Sexual reproduction in flowering plants

Give the four layers surrounding the pollen sac with their function

1. Epidermis 2. Endothecium 3. Middle layer 4. Tapetum First three layers are protective in function and help in dihescence of anther to release pollen grain . Tapetum is inner most layer that nourishes the developing pollen grain

What are the three types of endosperm development?

1. Free nuclear type: PEN undergoes repeated divisions and nuclei produced get arranged at the periphery leaving a large central vacuole. 2. Cellular type: the nucleus divides and every division is followed by cytokinesis making the nucleus cellular from the very beginning. 3. Helobial type: first mitotic division is followed by cytokinesis forming two unequal cells. The rest of the divisions are free nuclear, making the endosperm cellular later by cytokinesis.

State the economic importance of pollen grains

1. Pollen grains are rich in nutrients. Pollen tablets are used as food supplements. Pollen tablets and syrups increase the performance of athletes and race horses. 2. Pollen grains can be stored for years in liquid nitrogen at -196 degrees. They are used as pollen banks in crop breeding programs. 3. Pollen grains of some plants (eg: parthenium, carrot grass) are allergic for some people. It leads to chronic respiratory disorders such as asthma and bronchitis.

Explain apomixis

Apomixis is a special mode of reproduction in which seeds are formed without the fusion of gametes. A diploid cell of the ovule, either from the nucellus or megaspore, develops into the embryo. The remaining ovule develops into a diploid seed and the ovary into a fruit.

Why is pollen grain known as the male gametophyte?

Each pollen grain has two male gametes, hence, it is known as the male gametophyte

What is monosporic development?

Embryo sac formation from a single megaspore is known as monosporic development.

Explain the exine of pollen grains

Exine is the outer wall of the pollen grain. It is thick and tough, often with spinous outgrowths. It is made of one of the toughest, most biologically resistant materials SPOROPOLLENIN. This is important because it enables the pollen grain to survive in unfavorable conditions such as high temperature, alkaline of acidic medium etc. The region where exine is thin or absent is known as germ pore. The pollen tube emerges from here at the time of pollen germination.

Explain the process of pollen germination and fertilization

If the stigma is receptive and the pollen is compatible, then germination begins. The intine along with its contents emerges from the germ pore as germ tube which grows into pollen tube. The pollen tube is directed towards the ovary by chemicals (calcium-boron-inositol sugar complex). This process is positively chemotropic and negatively aerotropic. The sugar secreted by the stigma acts as stimulus. The pollen tube produces enzymes that digest the tissue of stigma and style for passage of pollen tube into ovule. In 2 celled stage plants, the generative nucleus divides to form two male gametes and the vegetative nucleus disintegrates. In 3 celled stage plants, the pollen tube already carries two male gametes. The pollen tube enters the ovule through the micropyle. It enters the synergids through filiform apparatus. One gamete moves to the egg cell and fuses with its nucleus to form the zygote. The second gamete fuses with the two polar nuclei to form triploid or primary endosperm nucleus.

What is pollen pistil interaction?

It is a dynamic process involving pollen recognition followed by promotion (or inhibition) of the pollen.

60% of angiosperms shed pollen grains at two celled stage. What does this imply? What about other angiosperms?

Most pollen grains shed their pollen grains at the two celled stage. They consist of a vegetative cell, which is bigger, has a largely irregular nucleus and is an abundant food reserve, and a generative cell, which is smaller, floats in the cytoplasm of the vegetative cell and has a dense cytoplasm and nucleus. Other angiosperms shed their pollen grains at the three celled stage, where the generative cell undergoes mitosis to form two male gametes. The three cells are, thus, the two male gametes and the vegetative cell.

Explain parthenocarpy

Parthenocarpy involves the development of fruit from the ovary without fertilization. The fruit formed is seedless as ovules do not develop into seeds. Apomixis forms seeds while there are no seeds in parthenocarpy.

What is perisperm? Differentiate between perisperm and endosperm

Perisperm is the nucellus that remains after fertilization. Perisperm: Represents persistent remains of nucellus (of ovule) to the seed It is a part that belongs to the seed It is usually dry It is diploid in nature Eg: black pepper Endosperm: It develops from the primary endosperm nucleus It contains reserve food materials It is usually in fluid form It is triploid in nature Eg: wheat

What is polyembryony? Why does it occur?

Polyembryony is the presence of many embryos in the same seed. It occurs due to: - presence of more than one egg cell in the embryo sac or more than one embryo sac in the ovule. - embryos formed from synergids and antipodal cells - number of embryos develop from the nucellus and integuments

Where is the filiform apparatus contained and what is its function?

The filiform apparatus is contained in the synergids of the micropylar end. It guides the pollen tube during fertilization.

Explain megasporogenesis

The diploid megaspore mother cell undergoes meiosis to form megaspore tetrad. Three of the megaspores degenerate leaving one functional megaspore. The nucleus of the functional megaspore divides to form 8-nucleate embryo sac.

Why are the divisions of the nucleus to form the embryo sac 'free nuclear'?

The division of the nucleus to form embryo sac is free nuclear, i.e, it is not followed immediately by cell wall formation.

Explain the intine of pollen grains

The intine is the thin and smooth inner layer. It is made of cellulose and lectin. At the time of pollen germination, it gives rise to the pollen tube.

Explain the formation and release of pollen grains from the microsporangium

The microsporangium consists of compactly arranged homogenous diploid cells called sporogenous cells or microsporocytes. These microsporocytes undergoes mitotic division to form the microspore mother cell or pollen mother cells (diploid). These undergo meiosis (MICROSPOROGENESIS) to form microspore tetrad. The anther matures and dehydrates and the microspores dissociate from one another. Each forms an inner and outer wall (intine and exine) and develops a larger vegetative nucleus and smaller generative nucleus. The microspore matures and is considered the male gametophyte due to later formation of 2 gametes from division of generative nucleus. The anther splits along the line of dihescence to release the mature pollen grains.

Explain the structure of an ovule

The stalk connecting the ovule to the ovarian wall is known as funicule. It transports food and water to the ovule. The junction between the body of the ovule and the funicle is known as the hilum. The multilayered body of the ovule is the nucellus. The nucellus contains reserve food material. Within the nucellus is the megastore mother cell. One or two protective envelopes enclose the nucellus and these are the integuments. These integuments are not found in one small pore at the end known as micropyle, which contains one egg cell with a synergid on either side. Opposite to the micropyle is the chalaza or the chalazal end, which consists of three antipodal cells.

Explain the development of embryo till formation of seed

The zygote divides transversely to form two cells: the suspensory cells near the micropyle and the embryo cell further away from the micropyle. The embryo cell undergoes mitotic division to form pro embryo, which undergoes division through various stages (globular, heart shaped etc) till the formation of mature embryo. The remaining chain of cells is formed by division of suspensory cells. This is a suspensor with basal cell at the end. The elongation of the suspensory cell pushes the embryo further into the endosperm for nutrition. A few cells of the embryo near the suspensor develop into hypocotyl and radicle. Other cells give rise to plumule and cotyledons.

Explain double fertilization and triple fusion

Two polar nuclei and one male gamete fuse - fusion of three haploid nuclei - triple fusion Two sets of fertilization occur simultaneously - syngamy (egg cell nucleus and male gamete) and triple fusion - double fertilization