Pollen

Pollen is a fine to coarse powder consisting of microgametophytes (pollen grains), which carry the male gametes of seed plants. Each pollen grain contains one or two generative cells (the male gametes) and a vegetative cell. The group of three cells is surrounded by a cellulose cell wall and a thick, tough outer wall made of sporopollenin.

Related Topics:
Microgametophytes - Seed plants - Cellulose - Sporopollenin

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Pollen is produced in the microsporangium (anther of an angiosperm flower or male cone of a coniferous plant). Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see photomicrograph at right). Most, but certainly not all, are spherical. Pollen grains of pines, firs, and spruces are winged. The smallest pollen grain, that of the Forget-me-not plant (Myosotis sp.), is around 6 µm (0.006 mm) in diameter. The study of pollen is called palynology and is highly useful in paleontology, archeology, and forensics.

Related Topics:
Anther - Angiosperm - Flower - Cone - Coniferous - Forget-me-not - µm - Mm - Palynology - Paleontology - Archeology - Forensics

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Except in the case of some submerged aquatic plants the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. Germination of the microspore begins before it leaves the pollen-sac. In very few cases has anything representing prothallial development been observed; generally a small cell (the antheridial or generative cell) is cut off, leaving a larger tube-cell.

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The transfer of pollen grains to the female reproductive structure (pistil in angiosperms) is called pollination. This transfer can be mediated by the wind, in which case the plant is described as anemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, often with air-sacs, and generally have inconspicuous flowers. Entomophilous (literally insect-loving) plants produce pollen that is relatively heavy and sticky, for dispersal by insect pollinators attracted to their flowers. When placed on the stigma, under favourable circumstances, the pollen-grain puts forth a pollen-tube which grows down the tissue of the style to the ovary, and makes its way along the placenta, guided by projections or hairs, to the mouth of an ovule. The nucleus of the tube-cell has meanwhile passed into the tube, as does also the generative nucleus which divides to form two male- or sperm-cells. The male-cells are carried to their destination in the tip of the pollen-tube.

Related Topics:
Pistil - Pollination - Flower

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