Scanning electron micrograph of the flower spike of timothy grass, Phleum pratense. The picture shows individual flowers, comprising anthers (paired, light brown) and the feathery female stigmas (pale green), attached to which are pollen grains (orange). The anthers are 1.5mm long; the pollen grains are 30 microns in size. Grasses are wind pollinated. This is a random process, so the anthers must shed large amounts of pollen to be carried by air currents to reach other plants in flower. Aerial pollen was first recognised as a cause of allergic rhinitis ("hay fever") in 1859. Pollen of timothy grass is the principal cause during the hay fever season. Each pollen grain has a pore in its wall through which the tube carrying the male gametes emerges. Chemicals localised to the pore are the source of the allergic reaction. The anthers in this picture have shed their pollen, and are empty.