Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants

Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants Solutions to each chapter is provided in the list so that you can easily browse through different chapters Assam Board HS 2nd Year Biology Chapter 2 Sexual Reproduction in Flowering Plants Question Answer.

Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants

Join Telegram channel

Follow us:

Also, you can read the AHSEC book online in these sections Solutions by Expert Teachers as per AHSEC (CBSE) Book guidelines. These solutions are part of AHSEC All Subject Solutions. Here we have given Assam Board Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants Solutions for All Subjects, You can practice these here.

Sexual Reproduction in Flowering Plants

Chapter – 2

REPRODUCTIONS

Very Short Answer Type Questions

Q.1. Define reproduction? 

Ans : Reproduction is a process by which an organism gives birth to individuals of its own kind and thereby continues to maintains its race. Through reproduction an organism handed down its genetic identity to their offsprings. 

Q.2. What is incompatibility? 

Ans : Inability to reproduce. 

Q.3. What is malacophily? 

Ans : Pollination caused by snail is called malacophily. 

Q.4. What types of flowers favours cross pollination? 

Ans : Bisexual flowers. 

Q.5. What types of pollination is found in a self incompatible plants? 

Ans : Cross-pollination. 

Q.6. Why sunflower is best suited for insect pollination? 

Ans : Sunflower is advise shaped inflorescence containing a large number of flowers together. A single insect may pollinate the entire flowers by roaming all around in search of honey and nectar. 

Q.7. What do you understand by double fertilization? Who discovered it? 

Ans : Out of the two male gametes developed design microgametogenesis,one fertilizes the egg and the other fertilizes the secondary nucleus. The fertilized egg gives rise to embryo and the fertilized secondary nucleus gives rise to endosperm. As fertilization occurs twice it is called double fertilization. 

Q.8. What is sporopollenin? 

Ans : It is a special substance present in the test of pollen grain which protects the pollen grain from external influence such as heat acid enzyme action etc. 

Q.9. What is the first cell to develop is female and malegametophyte of flowering plants? 

Ans : Functional megaspore and pollen grain.

Q.10. Name two plants in which pollination occurs with the help of 

(i) Birds 

(ii) Bats. 

Ans : (i) Coral tree (ii) Silk cotton.

Q.11. What is triple fusion? 

Ans : Secondary nucleus contains two nuclei one from syner gids and the other from antipodal cells. Thus it is already diploid (2n). So when the second male gamete fuses with it during fertilization it become triploid nucleus (3n). This is called triple fusion. 

Q.12. What do you meant by Parthenogenesis and Parthenocarpy? 

Ans : When fruits develop without fertilization it is called parthenogenesis. It leads to production of seedless fruit. Production of seedless fruits is called parthenocarpy. 

Q.13. Arránge the following terms in correct developmental sequence – Pollen grain, Sporogenous tissue, Microspore tetrad, Pollen mother cell, Male gametes. 

Ans : Sporogenous tissue-pollen mother cell-Micro spore tetrad-pollen grain- Male gamete. 

Q.14. How does endosperm in angiosperms become triploid? 

Ans : Secondary nucleus in the ovule is a diploid one (2n). When during fertilization the second male gametes fuses with the diploid secondary nucleus the secondary nucleus transforms into triploid (3n) nucleus. This triploid nucleus is called endosperm nucleus. The endosperm nucleus gives rise to embryo. 

Q.15. Mention the scientific term used for modified form of reproduction in which seed are formed without fusion of gametes. 

Ans : Apomixis. 

Q.16. A bilobed dithecous anther has 100 microspore mother cells per microsporangium. How many male gametophytes this anther can produce? 

Ans : 400 male gametophytes and from each male gametophyte two male gametes are produced. So a total of 800 male gametes will be produced. 

Q.17. Why should a beekeeper keep beehives in crop fields during flowing periods? 

Ans : Two reasons one the bees get pollen and nectar to make honey; second, crop field gets enough insects to pollinate the flowers. 

Q.18. If the chromosome number of somatic cell of a plant is 24. What would be the chromosome number of male gamete and endosperm cells of the plant. 

Ans : 12 and 36 chromosomes respectively. 

Q.19. What are the constituents cells of the egg apparatus in an embryo sac? 

Ans : Two synergids and one egg cells. 

Q.20. What are the three cells found in a pollen grain when it is ready to shed? 

Ans : One vegetative cell or to be cell and one generative cell which gives rise to two male gametes. 

Q.21. Name the parts of the pistil which develop into fruits and seeds? 

Ans : Ovary and ovule .

Q.22. Name the cultivated plant in which neither fruits nor seeds are formed? 

Ans : Potato, ginger, turmeric, sugarcane. 

Q.23. Named the activities : 

(i) Removal of anthers from the bisexual flower bud to make it a female flower. 

Ans : Emasculation.

(ii) Sprinkling of desired pollen grains over the stigma of a flower. 

Ans : Dusting of pollen grain over stigma. 

Q.24. What are archesporial and tapetum layers? 

Ans : Archesporial layer gives rise to sporogenous cells and tapetum provides nourishment to sporogenous cells. 

Q.25. Differentiate true fruits from false fruits. 

Ans : The fruits which develop from ovary are called true fruits and the fruits which develop from any part of flower other than ovary are called false fruits. 

Q.26. How many haploid cells are present in a mature female gametophyte of a flowering plant? 

Ans : Female gametophyte has eight cells which are haploid. Two synergids, an egg, two polar nuclei and three antipodal cells. 

Q.27. Mention the ways by which seeds are develop without fertilization? 

Ans : The way by which seeds are developed without fertilization is called parthenocarpy. 

Q.28. How do flowers reward their insect pollinators? 

Ans : The insect pollinating flowers reward their pollinators by providing water or honey which are formed at the base of flower. 

(B). Fill up the blanks: 

Q.1. Morphologically flower is a modified_____

Ans : Stem. 

Q.2. Transfer of pollen grains from anther to the stigma of another flower of the same plant is called_____

Ans : Geitonogamy. 

Q.3. The wind pollinated flowers are generally _____ Sexual in nature. 

Ans : Unisexual. 

Q.4. Entry of pollen tube into ovule through micropyle is known as ____

Ans : Porogami fertilization.

Q.5. Pollen grains are able to tolerate extremes of temperature and desiccation as its exine is made up of_____

Ans : Sporopollenin.

Q.6. Anthesis is a condition in which _____ takes place. 

Ans : Flower. 

Q.7. The ovule is attached to the placenta by _____

Ans Funicle. 

Q.8. _____ is the first cell of Embryo sac. 

Ans : Functional megaspore. 

Q.9. Hilum is the point at which ____ of ____ remain attached.

Ans : Stem, seed. 

Q.10. ____ is the formation of embryo from vegetative cells derived from Zygote. 

Ans : Polyembryony. 

Q.11. Parthenocarpy is the development ____  of is an unfertilized flower, resulting in a _____ fruit. 

Ans : Fruit, seedless fruit.

Q.12. Apomixis is unusual sexual reproduction where there is no ____ and _____ 

Ans : Gametes, fertilization. 

Q.13. Egg apparatus of empryosac consists of ____ cells.

Ans : Three cells. 

Q.14.  Definite or secondary nucleus of embryo sac is the product of ____ and _____ cells. 

Ans : A antipodal cell, a synergid cell. 

Q.15. In many dicots, food reserve in the ____ and consumed by the developing ____

Ans : Endosperm, embryo. 

Q.16. ____ of pistil forms fruits and ____ in ovary form seeds. 

Ans : Ovary/ base, Ovules/eggs.

Q.17. Milky water of tender coconut fruit is ____ 

Ans : endocarp. 

(C). Select the true and false statement : 

Q.1. In angiosperms, a mature male gametophyte is formed from a pollen mother cell through one meiotic and two mitotic divisions. 

Ans : True.

Q.2. Tapetum is the innermost layer of pollen sac which help in protection of sporogenous cells. 

Ans : False.

Q.3. Endosperms developed from secondary nucleus which is present in the middle of embryosac is diploid in chromosome set.

Ans : False.

Q.4. Apomixis is an event where a new plant is develop without the fusion of gametes.

Ans : True. 

Q.5. Xenogamy, the type of pollination brings genetically different types of pollen grains to the stigma. 

Ans : True.

Q.6. Fruits are formed in all phanerogams. 

Ans : False.

Q.7. Commonly in a mature fertilized ovule n, 2n and 3n, condition is respectively found in synergids, zygote and endosperm cells. 

Ans : True.

Q.8. A bisexual flower which never opens up in its life span is known as hemogamy. 

Ans : False. 

Q.9. Wind pollinated flowers are small producing large numbers of pollen grains. 

Ans : True.

Q.10. The egg apparatus of an angiosperm embryosac comprises of an egg cell and two antipodal cells. 

Ans : False.

Q.11. A monocarpic plant is one which has only one carpel. 

Ans : True.

Q.12. The role of double fertilization in angiosperms is to produce only endosperms. 

Ans : False.

Q.13. If any somatic cell of sporophyte produces gametophyte without reduction division is called parthenogenesis. 

Ans : True.

Q.14. The egg apparatus of angiosperms embryo sae comprises of an egg and two synergids. 

Ans : False.

II. QUESTION FOR 2 MARKS : 

Q.1. Draw a diagram of LS of the angiosperm ovule and label the different nuclei present in it. 

Ans :                  

Q.2. What do you meant by sporogenesis? Differentiate microsporogenesis from megasporogenesis? 

Ans : Process of development of male and female spores in plants from mother cell of anther and ovule through reduction division is called sporogenesis. Development of pollen grains is called microsporogenesis and development of functional megaspores is called megasporogenesis. Microsporogenesis takes place in anther and megas porogenesis takes place in ovule. 

Q.3. What is tripłe fusion? Where it take place? Name the nuclei involved in this event? 

Ans : Fusion of one definitive nucleus (2n) and a male gamete (n) which gives rise to endosperm nucleus (3n) is called triple fusion. This takes place in embryosac. 

Q.4. What is bagging technique? Mention its usefulness in the programme of plant breeding. 

Ans : In hybridization the pollen grains are not allowed to fertilize the stigma of the same flower and to achieve this the anther is removed before it is matured. The stigma is then covered by a bag to prevent pollination by unwanted pollen grains. When stigma becomes matured pollen taken from desired plants are brushed upon the stigma. The bag is again put over the sigma till fertilization is over. 

Q.5. Why apple is called a false fruit? Mention the parts of the flower from which this false fruit is develop.

Ans : Apple is not formed from ovary and therefore it is called false fruit. It is formed from thalamus. 

Q.6. Define emasculation? When and why the plant breeder employ this technique? 

Ans : See answer to question No. 4 above. 

Q.7. ‘Angiosperms perform double fertilization.’ Explain the statement. 

Ans : In angiosperm fertilization occurs twice. First, one male gamete fertilizes the egg cell which gives rise to embryo and second, another male gamete fertilizes the secondary nucleus to give rise to endosperm. As fertilization occurs twice it is called double fertilization. 

Q.8. What is sexuality of plants? Give one example each from both monoecious and dioecious plants. 

Ans : Fusion of male gamete with the egg cell of a plant is called sexuality in plant. 

Example : Monoecious plant-sweet gourd 

                 Dioecious plant – papaya. 

Q.9. What are cells present in a developed embryo sac of angiosperms. Named them and mention the chromosome set found in them. 

Ans : Antipodal cells, synergids, egg cell and secondary nucleus. In antipodal cells, synergids and egg cell the chromosome number is haploid (n) and in secondary nucleus the chromosome number is diploid (2n). 

Q.10. What is pollination? Briefly discuss its need and significance in Angiosperms. 

Ans : The transfer of pollen grains from anther to the stigma is called pollination. 

Pollination is essential for fertilization to occur. Fertilization gives rise to seeds and fruit. In the absence of pollination the ovary withers away and fruit and seeds are not formed. 

Q.11. Give one example from each of the following characters –

(a) Flowers having both androecium and gynoecium.

Ans : Mustard. 

(b) A plant bearing both male and female flowers in the same body. 

Ans : Sweet gourd.

(c) A plant where fruit is not formed from ovary.

Ans : Apple. 

(d) An angiosperm in which neither fruits nor seeds are formed.

Ans : Potato. 

Q.12. Which part of a seed is called the first food for the developing embryo? What is the primary sources of this food? 

Ans : Endosperm. Endosperm is stored in cotyledons. 

Q.13. What are functions of the cells present in a developed pollen grain which is ready to shed? 

Ans : One vegetative cell and a generative cell. In some cases before the pollen grain sheds the generative cell divides to form two male gametes. 

Q.14. Draw the LS of a pistil of angiosperms which is ready to take part in the process of fertilization. Label the different structure present there.

Ans :

Q.15. Distinguish between cutting and Layering. Give one example from each of them practiced in the crop plants. 

Ans : When a stem is cut into a piece and put into soil for regeneration it is called cutting. The cut end develops root when a long slender stem without removing from the plant is bend down to touch soil at some point and over which a lump of soil is placed rooting takes place at the point. This mode of propagation is called layering. 

Example : Cutting : Chinarose, rose, mango.  

                 Layering : Lemon, Ixora. 

Q.16. Banana is a parthenocarpic fruit where as Oranges show polyembryony. How both can be differentiate with respect to seeds? 

Ans : Banana is called parthenocarpic fruit where nor seed is formed and oranges are showing polyembryony which can be defined as the occurrence of two or more embryos in one ovule. Here cells of nucellus and integuments have also been observed to develop into embryos. e.g. citrus etc. 

Q.17. “Incompatibility is a natural barrier in the fusion of gametes”. Justify the statement. 

Ans : Incompatibility is the inability of functional male and female gametes to effect fertilization in particular combination. 

The self-incompatibility may be as follows: 

(a) Heteromorphic structure: Here, some species produces more than one morphological type of flowers. Distyly and Tristyly in primula which is for incompatibility. It is found in sweet potato also. 

(b) Como.morphic structure : Here incompatibility is not connected with morphological differences of flowers. The incompatibility may be due to the genotype of the plants on which it is produced (sporophytic control) or by its genotype (gametophytic control). 

The sporophytic control incompatibility is reported in Radish and Brassica while the gametophytic control incompatibility is recorded in Nicotiana, Trifolium, Solanum etc. Hence we can say that incompatibility is a natural barrier in the fusion of gametes.

Q.18. What are the strategies made by some flowers to prevent self pollination? 

Ans : To prevent the self pollination by the flowers the following strategies are made: 

(a) They never prefer autogamy, but prefer dichogamy. 

(b) They never follow the homogamy but preferring herkogamy.

(c) They never exhibit cleistogamy but produces unisexuality and flower is open. 

(d) Flowers have heterostyly with self sterility. 

III. QUESTIONS FOR 3 MARKS : 

Q.1. What is self incompatibility? Why does self pollination does not lead to seed formation is self incompatible flower. 

Ans : When the pollen grain of a flower cannot fertilization the ovule of the same flower it is called self-compatibility. It is due to sterility towards each other. But the same pollen grain can fertilize the ovule of another flower. This is an adaptation to prevent in breeding. 

Q.2. What do you mean by pollination. Discuss its importance in flowering plants. Describe the role of various agencies responsible for the phenomenon. 

Ans : Pollination means transfer of pollen grains from anther to stigma of same or another flower. Pollination leads to fertilization which is essential for the development of seeds a fruits. Each poller grain gives rise to tvo formation. Both the gametes take part in fertilization that leads to formation of embryo and endosperm of the seed and the ovary in which the embryo and the endosperm are located is transform into fruit. 

Role of Agents in Pollination : 

(a) Insects : Pollination done by insects are called entomophily and the flowers are called entomophilous. They are colorful and with good smell and nectar. 

(ii) Wind : Pollination is brought about by wind in flowers which are small and inconspicuous. They are not coloured or showy. They have not nectar, scent etc. 

(iii) Water : Pollen grains of aquatic plant may be carried by water current and may get in touch with the stigma at the surface of water. The plants are Hydrophilous and the pollination is called hydrophily. 

(iv) Animals : Pollination may be brought about by various animals. When pollination done by bat is called chiropterophily. When the pollination takes place through birds then it is called ornithophily and when by snail called malacophily. 

Even the pollination may take place by squirrel, monkey and man. Here a number of adaptation may be found in the process of pollination. Sometimes the pollen grains may be sticky or with scented etc. 

Q.3. What is vegetative propagation. State the importance of vegetative propagation in horticulture practices, giving examples. 

Ans : When a part of plant body is detached from the main body and regenerates to give rise to an independent plant it is called vegetative propagation.

Farmers use the method of vegetative propagation in many agricultural; practice such as in cultivation of potato, ginger, turmeric, sugarcane, sweet potato, strawberry, dahlia, lemon, litchi, pineapple, mango and many more. 

Q.4. Define triple fusion. What is the product of this process. What does the product develop into? 

Ans : The secondary nucleus contains diploid nucleus (2n). During fertilization one male gamete (n) fertilizes the egg and the other male gamete (n) fertilizes the secondary nucleus (2n). As a result the product of fertilization between secondary nucleus and a male gamete the endosperm nucleus. Becomes triploid (3n) this is called triple fusion. Endosperm nucleus develops into endosperm.

Q.5. Match the items of column ‘A’ with the items of column ‘B’ of the following- 

Ans :

Q.6. What is agamospermy? How is agamospermy different from parthenogenesis and parthenocarpy? 

Ans : Development of embryo without fertilization is called agamospermy. Parthenogenesis means development of ovary and fruit without fertilization. 

Q.7. What is meant by emasculation? When and why does a plant breeder employ this technique? 

Ans : In hybridization the pollen grains are r.not allowed to fertilize the stigma of the same flower and to achieve this the anther is removed before it is matured. The stigma is then covered by a bag to prevent pollination by unwanted pollen grains. When stigma becomes matured pollen taken from desired plants are brushed upon the stigma. The bag is again put over the sigma till fertilization is over. 

Plant breeders introduce desired character into plants by preventing self pollination and also cross pollination by pollen grains of undesirable plants by the technique emasculation. plant breeders artificially pollinate the pistil by pollen grains which are taken from a desired characters. 

Emasculation technique is used to produce desired quality seeds or fruits in short period or time spam. 

Q.8. What is poly empryony and how can it be commercially exploited? 

Ans : A seed carrying more than one embryo is called polyembryony. Polyembryony can be used prevent reversibility of hybrid character. 

Q.9. Write the characteristic features of anther, pollen and stigma of wind pollinated flowers. Discuss briefly how do flowers reward their insect pollinators. 

Ans : Pollen grains are flat, thin basifixed, having long filament. The stigma are feathery. The flower rewards the insect by allowing them to collect honey and nectar. 

Q.10. Draw a longitudinal section of a post pollinated pistil showing entry of pollen tube into a mature embryosac of an angiosperm ovary. Label all the constituent bodies found there. 

Ans :              

Q.11. Discuss the need and significance of pollination in plants. 

Ans : The pollination is essential for the transfer of male gametes to the female sex part i.e. gynoecium a stigma. 

Moreover pollination plays a significant role for the formation of Zygote by the process of fertilization in the ovary. The main significance of pollination is that it can produce the seeds with different genetical makeup.

Q.12. What do you meant by dormancy of seeds? Mention the advantages of seed dormancy in crop plants. 

Ans : Dormancy can be defined as the condition of seeds when it fails to germinate even though conditions necessary for germination are provided. The main advantages of dormancy of seed of crop plant is that we can store the plant seeds for a longer period and it can germinate when we need to grow it. 

Q.13. What is apomixis? How is it different from polyembryony? 

Ans : Apomixis is the formation of new individuals through asexual reproduction without involving the formation and usion of gametes. In polyembryony two or more embryos in one ovule may be formed by the cleavage where fusion of gametes is must or can say that it is a made of sexual reproduction where cleavage is followed. But apomixis is an asexual method of reproduction where it mimics the sexual method. 

IV.QUESTION FOR 5 MARKS (EXPLANATORY) 

Q.1. What is embryo? Discuss the stages of development of a dicot embryo. How you can differentiate a dicot embryo from a monocot embryo? 

Ans : Embryos are baby plants remain in dormant stage within the ovule. It has two axis called radicle and plumule. Radice give rise to future root system and the plumule given rise to future shoot. 

Embryo Formation : The fertilized egg cell cloths itself with a cellulose wall and becomes a zygote or oospore. Now the oospore divides into two cells by a transverse wall. Of the two cells thus formed one is basal cell and the other is terminal cell. The basal cell divides in one direction to form an elongated structure called haustorial suspensor. The suspensor pushes the young embryo deep inside the embryo sac. The end cell of suspensor which is called basal cell of the suspensor remains bulbous and acts as absorbing organ for the growing embryo. The terminal cell which acts as embryonal cell first divides by two consecutive vertical division to form a four-cell structure called quadrant then by a transverse division the number of cells rises to eight. 

This stage is called octant stage. The octant mass divides by a wall parallel to the curved surface forming a posterior or hyponasal octant and an anterior or epibasal octant. The epibasal cells by further differentiation form the plumule and cotyledons, while the parabasal cells form the hypocotyl. Plumule gives rise to shoot hypocotyl gives rise to radicle, the future root. The peripheral cells of the proembryo divide by a wall parallel to its curved surface. This layer of cells divide vertically and periblem (future cortex). The cells of the suspens or which is in contact with the embryo cell is called hypophysis. It forms the root. The embryo is further differentiated into cotyledons. The suspensor later degenerates and due to the curving of the ovule the embryo also becomes curved. 

The process of embryo formation described above is of dicotyledonous seed. The process of embryo formation in monocotyledonous seed is almost similar upto octant, stage. Difference appears only in later stages during the formation of cotyledons. In dicots two cotyledons are formed and in monocots only one cotyledon is formed. While in dicots the opposite cells of the terminal quadrant take part in the formation of two cotyledons, in monocots, 2-4 terminal cells take part in cotyledon formation. The other terminal cells take part in shoot formation. In grass family (monocotyledon) the cotyledon is situated laterally. It called scutellum. In monocoty ledons, the radicle is enclosed by a sheath called coleorhiza, and plumule is enclosed by another sheath called coleoptile.

Q.2. ‘Incompatibility is as natural barrier in the fusion of gametes.” Justify the statement. 

Ans : Incompatibility many occur due to many reasons. Sometime the microgamete and sometime macrogamete may be incompatible to each other or one or may be sterile. Sometime to prevent self pollination flowers adapt some contrivances so that only cross pollination takes place. These contrivanes are actually natural barrier which prevent self pollination. Thus the plant create self incompatibility. The natural barries one of various types such as unisexuality, self-sterility dichogamy, heterostyly, herkogamy etc. Of these unisexuality is the best adaptation of incompatibility. Because of incompatibility, cross-pollination occurs in most bisexual flowers. 

This leads to exchange of genetic quality. Which causes improvement in the genetic quality of plants and leads to further evaluation. 

Q.3. ‘Pollination is an important phenomenon in the life cycle of a flowering plant.’ Discuss the statement and describe briefly about the role of different agencies responsible for the events. 

Ans : Without pollination fertilization cannot takes place. And without fertilization fruits and seeds are not formed. Therefore pollination is essential for not as production of fruits and seeds but also for continuity of species through sexual reproduction pollinations essentially a part of sexual reproduction is plants. 

Pollination is the only method without which sexual reproduction cannot take place. Pollen Grains donate male gametes. The union of male and female gametes restore diploid chromosome number in a species. This is essential for continuity of the species. Pollen-grains reach the stigma by various methods devised suitably by each species. 

Plants have evolved various mechanisms for carrying but pollination. Morphology of the flowers have been adapted according to the nature and morphology of the pollinating agents. Nature favours cross-pollination and therefore flowers adapt themselves in various ways so that various natural agents can help pollination, the natural agents involved in pollination are: 

(i) Insect : Pollination done by insects is called entomophily and the insect pollinated flower is called entomophilous flowers. The insect loving flowers have various adaptation by which they attract insects and use them as conveyors of pollen-grains for pollination for insect pollination flowers must be. 

(a) Conspicuousness : When flowers are small they aggregate into inflorescence to become conspicuous which then can attract insects. 

(b) Coloured : Coloured and large showy flowers have great attraction for butterflies, bees and other types of insects. 

(c) Have Nectar : Nectar has special attraction for bees. Many types of flowers are equipped with nectar glands which secret nectar at the base of the corolla.

(d) Scented : Nocturnal flowers generally emit sweet scent which (p) attracts insects at hight when colour fails to attract them. 

(e) Have Sticky Pollen-grains : The pollen grains of entomophilous flowers are generally either sticky or are provided with spines so that the pollen grains can easily stick to the body of the insects and are then carried by them easily. 

(ii) Wind : Pollination is brought about by wind in flowers which are (1) small and inconspicuous. They are not coloured or showy. Such flowers do not produce any scent or nectar. 

(iii) Water : Pollen grains of aquatic plants may be carried by water current and may get in touch with the stigma at the surface of water. All aquatic pants are not hydrophilous. Hydrophily is but the rule in submerged plants. The submerged plants generally lift there female flower to the surface of water for pollination. 

(iv) Animals : Pollination may be brought about by various animals. When pollination is done by bats it is called chiropterophily. Bombax, Bauhinia etc. plants are pollinated by bats. Pollination in plants like Erythrina, Bombax, Bignonia etc. are brought about by birds, squirrels etc. Pollination by birds is called ornithophily. Pollination in aquatics are caused by snails. Such pollination is generally called malacophily. 

Q.4. Describe the sequence of the events that lead to the development of a 3 celled pollen grain from microspore mother cell in angiosperms. 

Ans : The process of formation of male reproductive unit (microspore or pollen-grain) and female reproductive unit (megaspore) is called sporogenesis. Development of microspores is called microsporogenesis. 

Each microsporophyll develops as a smail outgrowth known as microsporophyll primordium on the receptacle or thalamus. Due to apical growth it elongates and gets differentiated into filament and two anther lobes. 

A transverse section through a young anther shows that it comprises of a homogeneous mass of cells limited by a well defined epidermis. The cells are meristematic in nature. A hypodermal layer having larger cells with dense cytoplasm and prominent nuclei forms below it. This layer is called archesporial layer. In archesporial layer four cells become very much prominent-one in each developing lobe. These cells are called archesporial cells. Each such cell divides by a periclinal wall (parallel to outer wall) forming an outer primary wall cell and an inner primary sporogenous cell. 

The primary wall cell by periclinal and anticlinal divisions forms 2-6 layers of the wall of the microsporangium. The primary sporogenous cell by repeated division forms sporogenous mass. Of the concentric layers of the microsporangium wall the outermost one is called epidermis or exothecium and the innermost one is called tapetum. In between these two layers lies the endothecium. Tapetum is the nourishing layer which supplies nutrition to the sporogenous mass. 

The Primary sporogenous cells are diploid (2n) which may undergo first a mitotic division to produce a large number of diploid nuclei or may directly behave as the microspore mother cell (MMC). Each MMC by meiotic division gives rise to four haploid microspores. Initially all the four microspores remain enclosed by a common wall. This four-celled structure is called tetrad. A mature microspore is called pollen- grain. As each MMC gives rise to four pollen-grains a large number of pollen-grains are produced inside the pollen-sac or microsporangium. All sporogenous cells may not function as MMC. Some may function as nutritive materials for growth of MMC.

Development of Pollen grain into Male Gametophyte : 

(a) Pre-pollination Development : Microspore or the pollen is the first cell of the male gametophyte. The process of formation of male gametophyte starts with the division of the nucleus of the microspore which are still within the microsporangium. With the division of the nucleus into two the microspore also undergoes division into two cells. The larger one is called the vegetative cell or the tube cell and the smaller one is called the generative cell. Further changes in male gametophyte occur after pollination. 

(b) Post-pollination Development : After pollination, the pollen grains are stimulated by the secretion of the stigma to germinate. At the time of germination the exine breaks and the intine forms pollen tube. The vegetative nucleus now enters the pollen tube. It is now called tube-nucleus. The generative nucleus also enters the pollen tube and divides to form two nuclei before the tube reach micropyle. These two nuclei are called male gametes. The tube nucleus ultimately degenerates. The male gametes are now ready to unite with the egg formed by the process of megagametogenesis. In 70% plants post-pollination development takes place. The remaining plants, the generative nucleus divides into two male gametes of microgametes before the pollination take place. 

Q.5. What is allogamy? Discuss the contrivances of allogamy stating the most convenient method of achieving allogamy. 

Ans : Allogamy is the transfer of pollen grains from one flowers to another flower of the same or different plants. The flowers involved are bisexual flowers. The type of pollination is called cross Pollination. Nature favours cross-pollination and therefore to avoid self-pollination flowers and apt various ways, that is to say some favour able conditions called contrivances are created so that only cross pollination takes place. 

The contrivances are : 

(i) Dicliny or Unisexuality : In unisexual flowers the stamens and carpels are found in separate flowers. So self-pollination can never take place in unisexual flowers. Flowers bearing stamens and flowers bearing carpels may to borne by the same plant (monoecious) or by different plants (dioecious). In both the cases the pollen must be transferred from the stamen of one flower to the stigma of another flower. 

(ii) Self-sterility : Pollen grains of some flowers cannot fertilize the ovules of the same flower. The same pollen grains again can fertilize the ovules of another flowers. Example tuberosum, Nicotiana etc. 

(iii) Dichogamy : In many flowers anthers and stigma of the same flower mature at different time and thus prevent self-pollination. In such flowers the anther may mature earlier than stigma and in some others the stigma may mature earlier. The former condition is called rose, cotton, sunflower etc. are protandrous flowers. Ficus, Mirabilis, Magnolia etc. are protogynous flowers. 

(iv) Heterostyly: Some flowers show dimorphism with respect to the relative length of the stamens and the styles. In some of the flowers the style is longer than the stamens and in some of the flowers the stamens are longer than the style. Cross-pollination takes place between long stamens and long stigma and short stamens and short stigma. Thus self-pollination does not take place in such dimorphic. flowers. Example – Primrose. 

(v) Herkogamy : In some flowers some barriers exist which prevent self-pollination. In orchid and Calotropis the pollen grains are held together in anther lobe as a group called pollinium. The two pollinia of adjacent anther lobes joined together to a sticky disc called corpusculum. The pollinia cannot move out unless these are taken out by insects and deposited in another flower. 

In some flowers again the stamens and the stigma may lie at some distance from each other, the anther may be inserted within the corolla tube and the stigma far exerted or the anther far exerted and the style inserted. In certain flower (bleeding heart) the style bend downward and move away when the anthers mature. Then afterward when the pollen grains have been removed by insects the stamens roll down and the style stands erect. In such flowers therefore self-pollination cannot takes place. 

Q.6. What do you meant by fertilization? Discuss the significances of the event giving traces on post fertilization changes leading to the formation of seeds in angiosperms. 

Ans : Fertilization is the union of two dissimilar gametes in the process of sexual reproduction. In plants, pollination leads to fertilization. After reaching the stigma, the contents of the pollen-grain comes out through the germ pore in the form of pollen-tube. 

Pollen-tube pushes its way through the style and the wall of the ovary while carrying the tube nucleus and the generative nucleus with it. The tube nucleus degenerates after formation of the tube. The generative nucleus then divides to form two male gametes. The pollen-tube after running through the style enters the ovule to reach the embryo-sac. After the pollen-tube reaches the embryo-sac its tip dissolves and the two male gametes are liberated. Of the two male gametes, one fuses with the egg- cell of the embryo-sac while the other pushes further inside to meet the fusion product (2n) of the two polar nuclei called definitive nucleus. The synergids and antipodal cells of the embryo-sac do not have any function after fertilization hence they disappear sooner or later. 

After fertilization the egg-cell covers itself with a cell-wall and becomes an oospore. The definitive nucleus after fertilization transforms into an endosperm nucleus. The oospore gives rise to the embryo, the ovule to the seed and the ovary to the fruit.

In many plants such as in banana, papaya, orange etc. the fruits may develop without fertilization. Development of fruit without fertilization is called parthenocarpy. In true parthenocarpic fruits seeds are not formed or if formed they do not germinate. 

The definitive nucleus is already having diploid (2n) number of chromosomes in it. The fusion of the male gamete (n) brings the chromosome number to triploid (3n) and sometime due to presence of another polar nucleus to tetraploid (4n) condition. The fusion product is now called endosperm nucleus. 

Significance of Fertilization : 

(i) Oospore gradually develops into embryo and it can germinate to seedling. 

(ii) Ovule is transformed into seed. 

(iii) Ovary forms fruit. 

(iv) Fertilization helps to maintain the parental characters or diploidy (2n) of the sporophytic or plant body. 

Formation of Seed : 

(a) Embryo Formation : The fertilized egg-cell clothes itself with a cellulose wall and becomes a zygote or oospore. Now the oospore divides into two cells by a transverse wall. Of the two cells thus formed one is basal cell and the other is terminal cell. The basal cell divides in one direction to form an elongated structure called haustorial suspensor. The terminal cell which acts as embryonal cell first divides by two consecutive vertical division to form a four-cell structure called quadrant. Then by a transverse division the number of cells rises to eight. 

This stage is called octant stage. The octant mass divides by a wall parallel to the curved surface forming a posterior or hyponasal octant and an interior or epibasal octant. The epibasal cells by further differentiation forms the plumule and cotyledons, while the parabasal cells form the hypocotyl. Plumule gives rise to shoot, hypocotyl gives rise to radicle, the future root. Large number of nuclei. Cytoplasm is deposited around each nucleus. After this stage the further development is different in different types of plants.

(b) Formation of Endosperm : Endosperm nucleus is formed by the union of one male gamete (n) with the secondary nucleus (2n) giving it a triploid nature. The nucleus then undergoes division to give rise to a endosperm grows it fills up the nucellus. In some seeds the endosperm is completely utilized by the developing embryo so that a mature seed devoid of endosperm is formed. Such seeds are called non-endospermic or exalbuminous seeds (pea, gram). In others, the endosperm grows so vigorously that the growing embryo cannot completely utilize it. Such seeds are called endospermic or albuminous seeds, (rice, wheat). Endosperm be mealy as in rice, wheat etc. or may be oily as in mustard, castor may or may be watery as in coconut. 

(c) Other Changes : During the formation of embryo and endosperm the integuments of the ovary gradually dry up. The two dried integuments form the seed-coats. The funicle, hilum and the micropyle of the ovule give rise to stalk, hilum and micropyle of the seed respectively. 

Changes occuring in ovule during its transformation into seed : 

Q.7. Draw a labelled diagram of the sectional view of a typical anatropous ovule showing various stages of fertilization. 

Ans :

Q.8. What is embryo? Describe the development of dicot embryo with support of suitable diagrams. 

Ans : Embryos are baby plants remain in dormant stage within the ovule. It has two axis called radicle and plumule. Radide give rise to future root system and the plumale given rise to future shoot. 

Embryo Formation : The fertilized egg cell cloths itself with a cellulose wall and becomes a zygote or oospore. Now the oospore divides into two cells by a transverse wall. Of the two cells thus formed one is basal cell and the other is terminal cell. The basal cell divides in one direction to form an elongated structure called haustorial suspensor. The suspensor pushes the young embryo deep inside the embryo sac. The end cell of suspensor which is called basal cell of the suspensor remains bulbous and acts as absorbing organ for the growing embryo. 

The terminal cell which acts as embryonal cell first divides by two consecutive vertical division to form a four-cell structure called quadrant then by a transverse division the number of cells rises to eight. This stage is called octant stage. The octant mass divides by a wall parallel to the curved surface forming a posterior or hyponasal octant and an anterior or epibasal octant. The epibasal cells by further differentiation form the plumule and cotyledons, while the parabasal cells form the hypocotyl. Plumule gives rise to shoot hypocotyl gives rise to radicle, the future root. 

The peripheral cells of the proembryo divide by a wall parallel to its curved surface. This layer of cells divide vertically and periblem (future contex). The cells of the suspens or which is in contact with the embryo cell is called hypophysis. It forms the root. The embryo is further differentiated into cotyledons. The suspensor later degenerates and due to the curving of the ovule the embryo also becomes curved. 

Q.9. Discuss the following : 

(a) Endosperms development precedes embryo development. 

Ans : The formation of endosperm is initiated by mitotic division of the primary endosperm nucleus (3N). The formation of endosperm occurs usually prior to the Zygotic division. Endosperm accumulates food reserves and functions as the nutritive tissue for the developing embryo. Usually the zygote divides immediately after the first division of primary endosperm nucleus or if divides earlier than the first division of primary endosperm nucleus. After fertilization the zygote rests for a period that varies greatly in different taxa from a few hours to several weeks. 

There is no fundamental differences in the early stages of development of dicotyledonous and monocotyledonous embryo. But later stages of development do differ as the mature embryos differ considerably. 

(b) Production of seeds are advantageous to flowering plants. 

Ans : Seeds have been considered as physiological enigma of living world. They contain a miniature plant, a protective seed coat, reserve food for future growth and little long live RNA to guide future metabolism and growth. 

It is interesting to note that living cell dies if water content fall below 45-75% but seed continues to live with 10-15% water content. In seeds metabolism is suspended animation like in hibernating animals. In seed, all of a sudden growing embryo becomes dormant and stops growth. Food becomes immobile and respiration becomes anaerobic in mature seed. Seeds are able to tolerate high and low temperatures. However at the time of germination seeds destroy chemical inhibitors and start secreting growth hormones. 

Thus the production of seeds are advantageous to the flowering plants.

Dev Kirtonia

Hi, I’m Dev Kirtonia, Founder & CEO of Dev Library. A website that provides all SCERT, NCERT 3 to 12, and BA, B.com, B.Sc, and Computer Science with Post Graduate Notes & Suggestions, Novel, eBooks, Biography, Quotes, Study Materials, and more.