Sexual Reproduction In Flowering Plants

1. Vegetative cell (large). 2. Two male gametes (small).

The male gametophyte in angiosperms develops within the pollen grain:
1. Microspore Formation: Each microspore undergoes mitotic division to form a small generative cell and a larger vegetative cell.
2. Mature Pollen Grain: The generative cell divides mitotically to produce two male gametes, resulting in a three-celled structure (two male gametes + one vegetative cell).

1. The scientific terms are:
   • Figure A — Cross-pollination
   • Figure B — Autogamy (a type of self-pollination)
2. Two conditions necessary for autogamy:
   • (i) Anther and stigma must mature simultaneously (synchronous maturation).
   • (ii) Anther and stigma must be positioned closely to facilitate transfer of pollen grains.
3. • [(i)] Advantage: Maintains genetic purity of the species.
     Disadvantage: Leads to reduced genetic variation, which may limit adaptability. OR
   • [(ii)] One such plant is Viola (commonly known as pansy).
     Reason: It shows cleistogamy, where some flowers do not open and ensure autogamy as the only mode of pollination.

(A)

• ‘Right’ type of pollen is compatible with the stigma and initiates pollen germination; ‘wrong’ type is incompatible and does not germinate.
• After a compatible (right) pollen lands, it germinates to form a pollen tube through which male gametes travel.
• The tube grows through the style, enters the ovule via the micropyle, and releases two male gametes.
• One male gamete fuses with the egg (syngamy), the other with two polar nuclei (triple fusion), resulting in double fertilization.
• Pollen-pistil interaction is dynamic as the pistil selects suitable pollen and promotes or inhibits pollen tube growth accordingly.

OR
(B)

1. Oogenesis schematic:

• Oogonium (2n) → Primary oocyte (2n) → Secondary oocyte (n) + First polar body (n) → Ovum (n) + Second polar body (n)

1. Location of events:

• I. Corpus luteum – Ovary
• II. Tertiary follicle maturation – Ovary
• III. Fertilisation – Fallopian tube
• IV. Implantation – Uterus

Step 1: The outbreeding device shown in the table is self-incompatibility. Definition: Self-incompatibility is a genetic mechanism that prevents self-pollen (or pollen from genetically similar individuals) from fertilizing the ovule by inhibiting pollen germination or pollen tube growth on the stigma of the same plant.
Step 2: Disadvantage of absence: In the absence of self-incompatibility, the plant would undergo self-pollination, which would lead to:

• Reduced genetic variability.
• Increased chances of inbreeding depression (accumulation of harmful alleles).
• Reduced adaptability of the species in changing environments.

(A):
Step 1: For (i), megasporogenesis in an angiosperm is the process of forming megaspores in the ovule. It begins with a diploid megaspore mother cell (MMC) in the nucellus of the ovule. The MMC undergoes meiosis to produce four haploid megaspores. Typically, three megaspores degenerate, and one functional megaspore survives, which then develops into the embryo sac through megagametogenesis.
Step 2: For (ii), a mature embryo sac (female gametophyte) of an angiosperm is a 7-celled, 8-nucleate structure. It consists of: (1) the egg apparatus at the micropylar end with one egg cell and two synergids, (2) three antipodal cells at the chalazal end, (3) a large central cell with two polar nuclei, and (4) the overall sac surrounded by the integuments of the ovule. The diagram would show an oval structure with these labeled parts: egg cell, synergids, central cell, and antipodal cells.
OR
(B):
Step 3: For (i), the four phases of the menstrual cycle in sequence are: (1) menstrual phase, (2) follicular phase, (3) ovulatory phase, and (4) luteal phase.
Step 4: For (ii), the menstrual phase typically lasts 3 to 5 days, during which the uterine lining sheds if no pregnancy occurs.
Step 5: For (iii), estrogen peaks during the late follicular phase (around day 12–14) to stimulate the growth of the uterine lining and trigger the LH surge, while progesterone peaks during the luteal phase (around day 21) to maintain the endometrium for potential pregnancy.
Step 6: For (iv), the LH surge, occurring around day 14, triggers ovulation by causing the mature follicle to rupture and release the egg, a critical step for reproduction.
Thus, (A) describes megasporogenesis and the embryo sac structure, while (B) details the menstrual cycle phases, their duration, hormone peaks, and the role of the LH surge.

(a) (i) Structure of a Typical Monocotyledonous Embryo:

A typical monocotyledonous embryo (such as in maize or grass) has the following key components:

• Single Cotyledon (Scutellum): Located on one side of the embryonic axis, it absorbs nutrients from the endosperm.
• Embryonic Axis: Differentiated into:
  • Plumule: Future shoot tip enclosed in a protective sheath called the coleoptile.
  • Radicle: Future root tip enclosed in the coleorhiza.
• Mesocotyl: The region between the scutellum and the plumule.

(ii) Polyembryony in Citrus Fruit:

In citrus fruits, multiple embryos often arise within a single seed. This happens because, in addition to the zygotic embryo formed through fertilization, other embryos can develop from the nucellar cells of the ovule.

This phenomenon is called: Adventive Polyembryony

It is a form of apomixis where embryos arise asexually from diploid somatic tissues of the ovule, bypassing meiosis and fertilization.

OR

(b) (i) Male Sex Accessory Ducts and Their Structure:

The male accessory ducts serve to store, transport, and mature sperm. The main ducts include:

• Rete Testis: Network of tubules that collect sperm from seminiferous tubules.
• Vasa Efferentia: Thin ducts that transport sperm from rete testis to the epididymis.
• Epididymis: A long, coiled duct where sperm mature and are stored temporarily.
• Vas Deferens: A muscular duct that transports sperm from the epididymis to the ejaculatory duct.
• Ejaculatory Duct: Formed by the union of vas deferens and duct of seminal vesicle; opens into the urethra and discharges sperm and seminal fluid during ejaculation.

(ii) Role of FSH in Spermatogenesis:

Follicle-Stimulating Hormone (FSH) is secreted by the anterior pituitary gland and plays a crucial role in regulating spermatogenesis.

Functions of FSH:

• Stimulates Sertoli cells in the seminiferous tubules.
• Promotes the production of nutrients and growth factors essential for the development of spermatogonia into spermatozoa.
• Works synergistically with LH (Luteinizing Hormone), which stimulates Leydig cells to produce testosterone, further promoting spermatogenesis.

(a) The mode of abiotic pollination depicted is anemophily (pollination by wind).
(b) Exposed large feathery stigmas are needed to effectively trap airborne pollen grains from the environment, increasing the chances of successful pollination.
(c) The two important adaptations of pollen grains for wind pollination are:

• Pollen grains are lightweight and non-sticky, so they can be easily carried by wind.
• Pollen is produced in large quantities to compensate for the randomness and inefficiency of wind dispersal.

(d) The petals are small and non-green because they do not need to attract pollinators (like insects or animals), as the plant relies on wind rather than biotic agents for pollination.