Pollen

SEM image of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), oriental lily (Lilium auratum), evening primrose (Oenothera fruticosa), and castor bean (Ricinus communis).

Closeup image of a cactus flower and its stamens

Honey bee on sedum with pollen basket

Pollen sticking to a bee. Insects involuntarily transporting pollen from flower to flower play an important role in many plants' reproductive cycles.

Pollen, sometimes incorrectly called flower sperm, is a fine to coarse powder consisting of microgametophytes (pollen grains), which produce the male gametes (sperm cells) of seed plants.

The structure of pollen

Each pollen grain contains vegetative cells (only one in most flowering plants but several in other seed plants) and a generative cell containing a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose cell wall and a thick, tough outer wall made of sporopollenin.

Pollen is produced in the microsporangium (contained in the anther of an angiosperm flower, male cone of a coniferous plant, or male cone of other seed plants). 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 paleoecology, paleontology, archeology, and forensics.

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. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.

Pollination

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, sometimes with air-sacs. Non-flowering seed plants (e.g. pine trees) are characteristically anemophilous. Anemophilous flowering plants generally have inconspicuous flowers. Entomophilous (literally insect-loving) plants produce pollen that is relatively heavy, sticky and protein-rich, for dispersal by insect pollinators attracted to their flowers. Many insects and some mites are specialized to feed on pollen, and are called palynivores.

In non-flowering seed plants, pollen germinates in the pollen chamber, located beneath and inside the micropyle. A pollen tube is produced, which grows into the nucellus to provide nutrients for the developing sperm cells. Sperm cells of Pinophyta and Gnetophyta are without flagella, and are carried by the pollen tube, while those of Cycadophyta and Ginkgophyta have many flagella.

When placed on the stigma of a flowering plant, under favorable circumstances, a 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 micropyle of an ovule. The nucleus of the tube cell has meanwhile passed into the tube, as does also the generative nucleus which divides (if it hasn't already) to form two sperm cells. The sperm cells are carried to their destination in the tip of the pollen-tube.

Pollen as a carrier of ecological information in plants

A Russian theoretical biologist, Vigen Geodakyan (Geodakian), has suggested that the quantity of pollen reaching a pistillate flower can transmit ecological information and also regulate evolutionary plasticity in cross-pollinating plants. Plentiful pollen indicates optimum environmental conditions (for example a plant that is situated at the center of its natural range, in ideal growing conditions, with a large number of male plants nearby, and favorable weather conditions), whereas a small amount of pollen indicates extreme conditions (at the borders of its range, with a deficiency of male plants, and adverse weather conditions). Geodakian believes that the quantity of pollen reaching a pistillate flower defines the sex ratio, dispersion and sexual dimorphism of a plant population. High pollen quantity leads to a reduction of these characteristics and stabilization of a population. Small quantity leads to their increase and destabilization of a population.

Dependence of the secondary sex ratio on the amount of fertilizing pollen was confirmed on four dioecious plant species from three families – Rumex acetosa (Polygonaceae), Melandrium album (Cariophyllaceae), Cannabis sativa and Humulus japonicus (Cannabinaceae). (see summary of all these data in reveiw article).

Dependence of offspring phenotype variety on amount of pollen was observed by Ter-Avanesyan in 1949. All three studied species of plants (cotton plant, black-eyed pea, and wheat) showed dependence in the direction forecasted by the theory – fertilization with a small amount of pollen resulted in an increase in the diversity of the offspring. Ter-Avanesian writes that as a result of a limited pollination “instead of gomogenous sorts we get populations”.

Hay fever

Main article: Hay fever

Allergy to pollen is called hay fever. Generally pollens that cause allergies are those of anemophilous plants (pollen is dispers ed by air currents.) Such plants produce large quantities of lightweight pollen (because wind dispersal is random and the likelihood of one pollen grain landing on another flower is small) which can be carried for great distances and are easily inhaled, bringing it into contact with the sensitive nasal passages.

In the US, people often mistakenly blame the conspicuous goldenrod flower for allergies. Since this plant is entomophilous (its pollen is dispersed by animals), its heavy, sticky pollen does not become independently airborne. It is moved from one flower to the next by insects, so the only way to get goldenrod pollen into the nasal passages would be to stick the flower up one's nose. Most late summer and fall pollen allergies are probably caused by ragweed, a widespread anemophilous plant.

Arizona was once regarded as a haven for people with pollen allergies, although several ragweed species grow in the desert. However, as suburbs grew and people began establishing irrigated lawns and gardens, more irritating species of ragweed gained a foothold and Arizona lost its claim of freedom from hay fever.

Anemophilous spring blooming plants such as oak, birch, hickory, pecan, and early summer grasses may also induce pollen allergies. Most cultivated plants with showy flowers are entomophilous and do not cause pollen allergies.

See also

• Pollen count
• Pollen source
• Polyphenol antioxidant
• Evolutionary theory of sex
• Sex ratio

External links

Clumps of yellow pollen on a flower head.

• Daily Pollen Count
• Pollen and Spore Identification Literature
• Paleobotany and Palynology at the Florida Museum of Natural History
• Palynology at the University of Arizona
• Palynology at French Institute of Pondicherry
• Palynology at the University of Sheffield

• Palynology in Utrecht, the Netherlands
• The flight of the pollen cloud Outcrossing from transgenic maize and quantifying outcrossing rates
• The Evolutionary Theory of Sex: Mechanisms of Regulation of the Population Parameters