NCERT Exemplar Sexual Reproduction in Flowering Plants - 2 - Biology Class 12 - NEET

SHORT ANSWER TYPE QUESTIONS

Q.1. List three strategies that a bisexual chasmogamous flower can evolve to prevent self pollination (autogamy).

Ans. In bisexual chasmogamous flowers, self-pollination can be prevented by the following strategies:

1. Non-synchronisation of pollen release and stigma receptivity
   In some flowers, pollen is released before the stigma becomes receptive, or the stigma becomes receptive before pollen release. This lack of synchrony prevents self-pollination.

2. Different positioning of anther and stigma
   In some flowers, the anther and stigma are placed at different positions so that pollen cannot come in contact with the stigma of the same flower.

3. Self-incompatibility
   Self-incompatibility is a genetic mechanism that prevents pollen from the same flower or from other flowers of the same plant from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Q.2. Given below are the events that are observed in an artificial hybridization programme. Arrange them in the correct sequential order in which they are followed in the hybridisation programme.

(a) Re-bagging
(b) Selection of parents
(c) Bagging
(d) Dusting the pollen on stigma
(e) Emasculation
(f) Collection of pollen from male parent.

Ans. Correct sequence: (b) Selection of parents → (e) Emasculation → (c) Bagging → (f) Collection of pollen from male parent → (d) Dusting the pollen on stigma → (a) Re-bagging.

Explanation: Select appropriate parents first. Remove anthers (emasculation) from the flower chosen as female before they shed pollen. Immediately cover (bag) the emasculated flower to prevent unwanted pollen. Collect pollen from the male parent when it is viable and then dust it on the stigma of the emasculated flower. Finally, re-bag the pollinated flower to prevent contamination until fertilisation is assured.

Q.3. Vivipary automatically limits the number of offsprings in a litter. How?

Ans. In viviparous animals (most placental mammals, including humans), the embryo develops inside the female's body where space and maternal resources are limited. Because each developing embryo requires a substantial amount of maternal space, nutrients and physiological support, the female can carry only a limited number of embryos to term at one time. Thus vivipary naturally restricts the maximum number of offspring per pregnancy or litter.

Q.4. Does self incompatibility impose any restrictions on autogamy? Give reasons and suggest the method of pollination in such plants.

Ans. Yes. Self-incompatibility is a genetic mechanism that prevents self-fertilisation by rejecting pollen that is genetically similar to the pistil. It therefore restricts autogamy (self-pollination) because self-pollen either fails to germinate on the stigma or the pollen tube growth is inhibited in the style. To achieve fertilisation in self-incompatible plants, cross-pollination from genetically different individuals (xenogamy) is necessary. In practical terms, pollination by pollinators carrying pollen from other plants, or deliberate human-assisted cross-pollination (artificial hybridisation), overcomes the restriction. Mixed pollination (i.e., ensuring pollen from different genotypes reaches the stigma) is often used to obtain seeds in self-incompatible crops.

Q.5. In the given diagram, write the names of parts shown with lines.

Ans.

L.S. of an embryo of grass

Q.6. What is polyembryony and how can it be commercially exploited?

Ans. Polyembryony is the occurrence of more than one embryo in a single seed. In many species of Citrus and some varieties of Mango, nucellar cells (diploid maternal cells surrounding the embryo sac) may divide and give rise to additional embryos that develop alongside the zygotic embryo; these are called nucellar embryos. As a result, a single seed may give rise to several seedlings.

Commercial exploitation: Nucellar polyembryony produces seedlings that are genetically identical to the mother plant (clonal), which is useful for producing uniform, true-to-type plants without genetic segregation. If desirable hybrids can be made apomictic (i.e., produce clonal seeds by apomixis or nucellar embryony), farmers can reuse the same hybrid seeds in successive seasons and obtain uniform crops without buying new hybrid seeds every year. Polyembryony is therefore valuable for clonal propagation, rapid multiplication of elite genotypes, and rootstock production in horticulture.

Q.7. Are parthenocarpy and apomixis different phenomena? Discuss their benefits.

Hint: Yes, they are different. Parthenocarpy leads to development of seedless fruits. Apomixis leads to embryo development.

Ans. Yes, they are different phenomena.

• Parthenocarpy is the development of fruit without fertilisation of the ovules. Parthenocarpic fruits are seedless (or have reduced seeds). Benefits include production of seedless fruits (e.g., seedless grapes, seedless bananas) which are often preferred by consumers, and avoidance of seed-formation problems under poor pollination conditions.
• Apomixis is the formation of an embryo without fertilisation, producing a seed that gives rise to a progeny genetically identical to the mother plant (clonal seed production). Benefits include fixation of hybrid vigour (heterosis) in seed-propagated crops, production of true-to-type progeny without genetic segregation, and reduced need for repeated hybrid seed production by breeders.

Q.8. Why does the zygote begin to divide only after the division of Primary endosperm cell (PEC)?

Ans. After double fertilisation in angiosperms, one sperm nucleus fuses with the egg cell to form the zygote and the other sperm nucleus fuses with the two polar nuclei to form the primary endosperm cell (PEC). The PEC normally divides and produces endosperm tissue which serves as nutritive tissue for the developing embryo. Because the immature zygote has very limited food reserves, the endosperm begins to develop first to provide nourishment. Therefore, zygote division generally follows the initiation of endosperm development, ensuring a supply of nutrients for embryo growth.

Q.9. The generative cell of a two-celled pollen divides in the pollen tube but not in a three-celled pollen. Give reasons.

Ans. In a two-celled (bicellular) pollen grain, there are two cells: a vegetative (tube) cell and a generative cell. The generative cell is undivided at the time of pollen release and therefore divides later inside the pollen tube to form two male gametes (sperm cells). In a three-celled (tricellular) pollen grain, the generative cell has already divided within the pollen grain before pollen dispersal, producing two male gametes; therefore no further division of the generative cell is required in the pollen tube.

Q.10. In the figure given below label the following parts: male gametes, egg cell, polar nuclei, synergid and pollen tube.

Ans.

LONG ANSWER QUESTIONS

Q.1. Starting with the zygote, draw the diagrams of the different stages of embryo development in a dicot.

Ans.

Stages in embryo development in a dicot

Explanation: The diagram (above) typically shows the following stages in dicot embryogenesis: zygote, zygote undergoing first transverse division forming the basal and terminal cells, formation of the pro-embryo (octant stage), globular stage, heart-shaped stage (formation of two cotyledon primordia), torpedo stage and mature embryo with well-developed cotyledons and axis (radicle and plumule). The basal cell usually gives rise to the suspensor which anchors the embryo and helps in nutrient transfer; the terminal cell gives rise to the embryo proper.

Q.2. What are the possible types of pollinations in chasmogamous flowers. Give reasons.

Ans. A chasmogamous flower is a bisexual flower that opens and exposes its sexual organs (anthers and stigma). Such flowers can show the following types of pollination:

• Autogamy: Transfer of pollen from anther to stigma of the same flower. Although chasmogamous flowers open, self-pollination within the same flower can still occur if pollen falls onto the stigma. Continued autogamy may lead to inbreeding depression, which is why many mechanisms (dichogamy, heterostyly, self-incompatibility) reduce autogamy.
• Geitonogamy: Transfer of pollen from anther of one flower to stigma of another flower on the same plant. Even though genetically equivalent to autogamy, geitonogamy involves pollen transfer between different flowers and often requires a pollinator; it functions as a form of self-pollination at the plant level.
• Xenogamy: Transfer of pollen from anther of one plant to stigma of a flower on a different plant (cross-pollination). This is genetically the most favourable for maintaining variability and reducing inbreeding; chasmogamous flowers are adapted for xenogamy because their reproductive organs are exposed to pollinators and environmental agents (wind, insects, birds).

Q.3. With a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.

Ans.

The mature embryo sac

The typical mature (Polygonum-type) embryo sac is 7-celled and 8-nucleate and consists of:

• One egg cell located near the micropylar end; it fuses with a male gamete to form the zygote.
• Two synergids flanking the egg cell at the micropylar end; they help in guiding the pollen tube into the embryo sac and often degenerate after pollen-tube entry.
• Three antipodal cells at the chalazal end (usually ephemeral and short-lived in many species).
• One central cell containing two polar nuclei which fuse with the second male gamete to form the primary endosperm cell (PEC), leading to endosperm formation.

Role of synergids: The synergids possess a specialised structure called the filiform apparatus at their micropylar ends which increases the surface area and secretes chemical signals. The filiform apparatus guides and attracts the pollen tube towards the egg apparatus, facilitates entry of the pollen tube into the embryo sac, and helps in the release of male gametes. After pollen-tube entry, one or both synergids typically degenerate.

Q.4. Draw the diagram of a microsporangium and label its wall layers. Write briefly on the role of the endothecium.

Ans.

Enlarged view of one microsporangium showing wall layers

Structure and wall layers: A typical microsporangium (pollen sac) has a multilayered wall comprising, from outside to inside: epidermis, endothecium, middle layers (one or more), and tapetum (nutritive layer). The locule is the cavity where microspore mother cells (microsporocytes) undergo meiosis to form microspores which develop into pollen.

Role of the endothecium: Endothecium cells develop fibrous thickenings (lignified bands) which provide mechanical strength and assist in anther dehiscence. During maturation and drying, differential contraction of the endothecial thickenings helps the anther to split open (dehisce) along specific lines (stomium), releasing pollen grains.

Q.5. Embryo sacs of some apomictic species appear normal but contain diploid cells. Suggest a suitable explanation for the condition.

Ans. Many apomictic species produce embryo sacs that are morphologically similar to normal embryo sacs but are diploid rather than haploid. A likely explanation is that the megaspore mother cell (MMC) fails to undergo meiosis. If the MMC (which is diploid) undergoes mitosis instead of meiosis, all resulting nuclei and cells in the embryo sac will remain diploid. Such a diploid embryo sac can produce an embryo without fertilisation (apomixis), giving rise to clonal progeny.