Pollen Grains and Their Structure

Study Notes

Generally spherical in shape, measuring about 25-50 micrometers in diameter.
Composed of a two-layered wall with a hard outer layer called exine, made of sporopollenin, a highly resistant organic material.
Sporopollenin can endure high temperatures as well as strong acids and alkalis; no known enzymes can degrade it.
Germ pores are areas on the exine where sporopollenin is absent, allowing for pollen germination.
Exine patterns and designs vary greatly, contributing to the identification of different plant species.
The inner wall is called intine, composed of cellulose and pectin, surrounding the cytoplasm and plasma membrane.
Mature pollen grains typically consist of two cells: a vegetative cell and a generative cell.
The vegetative cell is larger with abundant food reserves, while the generative cell is smaller and spindle-shaped.
In over 60% of angiosperms, pollen is released as a two-celled structure; in others, the generative cell divides to form two male gametes before shedding.

Pollination occurs via two abiotic (wind, water) and one biotic (animals) agents; most plants rely on biotic agents.
Wind pollination is predominant among abiotic methods; requires light, non-sticky pollen grains for efficient transportation.
Wind-pollinated flowers frequently have exposed stamens and large feathery stigmas to capture airborne pollen.
Water pollination is rare, limited to about 30 genera, mainly among monocotyledons.

Seed formation allows independence from water for pollination and fertilization processes.
Seeds possess adaptive strategies for dispersal, aiding in colonization of new habitats.
Nutritional reserves in seeds support young seedlings until they can photosynthesize.
The hard seed coat provides protection for the embryo; seeds generate genetic variation through sexual reproduction.
Seeds are integral to agriculture, ensuring food availability and supporting crop propagation.

Seed viability post-dispersal varies significantly; some lose viability within months, while others remain viable for years or even centuries.
Notable viable seeds include a lupine seed estimated at 10,000 years old and a date palm seed found over 2,000 years old.

Apomixis allows some flowering plants to produce seeds without fertilization, acting as a form of asexual reproduction.
Polyembryony occurs when a single ovule contains multiple embryos, often originating from nucellar cell divisions.
Hybrid seeds require yearly production; apomixis can stabilize hybrid traits in progeny without segregation, reducing costs for farmers.
Research focuses on transferring apomictic genes into hybrid varieties to improve agricultural practices.

Flowers are centers of sexual reproduction in angiosperms, with stamens forming the male and pistils forming the female reproductive structures.
Anthers consist of four wall layers; microsporangia hold the sporogenous tissue that undergoes meiosis (microsporogenesis) to form microspore tetrads.
These microspores mature into pollen grains, representing the male gametophytic generation.