Rajasthan Board RBSE Class 12 Biology Chapter 4 Special Methods of Plant Reproduction
RBSE Class 12 Biology Chapter 4 Multiple Choice Questions
Development of new plants without fertilization –
Embryo development without fertilization of the egg is called –
(a) Haploid parthenogenesis
(b) Diploid parthenogenesis
(c) Adventive embryonic
(a) Haploid parthenogenesis
Formation of more than one embryo sac in ovule is found in which of the following:
Polyembryony is commonly found in –
(a) All monocots
(b) All dicots
Polyembryony caused by cleavage of zygote or proembryo is called, which polyembryony?
(a) Cleavage polyembryony
(b) General polyembryony
(c) Abnormal polyembryony
(d) Adventive polyembryony
(a) Cleavage polyembryony
RBSE Class 12 Biology Chapter 4 Very Short Answer Questions
A method of reproduction of plants, in which normal sexual reproduction is replaced by a form of reproduction in which meiosis and fertilization do not take place, is called apomixis.
Formation of more than one embryo in a seed is called polyembryony.
List the special methods of plant multiplication.
Higher plants normally reproduce by seed formation which normally involves meiosis and fertilization. Methods of plant reproduction without involving meiosis and fertilization are called special methods and are as follows:
- Vegetative reproduction
Write two advantages or importance of apomixis.
Apomixis does not involve meiosis and hence there is no segregation or recombinations hence genetically similar population can be raised by this method. This method is useful to produce hybrid seeds and also homozygous inbred lines.
RBSE Class 12 Biology Chapter 4 Short Answer Questions
Explain non recurrent apomixis and recurrent apomixis.
The process of formation of an embryo directly from egg cell without the process of fertilization is called non-recurrent apomixis, whereas in recurrent apomixis embryo arises either from diploid cells of the nucellus or from diploid cells of embryo sac formed without meiosis.
Differentiate between generative apomixis and somatic apomixis.
A generative apology is a type of apomixis in which embryo develops from diploid cells of embryo sac which is formed from the diploid cells of archesporium without meiotic division, whereas in somatic apomixis, an embryo develops from the diploid cells of the nucellus of the integuments.
Write two reasons of polyembryony.
Polyembryony may be due to:
- Cleavage of zygote or proembryo.
- Formation of more than one embryosac in one ovule.
Explain cleavage polyembryony.
Abnormal division in the oospore results in the formation of a mass of cells and cleavage in these may lead to the formation of more than one embryo. The proembryo may develop buds or outgrowths and this may lead to the formation of more than one embryo. In some cases the proembryo may show branching and embryo may begin to develop from each branch, resulting into polyembryony.
Explain the importance of micropropagation.
- It is a method of rapidly producing a population of genetically similar individuals.
- It can be used to produce disease-free plants.
- This is the only viable method of regenerating genetically modified cells.
- Greater number of plants can be developed in a small area.
RBSE Class 12 Biology Chapter 4 Essay Type Questions
Write a note on apomixis.
Apomixis is a special method of plant reproduction. In this method, the angiosperm plant reproduces without involving steps of usual sexual reproduction i.e. meiosis and fertilization. It can be sporophytic or gametophytic and non-recurrent or recurrent.
According to Hein’s Winkler (1908), the substitution of usual sexual reproduction by a form of reproduction which does not involve meiosis and syngamy is called apomixis.
According to some embryologists, vegetative propagation and agamospermy are also included under the category of apomixis.
According to valid definition apomixis is generally of two types:
- Sporophytic apomixis: This is also called adventive embryonic. In this apomixis, the embryo is formed from nucellus or integument or any diploid cell of the ovule. There is no meiosis and fertilization.
- Gametophytic apomixis: In this apomixis, an embryo develops from any cell of embryo-sac. It is of two types:
- When an embryo develops from the unfertilized egg then this process is known as parthenogenesis.
- When an embryo develops from any other haploid cell of embryo sacs such as accessory cells or synergids, then this process is called as apogamy.
Prof. Panchanan Maheshwari (1950) divided apomixis into following types as Non-recurrent apomixis and Recurrent apomixis. These are further divided into many types. We will study four types in brief.
- Non-recurrent Apomixis
- Recurrent Apomixis
- Adventive Embryonic
- Vegetative Apomixis
1. Non-recurrent Apomixis:
Non-recurrent apomixis is where an embryo arises directly from a normal egg cell without the process of fertilization. Egg cells are haploid and hence, the resulting embryo is also haploid. Non-recurrent apomixis is called haploid parthenogenesis.
This process was first of all studied by Jorgensen in 1928. If Embryo develops from any other cell of female gametophyte other than egg cell it is called haploid apogamy or endogamy. These types of apomixis are rare and are of genetic interest. This apomixis is found in Datura and Maize.
2. Recurrent Apomixis:
This apomixis is also called gametophytic apomixis. As meiosis is not completed. So in this apomixis, female gametophyte has chromosome number diploid as of mother plant.
It is of two types:
- Generative Aposopory: In this type of apomixis embryo sac develops from the diploid cell of archesporium. The diploid cell of embryo sac forms a diploid embryo.
- Somatic Apospory: In this apomixis, diploid cells of nucellus and integuments from diploid embryo sac and this diploid embryo sac forms a diploid embryo.
3. Adventive Embryonic:
In this process of embryo development, an embryo develops from any diploid cell of ovule outside of embryo sac.
Example: From nucellus or integumentary cells. This process is known as adventive embryony. In addition to the adventive embryonic process, the regular embryo within the embryo sac may also develop simultaneously resulting in polyembryony condition.
Example: Lemon, Mango, Opuntia.
4. Vegetative Apomixis:
In some plants like Propagules, some Allium, Agave and some grass species, vegetative buds (Bulbil and Propagules) are produced instead of flowers in the inflorescence. They germinate while on the plant itself. They can be reproduced without difficulty.
Importance of Apomixis:
- Apomixis does not involve meiosis so that there is no segregation or recombination. Plants obtained through this process are exactly similar to the mother plant. It is an effective tool to produce a clone.
- Apomixis results in rapid multiplication of genetically similar individuals without risk of segregation.
- Heterosis or hybrid vigour can be maintained permanently in crop plants.
- There can be an efficient exploitation of maternal effect from generation to generation.
- Homozygous inbred lines can be rapidly developed as they produce sectors of diploid tissues and occasional fertile gametes and seeds.
- This method is useful to produce hybrid seeds because it prevents loss of specific features in them.
Write a detailed account of polyembryony.
Development of more than one embryo in a seed is called polyembryony. It can be cleavage polyembryony or adventive polyembryony depending upon how is it caused.
Normally in plants, one seed contains only one embryo but sometimes more than one embryo develops in a single seed. Development of more than one embryo from ovule, seed or fertilized ovum is called polyembryony. Polyembryony is found in both plants and animals.
In 1719, Antony Von Leeuwenhoek first described polyembryony in Citrus fruit (Orange). It is a common feature in gymnosperms whereas it is found only in few genera of angiosperms such as lemon, blackberry, tobacco, Allium, Crotalaria etc. In most cases of polyembryony, one embryo only matures in one seed and other embryos are destroyed during embryo development.
Some plants such as Vanda, which is an epiphyte, many mature embryos are found at the time of seed germination.
Causes of Polyembryony in Angiosperm:
Polyembryony is a general feature of gymnosperm but it can occur in angiosperms due to following reasons:
- Cleavage in proembryo.
- Development of any other cell of embryo sac along with egg cell.
- Development of more than one embryo sac is one ovule.
- Overactivity of sporophytic cells of ovules.
1. Cleavage in Polyembryo:
In this type of polyembryony, there is cleavage of zygote or proembryo, which leads to the development of more than one embryo. In Orchid, Eulophia, there are three types of cleavage polyembryony.
- Abnormal division in oospore results in the formation of cellular mass, and cells towards chalazal end increase in number and form many embryones.
- There is the formation of buds or outgrowth from proembryo and this leads to the formation of many embryones.
- Filamentous Embryo: In this polyembryony branching occurs in proembryo and embryo develops from each branch.
2. Development of Embryo from cells of embryo sac other than the egg cell:
In this polyembryony, fertilized or unfertilized accessory cells or synergids develop into an embryo. They may be diploid if develop from fertilized (Aristolochia brachiate) and haploid (Argemone mexicana) if develop from unfertilized cells.
3. Development of more than one embryo sacs in the ovule:
In some angiosperms plants such as Casuarina Montana etc. more than one embryo sacs develope in ovule and each egg cell in each embryo sac after fertilization forms an embryo. In this way, more than one embryo can develop in one ovule.
4. Adventive polyembryony:
Sometimes cells of nucellus or integument get activated and form an embryo. This type of developing embryo is called adventive embryo and process as adventive polyembryony.
Significance of Polyembryony:
Significance of polyembryony in horticulture, cell biology, genetics and plant-breeding are as follows:
- Adventive embryo developed from nucellus are exactly similar to the mother plant.
- Plants produced from embryos developed from the cells of nucellus are full of vigour.
- Embryos developed from nucellus are disease-free.
For example, Virus-free clone of lemons is produced by polyembryony.
- The haploid embryo has its significance in cell biology.
- These haploid embryos can be treated with colchicine chemical to produce homozygous lines which have its importance in plant breeding experiment.