Class 12 VBQs Biology Sexual Reproduction in Flowering Plants
Very Short Answer Type Questions
Question. Hybrid seeds have to be produced year after year give reason.
Answer : Hybrid seeds have to be produced year after year because seeds collected from hybrid plants, if sown subsequently, do not maintain hybrid characters due to segregation of traits.
Question. Mention advantage of apomictic seeds to farmers.
Answer : In hybrid apomicts, there is no segregation of characters in hybrid progeny. So, farmers can keep on using the hybrid seeds to raise new crop year after year and they do not have to buy new hybrid seeds every year.
Question. Normally one embryo develops in one seed but when an orange seed is squeezed many embryos of different shapes and sizes are seen.Mention how it has happened.
Answer : In orange seed, embryos originate by adventive embryony from diploid cells of nucellus or integuments and thus, if orange seed is squeezed many embryos of different sizes can be observed.
Question. Mention the scientific term used for modified form of reproduction in which seeds are formed without fusion of gametes.
Answer : Apomixis is a mode of reproduction that produces seeds without fertilisation.
Question. Name the type of fruit apple is categorised under and why ? Mention two other examples which belong to the same category as apple.
Answer : False fruit, thalamus contributes to fruit formation:
Strawberry, Cashew (any other correct examples)
Question. Meiocyte of rice has 24 chromosomes. Write the number of chromosomes in its endosperm.
Answer : Number of chromosomes in meiocyte = 24 = 2n.
Haploid number (n) = 12.
Therefore, the number of chromosomes will be n × 3 = 36, because the endosperm in angiosperms is triploid (3n) as it is formed by the fusion of 2 polar nuclei (n) and one gamete.
Question. In the following figure of a fruit, label the part that is protective in function and the part that is responsible for producing new plants.
Question. In case of polyembryony, an embryo A develops from the synergids and the embryo B develops from the nucellus. State the ploidy of embryo A and B.
Answer : A- Haploid ; B- Diploid
Short Answer Type Questions
Question. (a) Why are seeds of some grasses called apomictic? Explain.
(b) State two reasons to convince a farmer to use an apomictic crop.
Answer : (a) Seeds of some grasses are called apomictic because they are produced without fertilisation and do not involve formation of zygote through gametic fusion.
(b) Two reasons to convince a farmer to use an apomictic crop are as follows:
(i) Production of infection free embryo for development of plant.
(ii) Production of better clones as adventive embryos are better clones than cuttings.
Question. Explain the different ways apomictic seed can develop. Give an example of each.
Answer : There are several methods of apomictic development in seeds. The two common ones are recurrent agamospermy and adventive embryony.
(i) Recurrent agamospermy : Agamospermy is the formation of seed that has an embryo formed without meiosis and syngamy. It is of two types, nonrecurrent and recurrent. In nonrecurrent agamospermy, the embryo is haploid. Therefore, the seed having it is non-viable. In recurrent agamospermy all the cells of embryo sac are diploid as it is formed directly either from a nucellar cell (apospory) or diploid megaspore mother cell (diplospory), e.g., Rubus, apple.
(ii) Adventive embryony : An embryo develops directly from a diploid cell other than egg like that of nucellus and integument, e.g., Citrus, Opuntia.
Question. (a) How is apomixis different from parthenocarpy?
(b) Describe any two modes by which apomictic seeds can be produced.
Answer : (a) Apomixis is a mode of reproduction which does not involve formation of zygote through gametic fusion whereas parthenocarpy involves formation of zygote through gametic fusion but produces seedless fruits. Asomixis produces seeds without fertilisation e.g., some species of Asteraceae and grasses parthenocarpy is production and development of seedless fruits e.g., banana, grapes.
Question. Suggest the advantage to a farmer for using apomictic seeds of hybrid varieties?
Answer : Advantages of apomictic seeds of hybrid varieties to farmer are as follows:
(i) It will reduce the cost on purchasing hybrid seeds every year.
(ii) Apomixis is genetically controlled so genes of apomixis can be introduced in hybrid varieties.
Question. Why are some seeds of Citrus referred to as polyembryonic? How are they formed?
Answer : Polyembryony is the occurrence of more than one embryo in a seed. In Citrus, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. So, their seed contains many embryos and are referred as polyembryonic seed.
Question. Fertilisation is essential for production of seed, but in some angiosperms, seeds develop without fertilisation.
(a) Give an example of an angiosperm that produces seeds without fertilisation. Name the process.
(b) Explain the two ways by which seeds develop without fertilisation.
Answer : (a) In the members of Family Asteraceae, seeds develop without fertilisation. This process is called apomixis.
(b) Two ways by which seeds develop without fertilisation are as follows:
(i) In some species, the diploid (2n) egg cell is formed without reduction division and develops into embryo without fertilisation.
(ii) In many varieties of Citrus and mango some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos.
Question. What is apomixis? What is its importance?
Answer : Apomixis is a mode of reproduction which does not involve formation of zygote through gametic fusion. It is a form of asexual reproduction that mimics sexual reproduction, in which seeds are produced without fertilisation. It is common in grasses and species of Family Asteraceae.
Apomicts have advantages in horticulture and agriculture, particularly hybrid seed industry.
(i) The hybrid varieties are more productive but production of these seeds is very costly and time consuming and such seeds do not maintain hybrid vigour in progeny due to segregation of characters/ genes during meiosis. If the hybrids are made into apomicts, there will be no segregation of genes due to no meiosis. Thus, apomictic hybrid seeds can be used year after year which will reduce the cost on purchasing hybrid seeds every year.
(ii) Adventive embryos are better clones than cuttings.
(iii) Apomictic embryos are generally free from infections.
Question. A non-biology person is quite shocked to know that apple is a false fruit, mango is a true fruit and banana is a seedless fruit. As a biology student how would you satisfy this person ?
Answer : Fruit is a ripened ovary where seed develops from ovule. Mango is a true fruit because it develops only from the ovary of the flower. Apple is a false fruit because here in this case along with ovary thalamus of flower also takes part in the formation of fruit.
Banana is a seedless fruit as it develops without the stimulus of pollination and fertilization. Such fruits are also called as parthenocarpic fruits.
Question. Why are some seeds of citrus referred to as polyembryonic ? How are they formed ?
Answer : Some seeds of citrus are referred to as polyembryonic because they contain more than one embryo. This phenomenon is called as polyembryony. In citrus, one embryo develops normally as a result of sexual reproduction and other additional embryos are produced from the cells of nucellus or integument apomictically. The cells of nucellus or integument surrounding the embryo sac protrude into it, divide and produce the embryos.
Question. Differentiate between pericarp and perisperm.
Answer : Pericarp – wall of the fruit (which develops from the wall of ovary)
Perisperm – persistent residual nucellus
Question. Explain the function of each of the following :
(ii) Germ pores
Answer : (i) Protects, the radicle of (monocot) embryo.
(ii) Allow germination of pollen grain / formation of pollen tubes.
Question. Banana is a parthenocarpic fruit, whereas oranges show polyembryony. How are they different from each other in respect to seeds ?
Answer : The banana is a parthenocarpic fruit as it develops without fertilization, whereas in oranges, additional embryos develop directly from diploid cells other than the egg, like nucellus and integument. In banana, the ovary may develop into the fruit without fertilization. The parthenocarpic fruits never contain seeds.
Question. List the changes that occur when an ovule matures into seed.
Answer : (a) Integuments of ovules harden as tough protective seed coats.
(b) The micropyle remains as a small pore in the seed coat.
(c) As the seed matures, its water content is reduced.
(d) Seeds become relatively dry (10-15 per cent moisture by mass).
(e) The general metabolic activity of the embryo slows down.
(f) The embryo may enter a state of inactivity called dormancy.
Question. (i) How does a farmer use the dormancy of seeds to his advantage ?
(ii) What advantages a seed provides to a plant ?
Answer : (i) For storage (dehydration) of seeds to be used as food, to raise the crop in the next season.
(ii) Seed formation is more dependable, better adaptive strategy for dispersal to new habitat, hard seeds provide protection to the young embryo, being a product of sexual reproduction they generate new genetic combinations / genetic variations / sufficient food reserve for the young seedling to be nourished.
Detailed Answer :
(i) Dormancy of mature seeds is crucial for storage of seeds. It can be used by farmers as food throughout the year and also to raise crop in the next season.
(ii) Importance of seeds to plants :
(a) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonize in other areas.
(b) They have food reserves. So young seedlings are nourished until they are capable of photosynthesis.
(c) The hard seed coat protects the young embryo.
(d) Being products of sexual reproduction, they generate new genetic combinations leading to variations.
Question. Double fertilization is reported in plants of both, castor and groundnut. However, the mature seeds of groundnut are non-albuminous and castor are albuminous. Explain the post fertilization events that are responsible for it.
Answer : Development of endosperm (preceding the embryo) takes place in both, developing embryo derives nutrition from endosperm Endosperm is retained / persists / not fully consumed in castor, endosperm is consumed in groundnut.
Detailed Answer :
Endosperm development precedes embryo development. The triploid primary endosperm nucleus (PEN) undergoes repeated mitotic divisions, without cytokinesis. At this stage of development, the endosperm is called free-nuclear endosperm.
Cell wall formation takes place later on. As a result the endosperm becomes partly or fully cellular. The cells of the endosperm store food materials, which are later used by the developing embryo.
In non-albuminous or non-endospermic seeds, the endosperm may be completely utilized by the developing embryo before the maturation of seeds as in pea, bean, groundnut etc. In albuminuous or endospermic seeds, a portion of endosperm persists in the mature seeds. e.g., castor.
Question. Explain any three advantages the seeds offer to angiosperms.
Answer : (i) Since reproductive processes such as pollination and fertilization are independent of water, seed formation is more dependable.
(ii) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
(iii) As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.
(iv) The hard seed coat provides protection to the young embryo.
(v) Being products of sexual reproduction, they generate new genetic combinations / variations.
(vi) Dehydration and dormancy of mature seeds are crucial for survival under adverse conditions. (Any three)
Question. Draw a labelled mature stage of a dicotyledonous embryo.
Question. Describe with the help of three labelled diagrams the different embryonic stages that include the mature embryo of dicot plants.
Answer : Following are the steps that occur during the development of an embryo :
First, the zygote starts dividing and gives rise to proembryo. The cells of this proembryo further divide, forming a globular, heart-shaped and finally mature embryo. A typical dicot embryo consists of an embryonal axis and two lateral cotyledons.
The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the cotyledons is called hypocotyl. It contains the radicle (root tip).
The root tip is covered by the root cap.
Question. (i) Give one example each of albuminous and nonalbuminous seeds.
(ii) Name the parts of the ovule and the embryo sac of an angiosperm that develop into :
(a) perisperm, (b) seed coats,
(c) endosperm, (d) embryonal axis.
Answer : (i) Example :
Non-albuminous seed : Pea, bean, mustard.
Albuminous seed : Castor, maize, coconut.
(ii) (a) Perisperm : Residual persistent nucellus.
(b) Seed coats : Integuments.
(c) Endosperm : Primary endosperm nucleus.
(d) Embryonal axis : Tigellum.
Question. In the figure of a typical dicot embryo, label the parts (1), (2) and (3). State the function of each of the labelled part.
Ans. Label 1 : Origin of plumule; plumule grows into shoot.
Label 2 : Cotyledons; food storage.
Label 3 : Origin of radicle; radicle grows into root.
Question. Explain how false, true and parthenocarpic fruits are different from each. Give one example of each.
Answer : (i) True fruit : It is the fruit which develops only from the ovary part of flower. e.g., maize, wheat.
(ii) False fruit : It is the fruit which develops from ovary but in addition other parts of the flower like thalamus, calyx or corolla also take part in fruit formation. e.g., apple, strawberry.
(iii) Parthenocarpic fruits : These are the fruits which develop without the act of fertilization.
Question. Parthenocarpy and apomixis have been observed in some plants. Give an example of each. State a similarity and a difference observed between the two processes.
|Fruit is formed without|
|Seed is formed without|
|Seedless fruits are produced||Fruits with seeds are produced|
Eg. : Banana / grapes /
any other correctly =
Eg. : species of Asteraceae
/ grasses / any other correctly =
Similarity : In both the processes, development takes place without fertilisation = 1
Long Answer Type Questions
Question. (i) Why does endosperm development precede embryo development in angiosperm seeds ?
State the role of endosperm in mature albuminous seeds.
(ii) Describe with the help of three labelled diagrams the different embryonic stages that include mature embryo of dicot plants.
Answer : (i) As it provides nutrition for the developing embryo. It is an adaptation to provide assured nutrition to the developing embryo.
Provides nutrition during and after germination.
(ii) The zygote (in the embryo sac) divides to form pro embryo and subsequently the globular, heart shaped and mature embryo as shown in the diagram.
Question. Write the changes a fertilized ovule undergoes within the ovary in an angiosperm plant.
Answer : A fertilized ovule undergoes the following changes in the ovary in an angiosperm plant :
Unfertilized ovule →Fertilized ovule → Seed
Integument →Seed coat
Outer integument →Testa
Inner integument →Tegmen
Nucellus →Absent or present as perisperm
Embryo Sac :
Antipodal cells →Degenerate
Synergid cells →Degenerate
Secondary nucleus →Endosperm
When the unfertilized ovule undergoes double fertilization, it makes fertilized ovule and then seeds. In this process funiculus and hilum are present. Outer integument makes testa and inner integument is tegmen. Chalaza and micropyle are present but nucellus is absent, partly persists as perisperm.
In embryo sac, antipodals and synergids degenerate.
The central cell makes endosperm and the egg changes into embryo.
Question. Draw a labelled diagram of L.S. of an embryo of grass (any six labels).
Question. Differentiate, giving one example of each, between the following :
(i) Parthenogenesis and parthenocarpy.
(ii) Perisperm and pericarp.
Answer : (i) Parthenogenesis and parthenocarpy :
|When the female gametes undergo development|
to form a new organism without fertilization, this
phenomenon is called parthenogenesis.
|Fruits that develop without fertilization|
are called parthenocarpic fruits and the process is
(ii) Perisperm and pericarp :
|In some seeds such as black pepper and sugarbeet, remnants of|
nucellus are persistent.
This residual, persistent nucellus is the called perisperm.
|The transformation of ovules into seeds and ovary into fruits proceeds simultaneously. The wall of the ovary develops into|
the wall of fruits called pericarp.
Question. (i) A capsicum flower has 240 ovules in its ovary. But, it produces a fruits with only 180 viable seeds.
(ii) Describe the development of an endosperm in a viable seed. Why does endosperm development precede embryo development ?
(iii) Give an example of an angiosperm seed that has a perisperm. Name the part the perisperm develops from.
Answer : (i) Less number of pollen grains / male gametes were available / all pollen grains did not germinates / all pollen grains did not form pollen tubes / many pollen were not compatible / 60 ovules not fertilised / only 180 fertilised
(ii) PEN undergoes successive nuclear divisions to give rise to free nuclei / free nuclear endosperm, cell wall formation occurs and the endosperm becomes cellular.
Cells of endosperm are filled with reserve food materials that are used for nutrition of developing embryo.
(iii) Black pepper / Sugar beet Nucellus
Question. (i) How is apomixis different from parthenocarpy ?
(ii) Describe any two modes by which apomictic seeds can be produced.
Answer : (i) Parthenocarpy : The formation of fruit without the act of fertilization is called parthenocarpy and the fruits so developed are called as parthenocarpic fruits. These fruits are seedless.
e.g., banana. While apomixis is a special type of sexual reproduction which does not involve meiosis and syngamy. In fact it is a form of asexual reproduction that mimics the sexual reproduction in which the seeds are produced without fertilization.
(ii) Apomictic seeds can be produced by any of the following methods.
(a) Apospory : It is the formation of diploid embryo sac directly from a cell of nucellus.
(b) Diplospory or generative apospory : It is the formation of diploid embryo sac from the megaspore mother cell without meiosis.
The diploid egg of the diploid embryo sac develops into embryo without syngamy i.e., parthenogenetically.
(c) Apogamy : The synergids or antipodals of the diploid embryo sac may develop into embryo parthenogenetically. (Any two)
Question. Explain the post-pollination events leading to seed production in angiosperms.
Answer : Post-pollination events leading to seed formation in angiosperms are as follows :
(i) After pollination, the pistil recognises the pollen whether it is of the right type (compatible) or of the wrong type (incompatible). Compatible pollens are accepted and germinate on the stigma to produce a pollen tube. Pollen tube grows and reaches the ovary and enters the ovule either through micropyle or chalaza or integuments.
(ii) The pollen tube bursts open in one of the two synergids to release the two male gametes. One male gamete fuses with the egg to form a diploid zygote or oospore (syngamy). The second male gamete fuses with the diploid secondary nucleus of the central cell to form a primary triploid endosperm nucleus (triple fusion). The whole process is termed as double fertilisation.
(iii) The primary endosperm nucleus gives rise to endosperm while the zygote develops into embryo.
(iv) The integuments of fertilised ovule harden to form the seed coat.
(v) The outer integument becomes hard and forms leathery testa or outer seed coat which ensures survival of seeds.
(vi) The inner integument, if persists, forms the tegmen.
(vii) The micropyle remains in the form of a fine pore on the surface of seed. Funicle is transformed into stalk of the seed. The hilum marks the point of attachment to the stalk. Micropyle facilitates the entry of oxygen and water into the seed.
Question. Explain any three advantages the seeds offer to angiosperms.
Answer : The three advantages that seeds offer to angiosperms are as follows:
(i) Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
(ii) They have suficient food reserves, young seedlings are nourished until they are capable to photosynthesise on their own and hard seed coat provides protection to the young embryo.
(iii) Seeds are product of sexual reproduction, they generate new genetic combinations leading to variations.
Question. Describe the development of endosperm after double fertilisation in an angiosperm. Why does endosperm development precedes that of zygote?
Answer : Endosperm is the food laden tissue formed after double fertilisation. It provides essential nutrients to the growing embryo and also the young seedling at the time of seed germination. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilisation or triple fusion i.e., fusion of a male gemete with secondary nucleus of the central cell. Based on the first and subsequent divisions of primary endosperm nucleus, the development of endosperm takes place in different ways and accordingly endosperm is of three types – nuclear, cellular and helobial.
Nuclear type : In the nuclear type of endosperm the first division of primary endosperm nucleus and few subsequent nuclear divisions are not accompanied by wall formation. The nuclei produced are free in the cytoplasm of the embryo sac and they may remain free indefinitely or wall formation takes place later.The multinucleate cytoplasm undergoes cleavage, and gives rise to multicellular tissue, maize, wheat, rice.
Cellular type : In this case, there is cytokinesis after each nuclear division of endosperm nucleus. The endosperm, thus, has a cellular form, from the very beginning because first and subsequent divisions are all accompanied by wall formation. e.g., Petunia, Datura, Adoxa etc
Helobial type: It is an intermediate type between the nuclear and cellular types. The first division is accompanied by cytokinesis but the subsequent ones are free nuclear. The chamber towards micropylar end of embryo sac is usually much larger than the chamber towards chalazal end. A large number of nuclei are formed in the micropylar chamber by free nuclear divisions while the nucleus of the chamber towards chalazal end divides to form a fewer free nuclei or may not divide at all, e.g., Order Helobiales and most monocots. Endosperm provides nourishment to the zygote. So, endosperm development precedes that of zygote.
Question. Differentiate between perisperm and endosperm giving one example of each.
Answer : Differences between perisperm and endosperm are as follows:
Question. Explain the development of a mature embryo from the embryo sac of dicot flower.
Answer : In a typical dicot flower, after double fertilisation, the zygote elongates and then divides by a transverse wall into two unequal cells. The larger basal cell is called suspensor cell. The other towards the antipodal end is termed as terminal cell or embryo cell. The suspensor cell divides transversely a few times to produce a filamentous suspensor of 6–10 cells. The suspensor helps in pushing the embryo in the endosperm. The first cell of the suspensor towards the micropylar end becomes swollen and functions as a haustorium. The last cell of the suspensor is known as hypophysis. Hypophysis later gives rise to the radicle and root cap. The embryo cell undergoes two vertical divisions and one transverse division to form eight cells arranged in two tiers– epibasal (terminal) and hypobasal (near the suspensor). The epibasal cells eventually form the two cotyledons and the plumule. The hypobasal cells produce the hypocotyl except its tip.
Stages in the development of a dicot embryo are represented diagrammatically as follows:
Initially the embryo is globular and undifferentiated.
Early embryo with radial symmetry is called proembryo. It is transformed into embryo with the development of radicle, plumule and cotyledons. At this time the embryo becomes heart-shaped. The rate of growth of the cotyledons is very high so that they elongate tremendously while the plumule remains as a small mound of undifferentiated tissue.
Question. Double fertilisation is reported in plants of both castor and groundnut. However, the mature seeds of groundnut are non-albuminous and castor are albuminous. Explain the post
fertilisation events that are responsible for it.
Answer : In angiosperms double fertilisation produces two structures – a diploid zygote and a triploid primary endosperm cell. The latter gives rise to tissue called endosperm. Zygote forms the embryo and endosperm provides nourishment to the growing embryo. With the growth of embryo the central part of the endosperm is utilised. In some seeds, the endosperm persists in the seed as food storage tissue. Such seeds are called endospermic or albuminous seeds, e.g., castor, cereals, etc. In others the endosperm is completely eaten up by growing embryo. The food for later development of embryo is then stored in cotyledons which become massive. Such seeds are called nonendospermic or exalbuminous, seeds e.g., groundnut, pea etc
Question. List the post-fertilisation events in angiosperms.
Answer : Soon after the act of double fertilisation, the flower begins to lose its shine. The petals, stamens and style either fall or wither away. The calyx, however, may persist in some cases (e.g., tomato,brinjal). The major events include –
(i) Development of endosperm from triploid primary endosperm nucleus in the central cell of embryo sac.
(ii) Development of embryo from diploid zygote.
(iii) Development of seed from ovule.
(iv) Development of fruit from ovary.
Question. List the changes each part of the fertilised ovule undergoes to develop into a seed.
Answer : The following changes take place in fertilised ovule to develop into a seed :
Question. Draw a diagrammatic sectional view of a mature anatropous ovule and label the following parts in it.
(a) that develops into seed coat
(b) that develops into an endosperm in an albuminous seed
(c) that develops into an embryo after fertilisation
(d) through which the pollen tube enters into the embryo sac
(e) that attaches the ovule to the placenta
Answer : Diagrammatic sectional view of anatropous ovule is as follows:
Question. L.S. of a maize grain is given below. Label the parts A, B, C and D in it.
Answer : A – Pericarp
B – Scutellum
C – Coleoptile
D – Coleorhiza
Question. Explain how false, true and parthenocarpic fruits are different from each other. Give one example of each.
Answer : False fruit is the fruit which is derived from the fertilised ovary and accessory floral parts e.g., thalamus also contribute to the fruit formation.
Examples : apple, strawberry, guava etc. True fruit is the fruit which is derived only from the ovary of a flower and is not associated with any non-carpellary part , for example : mango, tomato, etc.Parthenocarpic fruit is the fruit that is formed without fertilisation and does not bear seeds, e.g., pineappale, banana, etc.
Question. Differentiate between the following giving one example of each:
(a) Parthenogenesis and Parthenocarpy
(b) Perisperm and Pericarp
(a) Differences between parthenocarpy and parthenogenesis are as follows:
(b) Differences between perisperm and pericarp are as follows :
(a) Identify the figure.
(b) Name the initial cell from which this structure has developed.
(c) Draw the next mature stage and label the parts.
Answer : (a) The givenfigure represents globular embryo which is the developmental stage of dicot embryo.
(b) Zygote or oospore is the initial cell from which the given structure has developed.
(c) The next mature stage is heart shaped embryo, as shown below:
Question. A flower of tomato plant following the process of sexual reproduction produces 200 viable seeds.
Answer the following questions giving reasons.
(a) What would have been the minimum number of ovules present in per pollinated pistil?
(b) How many microspore mother cells would minimally be required to produce requisite number of pollen grains?
(c) How many pollen grains must have minimally pollinated the carpel?
(d) How many male gametes would have used to produce these 200 viable seeds?
(e) How many megaspore mother cells were required in this process?
Answer : Flower of tomato plant contains 200 viable seeds.
(a) Minimum number of ovules involved = 200 because following fertilisation one ovule matures into one seed.
Each microspore mother cell undergoes meiosis and gives rise to tetrad of four haploid microspores or pollen grains.
(b) Number of microspore mother cells required to produce 200 pollen grains is 200/4 = 50
(c) 200 pollen grains must have pollinated 200 carpels for production of 200 viable seeds.
(d) Male gametes involved are 200 × 2 = 400
Each pollen grain releases two male gametes, one fuses with the egg to form zygote and second male gametes fuses with polar nuclei to form endosperm nucleus.
(e) Megaspore mother cell undergoes reduction division to form four haploid megaspores. Only one megaspore remains functional and other three degenerate. Only the functional megaspore develops into female gametophyte. Hence, number of megaspore mother cell = 200.
Question. Draw a labelled diagram of L.S. of an embryo of grass (any six labels).
Answer : L.S. of embryo of grass is as follows:
Question. A flower of brinjal plant following the process of sexual reproduction produces 360 viable seeds.
Answer the following questions giving reasons:
(a) How many ovules are minimally involved?
(b) How many megaspore mother cells are involved?
(c) What is the minimum number of pollen grains that must land on stigma for pollination?
(d) How many male gametes are involved in the above case?
(e) How many microspore mother cells must have undergone reduction division prior to dehiscence of anther in the above case?
Answer : A flower of brinjal plant contains 360 viable seeds.
(a) Number of minimally involved ovules = 360. this is because following sexual reproduction, one ovule matures into one seed.
(b) Megaspore mother cell undergoes reduction division to forms four haploid megaspores. Only one megaspore remains functional and other three degenerate. Only the functional megaspore develops into female gametophyte. Hence, number of megaspore mother cell = 360.
(c) Compatible pollen grain after landing on stigma forms pollen tube which traverses through style of carpel and enters the ovule to release male gamete.
Here, minimum number of pollen grains involved is = 360.
(d) Each pollen grain releases two male gametes in an embryo sac. One male gamete fuses with the egg to form zygote and second male gamete fuses with polar nuclei to form endosperm nucleus. Hence, number of male gametes involved is = 360 × 2 = 720.
(e) Each microspore mother cell undergoes meiosis and gives rise to tetrad of four haploid microspores or pollen grains.
Number of microspore mother cell that have undergone reduction division prior to dehiscence of anther = 360/4 = 90
Question. Give reasons why
(a) most zygotes in angiosperms divide only after certain amount of endosperm is formed ?
(b) groundnut seeds are exalbuminous and castor seeds are albuminous ?
(c) micropyle remains as a small pore in the seed coat of a seed ?
(d) integuments of an ovule harden and the water content is highly reduced, as the seed matures ?
(e) apple and cashew are not called true fruits ?
Answer : (a) Endosperm is food storing tissue formed during the development of angiospermous seed which provides essential nutrients to the growing embryo and also young seedling at the time of seed germination.
(b) Groundnut seeds are exalbuminous as the seeds usually store food materials in cotyledons and the endosperm is used up by the developing embryo
Castor seeds are albuminous as they, have copious amounts of endosperm tissue to provide nutrition to the developing embryo.
(c) Micropyle remains as a small pore in the seed coat to facilitate entry of O2 and water into the seed during germination.
(d) Integuments of an ovule harden and the water content is highly reduced, as the seed matures to enable the seed to remain viable for a long time during the period of dormancy. The seed may enter a state of inactivity called dormancy and germinates only when the favourable conditions are available.
(e) Apple and cashew are not called true fruits as in these fruits, along with the ovary, thalamus also forms a part of fruit.