Sexual Reproduction In Flowering Plants

Step 1: Understanding the Concept:
In artificial hybridization, breeders want to ensure that only desired pollen grains are used for pollination.
Step 2: Detailed Explanation:
Definition: Emasculation is the process of removing the stamens or anthers from the flower bud of a female parent (if it is bisexual) using a pair of forceps.
Usefulness in Plant Breeding:
1. Prevents Self-Pollination: It ensures that the flower does not pollinate itself, which is crucial for creating specific hybrids.
2. Controlled Hybridization: It allows the breeder to pollinate the stigma with pollen grains collected from a specifically chosen male parent.
3. Genetic Purity: It maintains the genetic integrity of the experimental cross by eliminating unwanted natural pollination.
Step 3: Final Answer:
Emasculation involves removing anthers to prevent self-fertilization, allowing breeders to perform controlled crosses between desired parent plants.

Step 1: Understanding the Concept:
Many fruit-bearing trees, including apples, are entomophilous, meaning they require biotic agents (insects) for pollination.
Step 2: Detailed Explanation:
Bees are the primary pollinators for apple orchards.
A decrease in the bee population due to pesticides prevents the transfer of pollen from the anther to the stigma, leading to a failure in fertilization and subsequent fruit development.
The logical solution is to stop using harmful pesticides and restore the pollinator population by placing bee hives (apiculture) in the orchard during the flowering season.
Step 3: Final Answer:
Insect pollination is the affected process, and the remedy is reducing pesticide use and introducing hives.

i. Identification: Based on a standard 7-celled, 8-nucleate embryo sac:
A: Antipodal cells (located at the chalazal end).
B: Central cell (containing polar nuclei).
C: Egg cell (part of the egg apparatus at the micropylar end).
ii. Function of filiform apparatus: These finger-like projections in the synergids guide the entry of the pollen tube into the synergid.
iii. Formation of PEN: The secondary nucleus (formed by the fusion of two polar nuclei in the central cell) fuses with the second male gamete to form the Primary Endosperm Nucleus (PEN).
iv. Ploidy of antipodal cells: They are haploid ( ).

Step 1: Hydrophilous flowers are those that are pollinated by water.
Step 2: To protect the floral parts and pollen grains from damage due to constant water exposure, they are covered with a mucilaginous coating.
Step 3: This waxy/mucilaginous layer makes the floral parts un-wettable, ensuring that the pollen remains viable and light enough to be carried by water currents.

Step 1: In the development of a coconut, the primary endosperm nucleus undergoes successive nuclear divisions without immediate cell wall formation.
Step 2: This results in a large number of free nuclei suspended in fluid, known as free-nuclear endosperm.
Step 3: The surrounding white kernel (the "meat") is the cellular endosperm.

Step 1: During embryogeny in dicotyledonous plants, the zygote gives rise to the proembryo.
Step 2: The development follows a specific sequence of shapes: Globular Heart-shaped Mature embryo.
Step 3: The two "lobes" of the heart eventually develop into the two cotyledons characteristic of dicots.

Step 1: The process of formation of megaspores from the Megaspore Mother Cell (MMC) is called megasporogenesis.
Step 2: The MMC (which is diploid, ) undergoes meiosis (reduction division).
Step 3: Meiosis always results in the formation of a linear tetrad of four haploid ( ) megaspores.

Mode of Pollination: Pollination by insects is known as Entomophily.
Characteristics of Insect-pollinated Flowers:
Color: Flowers are brightly colored to be easily visible to pollinators.
Nectar: They produce nectar as a nutritional reward for insects.
Fragrance: Many flowers emit a strong scent to attract insects from a distance.
Sticky Pollen: Pollen grains have a sticky or spiny surface to adhere to the insect's body.
Sticky Stigma: The stigma is typically sticky to effectively capture pollen grains.

In a majority of flowering plants, out of the four megaspores formed after meiosis, three degenerate and only one remains functional. This single functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.

Step 1: A young anther is a bithecal and tetrasporangiate structure. In transverse section, it appears as a four-sided structure with four microsporangia.
Step 2: The anther wall consists of four layers: the Epidermis (outermost protection), Endothecium (helps in dehiscence), Middle layers, and the Tapetum (innermost nutrient-providing layer).
Step 3: The center of each microsporangium is occupied by Sporogenous tissue, which eventually produces pollen grains.

Step 1: Understanding the Concept:
Double fertilization requires two male gametes to fuse with two different structures within the same female gametophyte.
Step 2: Detailed Explanation:
- Fusion 1: 1st Male Gamete + Egg Zygote (Syngamy).
- Fusion 2: 2nd Male Gamete + 2 Polar Nuclei Primary Endosperm Nucleus (Triple Fusion).
The antipodal cells and the synergids do not participate in these fusions; they typically degenerate shortly after fertilization.
Step 3: Final Answer:
Since both the polar nuclei and the egg cell undergo fusion with male gametes, they are both participants in the process of double fertilization.

Step 1: Understanding the Concept:
In flowering plants, the megaspore develops into the female gametophyte, which is housed inside the ovule.
Step 2: Detailed Explanation:
The diagram shows a fully developed structure after three successive mitotic divisions of the functional megaspore.
It is a highly specialized structure containing precisely arranged cells ready for fertilization.
This specific arrangement (7 cells and 8 nuclei) is known as the "Polygonum type" embryo sac, which is the most common form in angiosperms.
Step 3: Final Answer:
The diagram represents a mature angiosperm embryo sac.

Step 1: Understanding the Concept:
The pollen tube must find its way through the style and into the tiny opening of the ovule (micropyle). Once there, it needs a specific signal to enter the embryo sac.
Step 2: Detailed Explanation:
Synergids are the "helper cells" located at the micropylar end.
They possess finger-like cellular thickenings known as the filiform apparatus.
This apparatus acts like a "beacon"; it secretes chemotropic chemicals that attract the pollen tube and provide a physical guide for the tube to enter one of the synergids to release its male gametes.
Step 3: Final Answer:
The filiform apparatus, located within the synergids, is responsible for guiding the pollen tube.

Step 1: Understanding the Concept:
Angiosperms undergo a unique process called "Double Fertilization."
Step 2: Detailed Explanation:
There are two polar nuclei located in the large central cell.
During fertilization, one of the two male gametes travels to the center of the embryo sac and fuses with these two polar nuclei.
Since this involves the fusion of three haploid nuclei ( ), it is called Triple Fusion.
The resulting cell is triploid ( ) and is called the Primary Endosperm Cell (PEC), containing the Primary Endosperm Nucleus (PEN).
This tissue grows into the Endosperm, which stores nutrients to feed the developing embryo.
Step 3: Final Answer:
The polar nuclei develop into the triploid endosperm, which serves as a nutritive tissue for the embryo.

Step 1: Understanding the Concept:
The embryo sac is polarized. One end points toward the opening of the ovule (Micropylar end) and the other toward the base (Chalazal end).
Step 2: Detailed Explanation:
1. Micropylar End: Contains the "Egg Apparatus." This consists of two synergids and one centrally placed egg cell.
2. Chalazal End: Contains three antipodal cells.
3. Central Region: Contains the large central cell with two polar nuclei.
Step 3: Final Answer:
The synergids and the egg are both located at the micropylar end.