The sex life of plants

Flowers are important to plants for reasons other than display in gardens. They are the most important method by which a flowering plant reproduces itself. Flowers are pollinated and then wither and die. In their place grow fruits containing seeds, which germinate and thus produce the next generation of plants.

Flowers are found mostly at the edge of a plant, often near the top. This allows them to be easily seen by insects or birds, or exposes them to pollen carried by the wind. The transfer of pollen from male to female parts of flowers is called pollination.

Inside the leaf-like sepal and ring of petals are stamens. These are the male parts of the plant, and vary in number from a few to some flowers to many in others. On the end of the small stalks are anthers which produce pollen grains. Inside the pollen grains are the sperm cells necessary for reproduction, and starch granules which contain allergens. in the centre of the flow are the female parts called carpels. At the top of the carpel is the sticky stigma ready to catch pollen grains. The stigma is connected to the ovary by the style, and inside the ovary are the ovules containing the eggs.

Pollination takes place when the pollen matures, the anther splits open and pollen is released. The pollen grains are physically transported to the stigma by air, water or animal carriers. When pollen lands on the stigma a tube extends from a small aperture in the pollen grain wall. The pollen tube penetrates the stigma and grows down the style to the ovary. Here the tip of the tube ruptures and sperm cells are transferred into the ovules to fertilize the egg. After fertilisation the ovule develops into a seed and often develops into a fruit.

People and pollen

Flowering plants are divided into ‘wind’ pollinating and ‘insect’ pollinating. AirWatch is concerned with wind pollinating plant types. During the flowering season numerous pollen grains are released from the flower and blown around in the air. During dry weather pollen is easily blown into eyes and nasal passages, causing hayfever in sensitive people.

During and after rain (often thunderstorms) some grains of pollen burst, releasing the allergen-containing starch granules. In Melbourne after rain, air samples have been shown to contain up to 50 times more starch granules than air sampled on a sunny day during the grass pollen season. The starch granules are small enough to be respirable and can enter the bronchi, where they may trigger allergic asthma.

Wind Pollination

The small flowers of grasses produce large amounts of wind-blown pollen. They also have large stigmas to catch this pollen. Grass pollen is the major pollen type in the air in spring and summer in cool temperate climates such as Melbourne.

See Appendix 3 for identification photographs of grass pollens and tree pollens.

Pollen Standards for grasses

The pollen count standards developed by Melbourne University and used across Australia by EPAs are based only on grass pollens.

The standards in the box below apply to pollen counts completed by The Botany Department at Melbourne University on a Burkhard volumetric trap. The Burkhard trap is elevated off the ground, always faces the prevailing wind, and samples at ~10 L/min. The pollen collects on a glass slide which has a sticky adhesive on the face. It is therefore relevant to note that the AirWatch sampling equipment behaves differently and may produce slightly different pollen counts.

We suggest that students sample for pollen over a number of days and compare their results with those published in the paper. If the results are consistently different, but by a similar margin each time, then a set of standards can be formulated for the AirWatch equipment.

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Modified: 16 May 2002