Reproduction in Plants Flashcards
describe the structure of
the anther
Annotated diagrams
required
Structure of the Anther:
Filament:
The filament is a slender stalk-like structure that supports the anther and positions it above the rest of the flower.
It provides a pathway for the pollen to be dispersed away from the anther.
Lobes:
The anther typically consists of two lobes or sacs, each known as a microsporangium (plural: microsporangia).
These lobes are usually attached to the filament by a connective tissue region.
Microsporangia (Pollen Sac):
Each lobe contains one or more microsporangia, which are the structures where pollen grains are produced.
Within the microsporangium, microspore mother cells undergo meiosis to produce haploid microspores, which then develop into pollen grains.
Connective Tissue:
The connective tissue region connects the lobes of the anther to the filament.
It may contain vascular tissue that supplies nutrients and water to the developing pollen grains.
Epidermis:
The outermost layer of cells covering the surface of the anther is the epidermis.
The epidermis protects the internal tissues of the anther and helps regulate gas exchange.
describe the formation of pollen grains
Annotated diagrams
required
Process of Pollen Grain Formation (Microsporogenesis):
Microspore Mother Cell (Microsporocyte) Formation:
Within each microsporangium of the anther, diploid microspore mother cells (also called microsporocytes) undergo differentiation.
The microspore mother cells are located within the sporogenous tissue of the anther.
Meiosis (Reduction Division):
Each microspore mother cell undergoes meiosis to produce four haploid microspores.
Meiosis consists of two consecutive divisions: meiosis I and meiosis II.
Meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids.
Tetrads of Microspores:
After meiosis, each microspore mother cell gives rise to a tetrad of four haploid microspores.
The microspores are initially connected by a layer of callose, a protective wall material.
Microspore Wall Formation:
Each microspore develops its own unique wall, which consists of two layers: the inner layer (intine) and the outer layer (exine).
The intine is composed of cellulose and pectin and is responsible for protecting the developing male gametophyte (pollen grain).
The exine is made of sporopollenin, a tough and resistant material that provides structural support and protection.
Pollen Grain Development:
As the microspore wall forms, the contents of the microspore undergo changes, including the development of two unequal cells: the generative cell and the tube cell.
The generative cell will give rise to the two sperm cells (male gametes) within the mature pollen grain.
The tube cell will form the pollen tube, which is essential for delivering the sperm cells to the female reproductive organs during fertilization.
describe the structure of
the ovule and the formation
of the embryo sac
Annotated diagrams
required
Structure of the Ovule:
Integuments:
The integuments are protective layers that surround the ovule.
They originate from the tissue layers of the ovary wall and enclose the nucellus (central region of the ovule) except for a small opening called the micropyle.
Nucellus:
The nucellus is the central region of the ovule containing the female gametophyte (embryo sac).
It consists of haploid cells and tissues that will give rise to the embryo sac.
Formation of the Embryo Sac (Megagametogenesis):
Megaspore Mother Cell Formation:
Within the nucellus, diploid megaspore mother cells (also called megasporocytes) undergo differentiation.
These cells are typically located deep within the nucellus, away from the micropyle.
Meiosis (Reduction Division):
Each megaspore mother cell undergoes meiosis to produce four haploid megaspores.
Meiosis consists of two consecutive divisions: meiosis I and meiosis II.
Degeneration of Three Megaspores:
In most angiosperms, three of the four megaspores degenerate, leaving only one functional megaspore within the ovule.
The degeneration of three megaspores ensures that only one functional megaspore contributes to the formation of the embryo sac.
Formation of the Embryo Sac (Female Gametophyte):
The functional megaspore undergoes multiple rounds of mitotic divisions (without cytokinesis), leading to the formation of the mature embryo sac (female gametophyte).
The mature embryo sac typically consists of seven cells arranged into three distinct regions: the egg apparatus, the central cell, and the antipodal cells.
explain the sequence of
events from pollination to
fertilization
Annotated diagrams
required
Sequence of Events from Pollination to Fertilization:
Pollination:
Pollination is the transfer of pollen grains from the anther of a stamen to the stigma of a pistil (or to the receptive surface of a compatible flower).
Pollination can occur through various mechanisms, including wind, water, animals (such as insects, birds, or mammals), or self-pollination.
Once a pollen grain lands on a compatible stigma, it may germinate and begin to grow a pollen tube.
Pollen Germination and Tube Formation:
Upon landing on the stigma, a pollen grain hydrates and germinates, forming a pollen tube.
The pollen tube grows down through the style (in the pistil) towards the ovary, guided by chemical signals from the pistil tissues.
Ovule and Embryo Sac Preparation:
Meanwhile, within the ovary, the ovule undergoes development, and the embryo sac (female gametophyte) forms from a megaspore.
The embryo sac typically contains the egg apparatus (comprising the egg cell and two synergids), a central cell with two polar nuclei, and three antipodal cells.
Pollen Tube Growth and Entry into the Ovule:
The pollen tube continues to elongate and eventually reaches the micropyle of the ovule.
The tip of the pollen tube releases sperm cells (male gametes) into the embryo sac.
Double Fertilization:
Once inside the embryo sac, one sperm cell fertilizes the egg cell, forming a zygote (2n).
Simultaneously, the other sperm cell fuses with the two polar nuclei of the central cell, forming a triploid cell (3n) called the primary endosperm nucleus (or central cell).
Zygote and Endosperm Development:
The zygote develops into the embryo, which eventually gives rise to the new plant.
The primary endosperm nucleus undergoes multiple divisions to form the endosperm, a nutrient-rich tissue that nourishes the developing embryo.
Seed Development:
The fertilized ovule develops into a seed, containing the embryo, endosperm, and protective seed coat.
The ovary matures into a fruit, which protects and disperses the seeds.
explain how cross-fertilization is promoted.
(Non-synchronous
maturation of stamens
(protogyny) and carpels
(protandry), separate
sexes (dioecy), insect
pollination, selfi-ncompatibility, and
sterility)
Cross-fertilization, also known as outbreeding, refers to the process where pollen from one individual is transferred to the stigma of a genetically different individual, leading to fertilization. This genetic diversity is beneficial for the offspring as it promotes genetic variation, which can enhance adaptation and survival. Several mechanisms promote cross-fertilization in plants:
Non-synchronous Maturation of Stamens and Carpels (Protogyny and Protandry):
Protogyny refers to the condition where the female reproductive organs (carpels) mature before the male reproductive organs (stamens).
Protandry is the opposite condition where the stamens mature before the carpels.
By having separate maturation times for male and female reproductive organs, plants reduce the likelihood of self-fertilization and promote cross-fertilization.
Separate Sexes (Dioecy):
Dioecy is a condition where individual plants have either male or female reproductive organs but not both.
In dioecious species, cross-fertilization is ensured because individuals cannot self-fertilize.
Dioecy promotes genetic diversity within populations by requiring pollination between different individuals.
Insect Pollination:
Many plants rely on insects, such as bees, butterflies, moths, and beetles, for pollination.
Insect-pollinated flowers often have adaptations such as bright colors, strong fragrances, and nectar rewards to attract insects.
Insects inadvertently transfer pollen from one flower to another while foraging for nectar, promoting cross-fertilization.
Self-Incompatibility:
Self-incompatibility mechanisms prevent self-fertilization by inhibiting the growth of pollen tubes or preventing the acceptance of self-pollen by the stigma.
Plants with self-incompatibility systems are unable to produce viable seeds when fertilized with their own pollen, thus promoting cross-fertilization.
Sterility:
Some plants exhibit sterility in their reproductive organs, either partially or completely.
Sterile stamens or carpels reduce the likelihood of self-fertilization and promote cross-fertilization by requiring pollen from other individuals for successful fertilization.
discuss the genetic
consequences of sexual
reproduction
(Self-fertilization and cross
fertilization)
. Self-Fertilization:
In self-fertilization, the fusion of gametes (sperm and egg) occurs within the same individual, leading to the production of offspring that are genetically identical or nearly identical to the parent.
Genetic consequences of self-fertilization include:
Loss of Genetic Diversity: Self-fertilization leads to a reduction in genetic diversity within populations because there is no introduction of new genetic variation from other individuals.
Increased Homozygosity: Homozygosity increases in offspring due to the inheritance of identical alleles from both parents. This can lead to the expression of deleterious recessive alleles and a decrease in overall fitness.
Fixation of Alleles: Self-fertilization can lead to the fixation of alleles within populations, where a single allele becomes the only variant present at a particular genetic locus.
2. Cross-Fertilization:
Cross-fertilization involves the fusion of gametes from genetically different individuals, leading to offspring with a combination of genetic material from both parents.
Genetic consequences of cross-fertilization include:
Increased Genetic Diversity: Cross-fertilization promotes genetic diversity within populations by introducing new combinations of alleles from different individuals. This enhances the adaptive potential of populations by providing a broader range of genetic variation for natural selection to act upon.
Heterozygosity: Offspring resulting from cross-fertilization are more likely to be heterozygous at genetic loci, which can confer advantages such as increased resistance to diseases and environmental stressors.
Recombination: Cross-fertilization facilitates genetic recombination during meiosis, where homologous chromosomes exchange genetic material through crossing over. Recombination generates new combinations of alleles on chromosomes, further increasing genetic diversity.
Comparison:
Self-fertilization tends to maintain the genetic composition of populations over time, leading to populations that are more genetically uniform and potentially less adaptable to changing environmental conditions.
Cross-fertilization, on the other hand, promotes genetic variation within populations, increasing the potential for adaptation and evolutionary change
explain the significance of
double fertilization in the
embryo sac
Double fertilization is a unique reproductive feature found in angiosperms (flowering plants), where two sperm cells are involved in the fertilization process within the embryo sac of the ovule. This process has several significant consequences and implications for plant development and reproduction:
Formation of the Embryo and Endosperm:
One sperm cell fertilizes the egg cell, resulting in the formation of a diploid zygote, which develops into the embryo.
Simultaneously, the other sperm cell fuses with two polar nuclei within the central cell, forming a triploid primary endosperm nucleus (3n).
The zygote gives rise to the new plant embryo, while the primary endosperm nucleus develops into the endosperm, a nutrient-rich tissue that supports embryo growth and development.
Nutrient Provision:
The endosperm serves as a nutrient reservoir for the developing embryo, providing essential nutrients such as carbohydrates, proteins, and lipids.
The triploid nature of the endosperm ensures that it contains a balanced genetic contribution from both parental gametes, optimizing nutrient provisioning for the developing embryo.
Genetic Regulation:
Double fertilization allows for precise genetic regulation and coordination between embryo and endosperm development.
The zygote and endosperm have different genetic constitutions (diploid and triploid, respectively), which regulate their developmental pathways and functions.
The endosperm undergoes multiple rounds of mitotic divisions to produce a multinucleate cellular tissue, providing continuous nourishment to the growing embryo.
Seed Development and Germination:
The fertilized ovule develops into a seed, containing the embryo, endosperm, and protective seed coat.
The endosperm plays a crucial role in seed development and germination by providing energy and nutrients for seedling growth after germination.
The embryo develops within the seed, utilizing nutrients stored in the endosperm for growth and development until it becomes self-sufficient through photosynthesis.
Adaptation and Evolution:
Double fertilization contributes to the reproductive success and evolutionary adaptation of angiosperms by ensuring efficient nutrient provisioning and seed development.
The co-evolution of double fertilization with other reproductive features, such as insect pollination and fruit formation, has facilitated the diversification and ecological success of flowering plants.
discuss the development of
the seed and the fruit from
the embryo sac and its
contents, the ovule and the
ovary
Fertilization and Seed Formation:
After double fertilization within the embryo sac, the zygote and the primary endosperm nucleus (triploid) initiate the development of the seed.
The zygote develops into the embryo, which consists of the embryonic axis (radicle, hypocotyl, and epicotyl) and one or two cotyledons (seed leaves).
The endosperm, derived from the primary endosperm nucleus, provides nutrients and support for embryo development. In some species, such as monocots, the endosperm persists in the mature seed and serves as a storage tissue.
The integuments of the ovule develop into the seed coat, which surrounds and protects the embryo and endosperm.
Maturation and Desiccation:
As the embryo and endosperm develop, the seed undergoes maturation and desiccation, during which water content decreases and the seed becomes dormant.
Maturation is accompanied by the accumulation of storage reserves, such as starch, proteins, and lipids, within the endosperm or cotyledons. These reserves provide energy and nutrients for germination and early seedling growth.
The seed coat hardens and becomes impermeable to water and pathogens, providing protection during dormancy and dispersal.
Fruit Development:
Concurrently with seed development, the ovary undergoes transformation into a fruit through a process called fruit ripening or maturation.
Fruit development involves changes in the ovary wall, including enlargement, differentiation, and the accumulation of sugars, pigments, and secondary metabolites.
Depending on the type of fruit, the ovary wall may develop into different layers, such as the exocarp (outer skin), mesocarp (fleshy or fibrous tissue), and endocarp (inner boundary around the seeds).
Fruits serve various functions, including protecting the seeds, aiding in seed dispersal, attracting seed dispersers (e.g., animals), and providing additional nutrients and protection for developing seeds.
Seed Dispersal and Germination:
Once mature, fruits facilitate seed dispersal through various mechanisms, such as wind, water, animals (endozoochory or epizoochory), or explosive mechanisms.
Upon dispersal, seeds may undergo dormancy, a period of arrested growth and development, until conditions are favorable for germination.
Germination begins when a viable seed absorbs water and swells, activating metabolic processes and embryo growth.
The radicle emerges first, followed by the hypocotyl and epicotyl, which give rise to the root system and shoot system, respectively.
Cotyledons or the first true leaves emerge, and the seedling establishes itself and begins photosynthesis to sustain growth and development.
discuss the advantages and
disadvantages of asexual
reproduction
(Explanation of the term
asexual reproduction.
Relate binary fission,
budding, asexual spore
formation, fragmentation
to asexual reproduction in
plants, for example,
ginger, meristems,
hormone stimulation,
details of the processes
involved in tissue culture
and the production of
cuttings.)
Advantages of Asexual Reproduction:
Efficiency: Asexual reproduction can be a very efficient means of reproduction because it does not require the time and energy needed to find a mate or undergo pollination.
Rapid Population Growth: Asexual reproduction allows for rapid population growth under favorable conditions since each individual can produce numerous offspring without the need for a partner.
Conservation of Energy: Asexual reproduction conserves energy that would otherwise be expended on courtship behaviors, mate attraction, and mating rituals.
Genetic Uniformity: Offspring produced through asexual reproduction are genetically identical to the parent. This can be advantageous in stable environments where the parent’s traits are well-adapted to prevailing conditions.
Disadvantages of Asexual Reproduction:
Lack of Genetic Diversity: Asexual reproduction produces genetically identical offspring, leading to reduced genetic diversity within populations. This lack of genetic variation can limit the ability of a population to adapt to changing environmental conditions.
Increased Susceptibility to Disease: Because asexual reproduction produces genetically identical offspring, entire populations may be susceptible to the same diseases or pests. A single pathogen or pest could potentially wipe out an entire population.
Accumulation of Deleterious Mutations: Asexual reproduction does not allow for the reshuffling of genetic material through recombination, which can lead to the accumulation of deleterious mutations over time.
Limited Dispersal: Asexual reproduction typically results in offspring that remain close to the parent organism. This limited dispersal can lead to competition for resources and increased vulnerability to environmental fluctuations.
Examples of Asexual Reproduction in Plants:
Binary Fission: Commonly observed in bacteria and some single-celled organisms, where the parent cell divides into two identical daughter cells.
Budding: Seen in organisms like yeast and Hydra, where a small outgrowth, or bud, develops on the parent organism and eventually detaches to become a new individual.
Asexual Spore Formation: Many fungi, algae, and some plants reproduce asexually by producing spores that develop into new individuals without the need for fertilization.
Fragmentation: Some plants, like certain species of algae and ferns, reproduce asexually by breaking apart into fragments, with each fragment capable of growing into a new individual.
Tissue Culture and Cuttings: In tissue culture, plant cells are grown in vitro under controlled conditions to produce genetically identical clones. Cuttings involve the removal of a portion of a plant, such as a stem or leaf, which is then encouraged to develop roots and grow into a new plant.
Key terms for reproduction
