VBQs Class 12 Biology Sexual Reproduction in Flowering Plants
Very Short Answer Type Questions
Question. Identify ‘A’ in the figure showing a stage of embryo development in a dicot plant and mention its function.
Answer : Cotyledons–Store food (for growth of embryo of the seed)
Question. Given below is a section of a Maize grain. Identify ‘A’ and state its function.
Answer : Coleoptile, protecting the shoot apex/plumule.
Question. Mention the fate of the components of the embryo sac after fertilization.
Answer : (i) The egg cell forms the zygote (2n) which give rise to embryo.
(ii) Synergids and antipodals degenerate.
(iii) Polar nuclei form primary endosperm nucleus (3n), which forms the endosperm.
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.
If you squeeze a seed of orange you might observe many embryos of different sizes. How is it possible? Explain.
Answer : Some nucellar cells / surrounding the embryo sac start dividing and protrude into the embryo sac and develop into embryos. In such species each ovule contains many embryos.
Question. Apple and cashew are not called true fruits, why ?
Answer : These fruits are not called true fruits because thalamus which is a part of flower other than ovary takes part in fruit formation. A true fruit is one which develops only from the ovary. Apple and cashew are thus false fruits.
Short Answer Type Questions
Question. Some angiosperm seeds are said to be albuminous, whereas a few others are said to have a perisperm.
Explain each with the help of an example.
Answer : Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development. e.g., wheat, maize, barley, castor, sunflower.
When remnants of nucellus are persistent it is said to have a perisperm. Example; black pepper, Sugar beet.
Question. Draw a sectional view of an apple and label the different parts of an ovary in it. Fruits develop from an ovary. Then why is apple referred to as a false fruit ?
Thalamus also contributes to fruit formation
Question. How do plants produce seeds through apomixis?
Explain with the help of an example.
Answer : In apomixis or agamospermy, seeds are formed without the fusion of gametes. Diploid cells of the nucellus or integuments develop into an embryo, giving diploid seeds with a genetic constitution identical to the parent. Apomixis takes place in orange and onion.
Question. Differentiate between albuminous and nonalbuminous seeds, giving one example of each.
Answer : Albuminous – (with residual) endosperm is not completely used up during embryonic development. e.g., wheat / maize / castor / sunflower.
Non albuminous – (with out residual) endosperm is completely consumed during embryonic development. e.g., pea / groundnut.
Question. For a layman, both apples and mangoes are ‘fruits‘.
Do you agree ? Give reasons in support of your answer.
Answer : No.
Apple – thalamus, (false fruit)
Mango – Ovary, (true fruit)
The parthenocarpic fruits never contain seeds.
Question. Do you think apomixis can be compared with asexual reproduction ? Support your answer, giving one reason. How is apomixis beneficial to farmers? Explain.
Answer : Yes, seeds are produced without fertilisation.
Production of hybrid seeds is costly, if hybrids with desirable characteristics can be made into apomicts, there is no segregation of characters in the hybrid progeny, farmer can continue using hybrid seeds year after year and not buy new seeds.
Yes, apomixis can be compared with asexual reproduction as like asexual reproduction, apomixis is the mechanism of seed production without involving the process of meiosis and syngamy.
Importance of apomixis to farmers: The method of producing hybrid seeds by cultivation is very expensive for farmers. Also, by sowing hybrid seeds, it is difficult to maintain hybrid characters as characters segregate during meiosis. Apomixis thus prevents the loss of specific characters into hybrid. If hybrids with desirable characteristics can be made into apomicts, there is no segregation of characters in the hybrid progeny and farmer can continue using hybrid seeds year after year and without buying new seeds. Also, it is cost-effective method for producing seeds.
Question. (i) How are parthenocarpic fruits produced by some plants and apomictic seeds by some others? Explain.
(ii) When do farmers prefer using apomictic seeds?
Answer : (i) Ovary develops into fruit without fertilisation.
Formation of seeds without fertilisation without reductional division/develop into embryo without fertilisation.
(ii) To maintain hybrid characters (year after year in a desired plant), to avoid buying hybrid seeds every year (expensive seeds).
Detailed Answer :
(i) Parthenocarpic fruit are fruits that develop without fertilisation. e.g., Banana. They can be induced artificially through the application of growth hormones and such fruits are seedless.
Apomictic seeds are produced in some species when the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation. In many species (e.g., many Citrus & Mango varieties) some of the nucellar cells surrounding the embryo sac divide, protrude into the embryo sac and develop into the embryos. In such species each ovule contains many embryos.
(ii) Farmers prefer apomictic seeds, when the hybrids are made into apomicts. It is because, in apomictic seeds, there is no segregation of characters in the hybrid progeny. Therefore,
the farmers can keep on using the hybrid seeds to raise new crop year after year.
Question. Describe the development of endosperm after double fertilization in an angiosperm. Why does
endosperm development precedes that of zygote?
Answer : (1) After triple fusion, the central cell develops to form primary endosperm cell which contain
triploid primary endosperm nucleus.
(2) The primary endosperm cell undergoes successive cell divisions to form triploid endosperm which has abundant food reserves.
(3) The primary endosperm nucleus undergoes successive nuclear divisions to form many free nuclei. This types of endosperm development is called free nuclear endosperm, after which cell walls are laid and the endosperm becomes cellular endosperm. E.g. coconut water is nuclear endosperm (containing many free nuclei). While white kernel around is cellular endosperm.
The endosperm development precedes that of zygote to ensure that endosperm containing abundant food reserves is formed earlier and can nourish the developing embryo.
Question. List the post-fertilization events in angiosperms.
Answer : The various post-fertilization events occurring in angiosperms are :
(i) Primary endosperm nucleus gives rise to endosperm. It is formed before the development of the embryo, as it provides nourishment to the developing embryo.
(ii) The zygote at the micropylar end undergoes successive divisions to form mature embryo.
(iii) Ovule is converted into seed.
(iv) Ovary develops into fruit.
Question. (i) Draw a diagram of a sectional view of monocot seed (grain).
(ii) Label and write the functions of coleoptile, coleorhiza, endosperm.
Coleoptile – sheath of plumule / protection of plumule
Coleorhiza – sheath of radicle / protection of radicle
Endosperm – filled with reserve food materials for nutrition of developing embryo
Question. In angiosperms, the zygote is diploid, while the primary endosperm cell is triploid. Explain.
Answer : A zygote is the product of syngamy, it is formed by the fusion of a haploid male gamete and a haploid female gamete, i.e., egg cell.
A primary endosperm cell is the product of the fusion of a secondary nucleus (2n) and a haploid male gamete (n).
Question. Differentiate between perisperm and endosperm, giving one example of each.
Answer : Differences between perisperm and endosperm :
|In some seeds such as black pepper, sugar beet, remnants of nucellus are|
also persistent. This residual, persistent nucellus is called perisperm.
|The secondary nucleus of the embryo sac after triple fusion becomes the primary endosperm nucleus and develops into|
the endosperm, e.g., castor, groundnut.
Question. With the help of an example of each explain the following ?
Apomixis, Parthenocarpy, Polyembryony.
Answer : (i) Apomixis : It is a type of an asexual process which imitates the sexual reproduction by formation of seed without fertilization. It can be referred to as special type of sexual reproduction which does not involve the process of meiosis and syngamy.
(ii) Parthenocarpy : The formation of fruit without fertilization is called parthenocarpy. Such fruits are seedless and called as parthenocarpic fruits.
(iii) Polyembryony : The occurrence of plural embryos in a seed is called polyembryony, e.g., citrus fruits like orange. Normally, one embryo is formed in a seed as a result of fertilization but additional embryos may develop apomictic.
Question. Apomixis resembles asexual reproduction, as well as mimics sexual reproduction in plants. Explain with the help of a suitable example.
Answer : Since there is no fertilisation in apomixis, it resemble asexual reproduction and development of embryo / seed / fruit formation is mimicing sexual reproduction.
In Citrus / Mango, some of the nucellar cells surrounding the embryo sac, act as diploid egg cell, which are formed without reduction division and develop into embryo, without fertilisation.
Question. If the meiocyte of a maize plant contains 20 chromosomes, write the number of chromosomes in the endosperm and embryo of the maize grain and give reasons in support of your answer.
Answer : Endosperm = 30 , Embryo = 20
Diploid meiocyte (20 chromosomes) form haploid gametes (10 chromosomes). \
Two haploid gametes fuse to form diploid (20)
zygote which develops into a (diploid = 20)
embryo / syngamy of two haploid gametes to form a diploid zygote.
One haploid gamete (chromosome 10) fuses with two polar nuclei (chromosome 10 + 10) to form (triploid – 30) endosperm nuclei (which divides to form endosperm) / Triple fusion of three haploid nuclei (1 gamete + 2 polar nuclei) to form a triploid endosperm.