In the dissemination of plant life, the atmosphere is nature's prime sower of the seed and thus vitally affects all living things. Air currents, by gripping the extended surfaces of winged larger seeds and the protuberances of the smaller pollens, lift, transport and release them at their destinations. As the size of particles decreases, the weight decreases more rapidly than the surface area and the tiny spores of molds and fungi require no special adaptation for air travel. Bacteria are even smaller, but since they have no power to invade the atmosphere except in association with other matter, their flight range is determined not by size but by the character of the matter with which they are associated.
"Dust and droplets wafted into the air include bacteria. The number of such temporary passengers on the wing depends upon the kind, the source and conditions which lift them into the air,
WELLS WF, WELLS MW. AIR-BORNE INFECTION. JAMA. 1936;107(21):1698–1703. doi:10.1001/jama.1936.02770470016004
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