraviolet radiation, helping preserve pollen viability during transport and storage. The adhesive nature of pollenkitt also facilitates the attachment of pollen grains to the bodies of insect pollinators, ensuring effective pollen transfer between flowers.

Tenuinucellate and crassinucellate ovules differ in the structure and extent of their nucellus tissue. In tenuinucellate ovules, the sporogenous cell is located in the hypodermal layer, just beneath the epidermis. These ovules have a single layer of nucellus tissue surrounding the megasporocyte, resulting in a very small nucellus. The nucellus is reduced and provides minimal nutritive tissue around the developing megaspore. In contrast, crassinucellate ovules have a sporogenous cell that originates from a sub-hypodermal position, located deeper within the tissue layers. These ovules possess a large and well-developed nucellus with many layers of cells, providing abundant nutritive tissue. The crassinucellate condition represents a more robust nucellus that can supply greater nutrition to the developing megaspore and female gametophyte. These structural differences have implications for the development of the female gametophyte and the nutrition available during megasporogenesis and megagametogenesis. Tenuinucellate ovules are found in families like Poaceae, while crassinucellate ovules are common in families like Ranunculaceae and Rosaceae.

Pollination in gymnosperms differs fundamentally from that in angiosperms in terms of the position of ovules and the mechanism of pollen deposition. In gymnosperms, the ovules are exposed and unenclosed, and pollen grains are deposited directly on the ovule surface. This is referred to as direct pollination. In contrast, angiosperms have enclosed ovules contained within the ovary, and pollination is an indirect process wherein pollen grains are first deposited on the stigma of the pistil or carpel. From the stigma, the pollen tube must grow through the style to reach the ovule for fertilization. This fundamental difference reflects the evolutionary advancement of angiosperms, where the carpel provides protection to the ovules and allows for more selective pollination mechanisms.

Heterostyly is a mechanism to promote cross-pollination.
* Different forms of flowers with different lengths of stamen and style.
* Pollination takes place between organs of same length.
a) Distyly. (Ex. Primula)
* Thrum-eyed flowers have short styles. Anthers of pin have short stamen.
* Anthers of thrum-eyed flowers and stigma of the pin are of the same height (both are long). This helps in effective pollination.
b) Tristyly (Ex. Lythrum)
3 kinds of flowers are there, with respect to the length of style and stamens. Flower of one type can’t pollinate their own type. They pollinate the other 2 types.

The characteristic features of entomophilous flowers are as follows:
* Flowers are generally large or if small they are aggregated in dense inflorescence. Example: Asteraceae flowers.
* Flowers are brightly coloured. The adjacent parts of the flowers may also be brightly coloured to attract insects. For example in Poinsettia and Bougainvillea, the bracts become coloured.
* Flowers are scented and produce nectar.
* Flowers in which there is no secretion of nectar, the pollen is either consumed as food or used in building up of its hive by the honeybees. Pollen and nectar are floral rewards for visitors.
* Flowers pollinated by flies and beetles produce foul odour to attract pollinators.
* In some flowers, juicy cells are present which are pierced and the contents are sucked by the insects.

Microsporogenesis is the process of formation of haploid microspores from a diploid microspore mother cell through meiosis. The process involves several distinct steps. Initially, the primary sporogenous cells undergo mitotic divisions to form sporogenous tissue, which consists of many cells capable of functioning as microspore mother cells. Each microspore mother cell (also called microsporocyte) is diploid and undergoes meiosis to produce a tetrad of four haploid microspores. During meiosis I, the first meiotic division separates homologous chromosomes, and during meiosis II, sister chromatids separate, resulting in four genetically distinct haploid cells. Following meiosis, the microspores initially remain held together in a tetrad arrangement by a callose wall, but they eventually separate and remain free in the anther locule. The separated microspores then develop into pollen grains through a process called microgametogenesis. In some plants, particularly orchids, the microspores remain held together by a filament-like structure called a pollinium. The pollinium is attached to a clip-like structure called the corpusculum through a Y-shaped filament known as the retinaculum or translator. This specialized structure facilitates pollination by insects, as the entire pollinium is transferred as a unit to the stigma of another flower, ensuring efficient pollen transfer and cross-pollination.

• Ovule of ovule (Megasporangium)
• It has a stalk and a body.
• stalk (funiculus) is at the base of ovule. It attaches ovule to the placenta.
• Hilum is the junction (point of attachment) between ovule and funicle.
• In an inverted ovule, the funicle is fused to the body of ovule. Thus a ridge called raphe is formed.
• Body of ovule has central mass of reserve food called nucellus.
• Nucellus is covered by 2 layers, called integuments.
• Integument covers the nucellus completely except at the top. This forms a pore called micropyle.
• Ovule with single integument is called unitegmic.
• At the base of body, nucellus, integument and funicle meet. This is called chalaza.
• Sac like structure in nucellus towards micropylar end is called embryosac (or) female gametophyte. It is formed from functional megaspore of nucellus.
• The nutritive inner intergument layer is called integumentary tapetum or endothelium.
• Tenuinucellate type ovule has hypodermal sporogenous cell. It has single layer of nucellar tissue.
• Crassinucellate type, ovule has subhypodermal sporogenous cell.
• Group of cells between chalaza and embryosac is called hypostase.
• Thick walled cells above micropyle are called epistase.

Steps in the fertilization of Angiosperms
1. Germination of pollen to form pollen tube in the stigma.
* Pollens fall on receptive stigma.
* Compatible pollen germinates to form a tube.
* This is helped by stigmatic fluid in wet stigma and pellicle in dry stigma.
* Compatibility is decided by recognition, rejection protein reaction, between pollen and stigma surface.
* Pollen undergoes hydration. Pollen wall proteins cire released.
* The entire content moves into pollen tube.
* Growth is at the cytoplasmic contents at the tip.
* The remaining part of pollen tube is occupied by a vacuole.
* It is cut off from tip by callose plug.
* The hemispherical, transparent pollen tip of pollen tube is called ‘cap block.
* The “cape block” disappears and the growth of the pollen tube stops.
2. Growth of pollen tube in the style.
* Hollow style glandular canal cells secrete mucilaginous substance. These secretions are nutrition for growing pollen tube. They control compatibility of style and pollen tube.
* In solid style the pollen tube grows through the intercellular space of transmitting tissue. Semisolid style is intermediate between solid and open type.
3. Entry of the pollen tube into the ovule.
* Propgamy – Pollen tube enters through the micropyle.
* Chalazogamy – Pollen tube enters through chalaza.
* Monogamy – Pollen tube enters through integument.
4. Entry of pollen tube into the embryo sac.
* Pollen tube enters embryosac at the micropylar end. It is guided by an obturator.
* Pollen tube enters into one of the synergids.
5. Double fertilization and Triple fusion.
* In Angiosperms, both the male gametes are involved in fertilization, it is called double fertilisation.
* One of the male gametes fuses with the egg nucleus (syngamy). Thus zygote is formed.
* The second gamete migrates to central cell. It fuses with polar nuclei (or) secondary nucleus. Thus primary Endosperm nucleus is formed. This involves the fusion of 3 nuclei so it is called Triple fusion.

Dicot and monocot seeds differ significantly in their structural organization and composition. Dicot seeds typically contain two cotyledons, which are the seed leaves that store or absorb nutrients. The seed coat of dicots consists of an outer layer called the testa and an inner layer called the tegmen. In some dicots like pea, the cotyledons themselves store reserve food materials such as starch and proteins, while in others like castor, the endosperm surrounding the embryo is the primary storage tissue. Dicot seeds lack specialized protective sheaths around the embryonic structures; they do not possess a coleoptile (sheath of the plumule) or a coleorhiza (sheath of the radicle). In contrast, monocot seeds contain only a single cotyledon, which is called the scutellum. Monocot seeds like paddy are typically one-seeded, and the seed is enclosed by a husk, which is the outer protective covering. The seed coat in monocots is a thin, brown, membranous layer that closely adheres to the grain. The scutellum is a specialized cotyledon that absorbs nutrients from the endosperm and supplies them to the embryo through an epithelial layer. Monocot seeds characteristically possess a coleoptile, which is a protective sheath surrounding the plumule, and a coleorhiza, which is a protective sheath surrounding the radicle. These structures protect the delicate embryonic organs during germination and emergence from the soil. These structural differences reflect the different evolutionary strategies and growth patterns of dicots and monocots.

In some plants, 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. Nitsch in 1963 classified the parthenocarpy into the following types:
* Genetic Parthenocarpy: Parthenocarpy arises due to hybridization or mutation.
Examples: Citrus, Cucurbita.
* Environmental Parthenocarpy: Environmental conditions like frost, fog, low temperature, high temperature etc., induce Parthenocarpy. For example, low temperature for 3-19 hours induces parthenocarpy in Pear. Chemically
* induced Parthenocarpy: Application of growth-promoting substances like Auxins and Gibberellins induces parthenocarpy.
* Significance: The seedless fruits have great significance in horticulture.
* Seedless fruits have great commercial importance.
* Seedless fruits are useful for the preparation of jams, jellies, sauces, fruit drinks, etc.
* A high proportion of edible parts is available in parthenocarpic fruits due to the absence of seeds.
12th Bio Botany Guide Asexual and Sexual Reproduction in Plants in Animals Additional Important Questions and Answers
I. Choose the correct answer

• It is a third type of tapetum.
• The cell wall is not lost.
• Cells protrude into the anther cavity, by amoeboid movement.
• It is connected to male sterility. It is not periplasmodial type.

Exine and intine are the two distinct layers that compose the pollen wall, each with different structural and chemical properties. The exine is the outer wall layer of the pollen grain. It is relatively thick and has a non-uniform structure with distinctive patterns and sculpturing on its surface. The exine is composed primarily of sporopollenin, a highly resistant biopolymer, along with cellulose and pollenkitt, which is a sticky substance that aids in pollen adhesion and dispersal. The intine is the inner wall layer of the pollen grain, located between the exine and the cytoplasm. It is much thinner than the exine and has a uniform structure without surface ornamentation. The intine is composed of pectin, hemicellulose, and cellulose, which are more readily degradable polysaccharides compared to sporopollenin. The intine is flexible and allows for the growth of the pollen tube during pollination and fertilization. The exine provides protection and species-specific identification features, while the intine provides flexibility and facilitates pollen tube growth.

Pollen grains can be preserved for extended periods through cryopreservation, a technique that involves storing pollen in liquid nitrogen at a temperature of minus 196 degrees Celsius. At this extremely low temperature, all metabolic activities within the pollen grains are halted, maintaining their viability in a dormant state for prolonged durations, sometimes for many years or even decades. This method is particularly valuable for preserving pollen from economically important plants such as crop varieties, fruit trees, and ornamental species. Pollen banks have been established in various research institutions and agricultural centers worldwide to maintain genetic diversity and preserve valuable germplasm. These pollen banks serve as repositories for breeding programs, crop improvement initiatives, and conservation of plant genetic resources. The cryopreserved pollen can be revived and used for controlled pollination, hybridization studies, and maintenance of plant varieties without the need for continuous cultivation.

The archesporium is a specialized cell or group of cells found in the ovule that plays a crucial role in megasporogenesis. In the ovule, a single hypodermal cell located in the nucellus becomes enlarged and differentiated to function as the archesporium. This cell is positioned just beneath the epidermis of the nucellus. In many plant species, the archesporium directly functions as the megaspore mother cell, undergoing meiosis to produce four megaspores. In other plants, the archesporium may first undergo mitotic divisions to produce several cells, one of which then functions as the megaspore mother cell. The archesporium is thus the precursor cell that initiates the process of megasporogenesis and ultimately leads to the formation of the female gametophyte or embryo sac. Its identification and behavior are important in understanding ovule development and reproductive biology in angiosperms.

Pollination in gymnosperms is direct, meaning pollen grains are directly deposited on the exposed ovules without the intervention of any stigma or style. The pollen lands directly on the nucellus or integument of the ovule. In contrast, pollination in angiosperms is indirect, where pollen grains are first deposited on the stigma of the pistil, which is the receptive part of the female reproductive organ. After landing on the stigma, the pollen grain germinates and develops a pollen tube that grows through the style to reach the ovule for fertilization. This fundamental difference reflects the structural complexity of angiosperm flowers compared to the simpler reproductive structures of gymnosperms. The presence of a specialized stigma in angiosperms allows for greater specificity in pollination and provides mechanisms for pollen recognition and compatibility.

• During the developement, the two cells of the basal cell undergoes several transverse division into form a six to ten called suspensor.
• The suspensor helps to push the embryo deep into the endosperm.

• The seeds of paddy is one seeded and is called Caryopsis.
• The embryo is small and consists of one shield shaped cotyledon known as scutellum.
• It is present towards lateral side of embryonal axis.

• The activation of micellar tissue or an\ other cells (sporopln lie ceils of the ovule) can produce more’ Ilian one embryo, known as poly embryonv.
• The seedlings formed from the nucellar tissues in citrus are found belter clones lor orchards.
• They are Disease resistant (virus free) and are preferred by Agriculturists than the normal seedlings.

Pin-eyed flowers and thrum-eyed flowers represent two distinct floral forms found in heterostylous plants such as Primula, demonstrating a reproductive strategy that promotes cross-pollination. Pin-eyed flowers possess a long style with the stigma positioned at the mouth of the flower, while the stamens are short and located lower within the flower. The stigmatic papillae in pin-eyed flowers are long, and the pollen grains produced are relatively small. In contrast, thrum-eyed flowers have a short style with the stigma positioned lower in the flower, while the stamens are long and positioned at the mouth of the flower. The stigmatic papillae in thrum-eyed flowers are small, and the pollen grains are relatively large. This reciprocal arrangement of reproductive organs ensures that pollen from thrum-eyed flowers is effectively deposited on the stigma of pin-eyed flowers and vice versa, promoting cross-pollination and genetic diversity.

Coleoptile and coleorhiza are protective sheaths found in monocot seedlings that cover and protect the delicate embryonic tissues during germination and early growth. The coleoptile is a tubular, protective sheath that surrounds the plumule, which is the embryonic shoot consisting of the epicotyl and the first few leaves. This structure protects the young leaves and growing point as they push through the soil during seed germination. The coleorhiza, on the other hand, is a protective sheath that covers the radicle, which is the embryonic root including the root cap. The coleorhiza protects the root tip and root cap as the radicle emerges from the seed and penetrates into the soil. Both structures are temporary and eventually rupture as the seedling develops, allowing the shoot and root to emerge and establish themselves in their respective environments.

• It is common among dicots. It is character¬ized by the presence of central core of elorgated highly specialised cells called transmitting tissue.
• This is equivalent to the lining cells of hollow style and does the same function.
• Its contents are also similar to the content of those cells. The pollen tube grows through inter-cellular spaces of the transmitting tissue.

Endospermous seeds, also called albuminous seeds, are seeds that contain endosperm tissue at the time of seed maturity. The endosperm serves as a nutrient-rich tissue that provides nourishment to the developing embryo during germination and early seedling growth. Examples of endospermous seeds include paddy, coconut, and castor. Non-endospermous seeds, also called ex-albuminous seeds, are seeds that lack endosperm at maturity because the endosperm is completely consumed by the developing embryo during seed development. In these seeds, the cotyledons become thick and fleshy, storing all the necessary nutrients for the embryo. Examples of non-endospermous seeds include pea, groundnut, and bean. The key difference lies in whether nutrient storage tissue persists in the mature seed or has been entirely utilized by the embryo during development.

• These are layers of specialized cells around the endosperm, in cereals.
• They have sphaerosomes (Ex. Barley, Maize) During germination, they secrete hydrolytic enzymes amylase,
• protease. They digest reserve food of endosperm.

• Eventhough Grafting is considered as artifica! method of vegetative reproduction, it is realtv used to produce plants combining favourable stem characteristics with root characteristics.
• The stem of the plant to be grafted is known as scion and the root is called stock.
• Here, one fnbrid is produced unlike in other method where manv number of plants are produced.

• In Bee orchid (ophyrus) the morphology of flower is similar to female wasp (colpa).
• Male wasp mistakes the flower for female wasp, and try to copulate. This pseudocopulation helps in pollination.
• In pea cotyledons store food.

• Fertilisation forms fruits and seeds.
• Pollination brings male and female gametes closer.
• Cross pollination produces variations, due to mixing of genes. Variations help the adaptation of plants to environment. It helps in specifiation.

Coconut endosperm plays a crucial nutritional and developmental role in plant tissue culture and embryo development. Coconut milk, which is the liquid endosperm, serves as a rich nutrient medium containing essential minerals, vitamins, amino acids, and growth-promoting substances. When used in tissue culture, coconut milk induces differentiation of embryos and promotes the development of embryoids and plantlets from various plant tissues, making it invaluable in micropropagation techniques. The coconut water represents the free nuclear stage of endosperm development, where nuclei are present without cell walls, while the white kernel portion represents the cellular endosperm where cell walls have formed around the nuclei. This dual nature of coconut endosperm, combined with its rich nutritional composition and natural growth-promoting factors, makes it an excellent medium for supporting plant tissue culture and regeneration of complete plants from explants.

• Seeds are attached to fruits by funiculus.
• The scar of funiculus is called hilum.
• Micropyle is the small pore below hilum.
• O2 and water enters seed for germination through micropyle.
• Each seed has outer thick seed coat.seed coat develops from the integuments of ovule.
• Testa is the hard outer coat.
• Tegmen is the thin membranous inner coat.
• In pea testa, tegmen are fused.
• Two cotyledons are laterally attached to embryonic axis.
• In castor endosperm has reserve food.
• One end of embryonal axis projecting beyond the cotyledons. It is called radicle (embryonic root)
• The other end of embryonal axis called plumule (embryonic shoot)
• Embryonic axis above the cotyledons is epicotyl.
• Cylindrical region between the cotyledons is hypocotyl.
• Epicotyle terminates in plumule. Hypocotyl ends in radicle

• Paddy is a one seeded caryopsis.
• The seed is enclosed by brown husk with 2 rows of glumes.
• The brown, membranous seed coat is attached to grain.
• Endosperm is the bulk of grain. It is the storage tissue.
• It is separated from embryo by epithelium.
• Embryo has one cotyledon called scutellum. It is later to embryonal axis.
• A short axis with plumule and radicle is protected by root ‘ cap.
• Coleoptile is a protective sheath of plumule.
• Coleorhiza is the protective sheath of radicle.
• Scutellum supplies food to embryo from endosperm through epithelium.

it is tile plant reproduction which does not involuo the union of male and female gametes.
A) Recurrent Apomixis.
Vegetative Reproduction and agamospernw.
B) Non recurrent Apomixis.
Alter meiosis, haploid embrvosoe is lormed.
It devleps into emhrvo without fertilization.
I) Vegetative Reproduction
Eiopagation of plants by parts other than seeds.
Ex. bulbil – I’riiiliaria imperialis.
Bulbs – Allium
Sucker – Chrysanthemum.
a) Agamospory
Embryos are rormed without syngamy and meiosis.
b) Advcntiver Embryony
Embrvo arises from diploid sporopln tic coll of nuceilus or integument, lt is called sporophytic budding, Gametophylic phase is completeh absent.
c) Diplospory (Generative apospory)
Megaspore mother ceils gives rise to diploid embrvosac without meiosis. ex. Eupatorium.
d.) Apospory
Nucellar cell develop into diploid emhryo sac. This is somatic apospory. Ex. Hieracium, parthenium.

Heterostyly and herkogamy are two distinct mechanisms that promote cross-pollination and prevent self-pollination in flowering plants, but they operate through different structural arrangements. Heterostyly involves the production of two or three different floral forms within a species, distinguished by differences in the relative lengths of stamens and styles. In distylous flowers, for example, some flowers have long styles with short stamens while others have short styles with long stamens. Pollination is effective only when pollen from stamens of one length contacts stigmas of the corresponding length, a phenomenon called legitimate pollination. Primula is a classic example of a distylous plant. Herkogamy, in contrast, is a mechanism where the stamens and stigmas are physically separated or arranged in such a way that self-pollination is mechanically prevented, regardless of flower form. In herkogamous flowers like Hibiscus, the stigmas project well above the stamens, making it physically impossible for pollen from the same flower to reach the stigma. Both mechanisms effectively promote cross-pollination but heterostyly involves floral polymorphism while herkogamy involves spatial separation of reproductive organs.

The genetic ability of a plant cell to produce entire plant in suitable condition is called Totipotency.
i) Tissue Culture
Growth of plant tissue in special cutture medium under suitable conditions is called tissue culture.
Ex. F.C steward of Cornell University developed a new carrot plant from the phloem parenchyma cell.
ii) Micropropagation
Regenerationof whole plant from a cell or tissue of vegetative structures.
* Advantages of Modern methods.
* Plants with desired characteristics are multiplied rapidly in short duration.
* Genetically identical plants are produced.
* Done at any season.
* Plants without viable seeds, difficult to germinate can be propagated.
* Rare, endangered plants are propagated.
* Meristem culture produces disease free plants.
* Cells are transformed by genetic modification.
Disadvantages of modem methods
* Labour intensive. It requires skilled workers.
* Maintenance of sterile condition increases cost.
* Genetically identical clones are susceptible to new diseases.
* Genetical changes in callus is not desirable for commercial use.

1. Anther Wall
a) Epidermis
* Protective single layer.
* Cells undergo anticlinal division to cope up enlarging internal tissue.
b) Endothecium
* Single layer of radially elongated cells.
* Bands of cellulose (or) lignin are seen in tangetial wall.
* At the junction of 2 sporangia these thickenings are absent. This region is called stomium.
* Hygroscopic nature of endothecium helps in dehiscence of anther.
c) Middle layer
* 2 to 3 layers next to endothecium.
* These are ephemeral. Disintegrate or crushed during maturity.
d) Tapetum
* It is dual in origin (from peripheral wall layer and connective tissue of anther lining.
* It nourishes sporogenous tissue, microspore mother cell, microspores.
* Cells are uninucleate, multinucleate with polyploid nucleus.
* It contributes to wall material, sporopollenin, pollen kitt, tryphine.
* It controls fertility or sterility of pollengrains. It is of 2 tvpes i) Secretory tapetum ii) Invasive tapetum
2. Anther cavity.
* It is filled with young microspores or mature pollengrains.
* Microspore mother cells form microspore by meiosis.
3) Connective.
* It is a colume of sterile tissue. It is surrounded by anther lobe. It has vascular tissue.

• Haploid microspore is the first cell.
• Development takes place at microsporangium.
• Microspore nucleus divides into vegetative and generative nucleus.
• Large vegetative cell and small generative cell is formed.
• At this 2 celled stage, pollens are liberated from anther.
• In some plants generative cell form 2 male gametes.
• Male gametophyte grows when the pollen reaches the right stigma.
• Pollen absorbs moisture and swells.
• Intine grows as pollen tube through germ pore.
• At the 2 celled stage, generative cells divides into 2 male cells at stigma.

Ovules exhibit remarkable diversity in their orientation, form, and position of the micropyle relative to the funicle and chalaza, resulting in six major types. Orthotropous ovules are straight with the micropyle at the distal end and the funicle, body, and chalaza arranged in a single vertical line, as seen in Piperaceae. Anatropous ovules are completely inverted, with the body of the ovule bent so that the micropyle and funiculus lie close to each other, and this type is most common in dicots and monocots. Hemianatropous ovules have the body positioned transversely at a right angle to the funicle, as found in Primulaceae. Campylotropous ovules have the body curved at the micropylar end with the embryo sac also curved, and the hilum, micropyle, and chalaza positioned relatively close together, characteristic of Leguminosae. Amphitropous ovules have a reduced distance between the hilum and chalaza with the nucellus forming a characteristic horse-shoe shape, found in Alismataceae. Circinotropous ovules, seen in Cactaceae, have a long funicle that surrounds the ovule body. These variations in ovule orientation reflect adaptations to different reproductive strategies and developmental patterns in various plant families.

• Functional megaspore is the first cell of embryosac or female gametophyte.
• Megaspore elongates along micropylar – chalaza! axis.
• Nucleus undergoes mitosis without wall formation.
• A central vacuole expands and pushed the nuclei towards the opposite poles.
• Each nucleus divide mitotically twice. Thus 4 nuclei are formed at each pole.
• Eight nuclei are in common cytoplasm.
• Of the 4 nuclei at micropylar end, 3 nuclei form 3 antipodal cells. Fourth one is the lower polar nucleus.
• Two polar nuclei fuse into secondary nucleus.
• Thus 7 celled, 8 nucleated embrovsac is formed.

1) Dicliny or Unisexuality. ’
In unisexual flowers, only cross pollination is possible,
i) Monoecious (Ex. coconut)
* Male and female flowers on same plant
* Autogany is prevent in castor, maize. Geitonogamy takes place.
Dioecious.
* Male and female flowers are on different plants.
* Both autogamy, geitonogamy are prevented.
2) Monocliny or Bisexuality.
i) Dichogamy.
Anther and stigma mature at different times.
* Protandry (Ex. Helianthus)
Stamens mature earlier than stigma
* Protogymy (Ex. Aristolochia)
Stigmas mature earlier than stamen.
ii) Herkogamy.
Arrangement of stamen, stigma are different. Thus self pollination is prevented.
Ex. Hibiscus – Stigma project above stamen.
iii) Heterostyly,
Flowers differ in the length of stamen and style.
Pollination takes place between organs of the same length.
a) Distyly.
Pin flowers have long style. Thrum eyed flowers have long stamens. This same height helps in pollination.
Pin flowers have short stamens. Thrum eyed flowers have short style.
This helps in pollination.
b) Tristyly (Ex. Lythrum)
Plant produces 3 kinds of flowers with respect to length of style and stamens.
iv) Self sterility / Self incompatibility. Pollengrain of one flower is unable to germinate in the stigma of the same.
Ex. Passiflora

• Flowers in pendulous, catkin like, spike inflorescence.
• Inflorescence axis elongates. So, flowers are brought above leaf level.
• Reduced perianth (or) Absent.
• Small, colourless flowers do not / secrete nectar. The are not scented
• Long, exerted, versatile filaments.
• Enormous quantitv of pollen grains.
• Minute, light, dry pollen easily cart ied by wind to long distances.
• Violent bursting of anthers release the pollengrains. Ex. Urlica.
• Protruding, feathery, branched stigma catch pollengrains.
• Flowers are produced before leaves. So, they are carried without hindrance.

• Maize is monoecious and unisexual.
• Male inflorescence is at the terminal.
• Female inflorescence is at the lateral lower level.
• Heavy pollens cannot be carried by breeze.
• Male inflorescence is shaken by wind. The released pollens fall vertically below
• Male inflorescence (Tassel) Female inflorescence (Cob)
• The long stigma (23 cm) projects beyond the leaves.
• Pollens dropping from tassel is caught by the stigma.

• Salvia is adapted for bee pollination.
• Bilabiate corolla has 2 stamens.
• Each anther has upper fertile lobe and lower sterile lobe separated by long connective. The anthers swing freely.
• The bee strikes against the sterile end of connective. So, fertile part of stamen descend. It strikes at the back of the bee.
• When the bee visits another flower, the pollen is rubbed on stigma. Thus pollination is

• The embryo develops at micropylar end of embryo sac.
• The zygote undergoes transverse division.
• An upper terminal cell and lower basal cell is formed.
• Divisions in zygote during development lead to the formation of embryo.
• Before mature stage, embryo undergoes globular, heart shaped stages.
• Mature embryo has a radicle, 2 cotyledons and a plumule.

Epigeal hydrophily is a specialized form of pollination that occurs at the water surface level, exemplified by the aquatic plant Vallisneria. Vallisneria is a submerged rooted hydrophyte that has evolved a remarkable mechanism for pollination despite living underwater. At the time of pollination, the female flowers are brought to the water surface by elongated coiled stalks that extend from the submerged plant. The female flower develops a cup-shaped depression on its surface that serves as a landing platform. Meanwhile, the male flowers detach from the plant and float freely on the water surface. As the male flowers drift on the water, they eventually settle into the cup-shaped depression of the female flower, where they come into contact with the stigma and effect pollination. After successful pollination, the coiled stalk of the female flower contracts and spirals, pulling the developing fruit back beneath the water surface where seed development continues. This elegant mechanism represents an adaptation that allows submerged aquatic plants to utilize wind and water currents for pollen dispersal while maintaining their aquatic habitat.

Conventional methods of vegetative propagation offer several significant advantages that make them valuable in agriculture and horticulture. Plants produced through vegetative propagation are genetically uniform and identical to the parent plant, ensuring consistency of desirable traits. New plants are produced quickly, allowing rapid multiplication compared to seed propagation. Vegetative propagation is particularly useful for plants that produce little or no seeds or whose seeds do not germinate readily. It is economical for commercial production, as exemplified by potato (Solanum tuberosum) cultivation. Plants with desirable characteristics such as disease resistance, high yield, or superior quality can be propagated and maintained through grafting and other vegetative methods. However, conventional vegetative propagation has notable disadvantages. Virus-infected plants will produce virus-infected new plants since the virus is transmitted through the vegetative propagules, perpetuating the infection. Bulky vegetative structures such as tubers, rhizomes, and corms are difficult and expensive to handle, transport, and store compared to seeds. Additionally, vegetative propagation does not allow for genetic recombination, limiting the development of new varieties through natural breeding processes.

Biosporic and tetrasporic megaspore development represent two different patterns in the formation of the female gametophyte or embryo sac in flowering plants. In biosporic megaspore development, only two of the four megaspores produced by meiosis participate in embryo sac formation, while the other two degenerate. The two functional megaspores undergo mitotic divisions to form the embryo sac structure. Allium is a classic example of a plant exhibiting biosporic megaspore development. In tetrasporic megaspore development, all four megaspores produced by meiosis are involved in the formation of the embryo sac. All four megaspores undergo mitotic divisions and their nuclei contribute to the formation of the mature embryo sac. Peperomia is a characteristic example of tetrasporic megaspore development. The fundamental difference between these two patterns lies in the number of meiotic products that participate in gametophyte formation, which results in different embryo sac structures and ploidy levels of the cells within the mature embryo sac.