Biology Test - 1

Concept:

• Pollen grains are the male gametophytes of higher plants.
• They develop from microspores that are formed from pollen mother cells by meiosis.
• Thus pollen grains are haploid in nature.
• They are mostly spherical in shape and about 20-25µm in diameter.
• They have 2 prominent wall layers:
  • Exine - It is the outer hard layer made up of sporopollenin.
  • Intine - It is the inner layer made up of cellulose and pectin.
• Germ pores - These are apertures on the exine, formed by the absence of sporopollenin in those areas.

Important Points

• Sporopollenin is known to be the most resistant organic material.
• It can tolerate high temperatures as well as strong acids or alkali.
• It can not be degraded by any known enzyme.
• It protects the pollen grains from adverse conditions.
• Presence of sporopollenin helps in preserving pollen grains as fossils.
• It is present in the exine layer of pollen grains.
• The absence of sporopollenin in the germ pores allows growth of pollen tube through the exine.

Key Points

• In angiosperms, the male reproductive organs are called stamens which consists of anthers.
• Anthers are usually bilobed and dithecous.
• Anthers consist of 4 microsporangia organized into 2 theca.
• These microsporangia are microspore-bearing structures that develop into pollen sacs.
• At maturity, the septum between 2 microsporangia disintegrates and the theca opens by a longitudinal slit.

Important Points

• A cross-section of anther shows the following:
  • Outer anther wall - consists of several wall layers that are mainly protective in function.
  • Inner sporogenous tissue - develop into microspore or pollen mother cells (PMC), which may undergo meiosis to form microspores.
• The anther wall layers can be broadly divided into 4 layers (from outer to inner):
  • Epidermis - It is the outermost cell layer of the anther.
  • Endothecium - It lies just inside the epidermis and consists of enlarged cells with secondary wall thickenings that help in anther dehiscence.
  • Middle layers - They are additional cell layers between endothecium and tapetum that are present in some species.
  • Tapetum - It is the innermost cell layer containing metabolically active cells that surrounds the developing microspores.
• The tapetum is nutritive, while the other wall layers are protective in function.

Therefore, the correct outer to inner sequence of the wall layers is:

Epidermis → Endothecium → Middle layers → Tapetum

• Plants use various agents for pollination.
• The agents of pollination may be:
  • Biotic - Living organisms like bees, wasps, butterflies, birds or even small animals like rodents.
  • Abiotic - Non-living agents like wind and water.
• Plants produce morphologically different flowers for different types of pollination.
• Each type of agent requires different features in the pollens to effectively cause pollination.

Important Points

• Wind pollination is the most common among abiotic pollinations.
• Features of wind pollinated flowers:
  • Pollens are light and non-sticky to be carried by wind currents.
  • Stamens are well-exposed so that pollens are easily dispersed by the wind.
  • Stigma is usually large and feathery to trap the pollens from the air.
  • Flowers usually have a single ovule in each ovary and numerous flowers are packed into an inflorescence.
• Example - Corn tassels represent feathery style that wave in the wind to trap pollens.
• It is also common in grasses.

Additional Information

• Water pollination -
  • The flowers use water currents for pollination.
  • Flowers may emerge on to surface to release the pollens which are then carried to the stigma by water currents. E.g.- Vallisneria.
  • In some plants, flowers may remain submerged and release long ribbon-like pollens that have a mucilaginous covering. E.g.- Zostera.
• Biotic pollination -
  • It is carried out by bees, flies, moths, wasps, butterflies, birds, bats or small animals.
  • Pollens are usually sticky or spiny to get stuck to the animal bodies.
  • Flowers are large, colourful and fragrant to attract the agents.
  • Flowers offer floral rewards in the form of nectar so that the agents visit repeatedly.
  • Some flowers may also provide a safe place to lay eggs in return for pollination.
  • Example - Yucca plant provides a place to lay eggs to the moth that pollinates it.

Key Points

• In Angiosperms, embryo sac is formed from a single megaspore.
• The embryo sac represents the female gametophyte in the plant.
• The embryo sac is 8-nucleate and 7-celled at maturity.
• It consists of the following cells:
  • Antipodal cells - There are 3 of these uninucleate cells present at the chalazal end that degenerate after fertilization.
  • Central cell - It is centrally located covering most of the embryo sac and contains 2 polar nuclei.
  • Egg cell - It is the female gamete that undergoes fusion with a male gamete.
  • Synergids - There are 2 synergid cells at the micropylar end with the filiform apparatus at their base.
• Filiform apparatus -
  • It consists of finger-like projections that assist the entry of pollen tube into the embryo sac.
  • It is formed by cell thickenings of the synergids at the micropylar tip of the embryo sac.

Additional Information

• Egg apparatus - It refers to the 3 cells, 2 synergids and 1 egg cell, grouped together towards the micropylar end of the embryo sac.
• Chalaza - It is the tissue that joins the nucellus and integuments in the embryo sac, at the opposite end of the micropyle.
• Antipodals - These are the 3 uninucleate cells at the chalazal end of the embryo sac.

• Asexual reproduction is a mode of reproduction in which new offspring are produced by a single parent without the involvement of gametes or fertilization.
• Offspring produced by such reproduction are genetically identical copies of the parents.
• Asexual reproduction is of the following types:
• Binary fission
• Budding
• Fragmentation
• Gemma-cup formation
• Spore formation
• Vegetative propagation
• Gemmule formation

​Explanation:

• Gemmules are small internal buds that are formed in Sponges. These are modes of asexual reproduction in sponges
• The gemmules are capable of producing the entire organism.
• The gemmules are thick-walled and they help the sponges to withstand unfavorable conditions, on the onset of favorable conditions the gemmules germinate to form the offspring

                Gemmules in sponges​

Key Points

• Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
• Pollination can be divided into 3 types:

1. Autogamy -
   • It is the transfer of pollen grains from the anther to the stigma of the same flower.
   • It requires the pollen release and stigma receptivity to be synchronous, to prevent cross-pollination.
   • The anthers and stigma lie close to each other to assure self-pollination.
   • The autogamous flowers can be of 2 types - chasmogamous and cleistogamous.
   • Chasmogamous flowers have normal flowers with exposed anther and stigma.
   • Cleistogamous flowers do not open at all, thus assuring self-pollination and seed set.
   • Some plants like Oxalis, Viola and Commelina produce both chasmogamous and cleistogamuos flowers.
2. Geitonogamy -
   • It is the transfer of pollen grains from the anther to the stigma of a different flower of the same plant.
   • It is functionally cross-pollination as pollinating agents are involved.
   • It is genetically self-pollination as both male and female gametes come from the same plant.
   • Example - Maize, Castor.
3. Xenogamy -
   • It is the transfer of pollen grains to the stigma of different plant.
   • It causes genetic variation and is a type of cross-pollination.
   • It is always found in unisexual plants where male and female flowers are borne on different plants.
   • Example - Papaya.

• Therefore, Helianthus is the correct answer.

Key Points

• Outbreeding devices refer to the adaptive methods of plants that prevent self-pollination and encourage cross-pollination.
• This is to prevent continued self-pollination, which may result in inbreeding depression.
• Some of the outbreeding devices are:
  • Self-incompatibility - Pollen germination and pollen tube growth is inhibited on the same plant.
  • Dicliny - It is the production of dioecious or unisexual flowers, where male and female flowers are produced on different plants. It prevents both autogamy and geitonogamy.
  • Protogyny & Protandry - These are processes where pollen and stigma maturity are not synchronized. Pistil develops before anther in protogyny and vice versa in protandry.
  • Heterostyly - Stigma is placed at a higher position than the anthers, so that pollens do not fall on them.
• Production of monoecious flowers means both male and female flowers are borne on the same plant.
• This may prevent autogamy but not geitonogamy, which is also self-pollination.
• Therefore, production of monoecious flowers is not an effective outbreeding device for plants.

Additional Information

Pollination can be divided into 3 types:

• Autogamy -
  • It is the transfer of pollen grains from the anther to the stigma of the same flower.
  • It requires the pollen release and stigma receptivity to be synchronous, to prevent cross-pollination.
  • The anthers and stigma lie close to each other to assure self-pollination.
  • The autogamous flowers can be of 2 types - chasmogamous and cleistogamous.
  • Chasmogamous flowers have normal flowers with exposed anther and stigma.
  • Cleistogamous flowers do not open at all, thus assuring self-pollination and seed set.
• Geitonogamy -
  • It is the transfer of pollen grains from the anther to the stigma of a different flower of the same plant.
  • It is functionally cross-pollination as pollinating agents are involved.
  • It is genetically self-pollination as both male and female gametes come from the same plant.
  • Example - Maize, Castor.
• Xenogamy -
  • It is the transfer of pollen grains to the stigma of different plant.
  • It causes genetic variation and is a type of cross-pollination.
  • It is always found in unisexual plants where male and female flowers are borne on different plants.
  • Example - Papaya.

Concept:

• The most important and unique characteristic of angiosperms is the participation of both male gametes in the act of fertilization. One male gamete fuses with the egg cell and forms the diploid Zygote. The process is called syngamy or generative fertilization. The  Zygote (2n)  develops into an embryo.
• The other male gamete fuses with two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is known as a triple fusion or Vegetative fertilization. 
• These two actions of fertilization are together called Double Fertilization.

Explanation:

• The endosperm is the food-laden tissue formed during the development of the angiospermous seed. In angiosperms, the endosperm develops from a triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion, or the fusion of a male gamete with the secondary nucleus of the central cell.
• It provides essential nutrients to the growing embryo and also to the young seedling at the time of seed germination. So for this reason it precedes the development of zygote (embryo) in angiosperms.

Additional Information

• The zygote remains dormant for a short period in a fertilized ovule as it waits for the development of endosperm so that it can derive nutrition from it.

Concept:

• Reproduction is the biological process in which the organism gives rise to young ones or offsprings similar to itself.
• It is responsible for the continuity of species generation after generation.
• Reproduction is mainly divided into two types:

1. Sexual reproduction: 

• Involves the formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex.
• These gametes fuse to form the zygote which develops to form the new organism.
• It is an elaborate, complex, and slow process as compared to asexual reproduction. (E.g. Mammals).
• The offsprings show characteristics of both male and female parents and show variation.

       2. Asexual reproduction:

• The reproduction in which an offspring is produced by a single parent without the fusion of gametes is called asexual reproduction. (E.g. Amoeba, Hydra)
• The offsprings are exact replica of their parents and genetically identical.
• Asexual reproduction is of many types: Sporulation, Fission, Fragmentation. Budding, Plasmotomy, vegetative propagation, Apomixis & Parthenogenesis

​

Explanation:

• Apomixis: ''apo'' = without & ''mixis'' = mixing. It is a form of asexual reproduction that results in the formation of seeds without fertilization (i.e without the fusion of male and female gametes). It is seen in plants of species Asteraceae & grasses.
• It is a form of asexual reproduction that mimics sexual reproduction.

Additional Information

Key Points

• After fertilization, the ovary develops into fruit and the ovules develop into the seeds.
• A seed consists of:
  • Seed coat - It can be divided into the outer testa and inner tegmen.
  • Embryo - It consists of the radicle, plumule, embryonal axis and cotyledon(s).
• Post fertilization, the endosperm develops along with the zygote.
• The primary function of endosperm is providing nutrition to the developing embryo.
• After the embryo matures, the endosperm may or may not be retained in mature seeds.
• Based on the above factor, seeds are divided into 2 groups:
  • Endospermous - The endosperm is present in the mature seeds. Example - Maize, coffee, castor.
  • Non-endospermous - The endosperm is not present in the mature seeds. Example - Pea, gram, beans.
• Monocots are usually endospermous with only a few exceptions.
• The endosperm in monocots is usually bulky and stores food.

Important Points