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Goldenrod pollen. Coloured scanning electron micrograph (SEM) of pollen grains from the goldenrod plant (Solidago sp. ). Goldenrod flowers produce large quantities of pollen from July to September. Most pollen grains are dispersed by the wind or insects. The spiky outer coats (exines) of these grains would appear to indicate that they are carried by insects, with the spikes designed to catch the insects' hair. Wind-dispersed pollen is a major cause of allergic hayfever and some cases of asthma. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/M. KALAB, CUSTOM MEDICAL STOCK PHOTO)