CLIMATIC FACTORS : THE AIR 107 



Wind-borne Pollen.— Many pollen grains are light and small or, 

 as in conifers, have bladder-like wings, which increase their buoy- 

 ancy. As a result, they may be carried for great distances by the 

 wind. The chances that an individual pollen grain will accomplish 

 its function must be extremely small. This uncertainty is compen- 

 sated for in quantity of pollen produced. Efdtman 97 gives the 

 pollen production for several wind-pollinated European species 

 from which the following are selected : Rumex cicetosct produces 

 30,000 grains per stamen, Acer platanoides, 1,000; pollen output per 

 staminate cone of gymnosperms may be judged by Pinus nigra, 

 1,480,000; Ficea excelsa, 590,000; and Jwiiperns communis, 400,- 

 000; production per flower of angiosperms ranges from Rumex 

 acetosa, 180,000, through Tilia cordata, 43,500, to Acer platan- 

 oides, 8,000. Such figures for single stamens and flowers serve to 

 explain the continuous and enormous rain of conspicuous pollen 

 that may fall in season, especially from conifers. Sidewalks, 

 porches, floors, tables— everything in the vicinity of a coniferous 

 forest— may be dusted with pollen. 



Not all noticeable pollen is locally produced, and a great deal of 

 evidence has been accumulated to show irregular and normal dis- 

 tributions. There is a story that, in the early days of the city of St, 

 Louis, it was at one time continuously showered with a yellow 

 dust, which gave residents some concern until botanists identified 

 it as pollen of Pinus palustris transported from the coastal plain far 

 to the south. Some quirk of pressure and wind was depositing the 

 pollen upon St. Louis. 



There are numerous records of pollen being transported long 

 distances. 97 Spruce, pine, and birch pollen was collected on light- 

 ships in the Gulf of Bothnia thirty and fifty-five kilometers off the 

 coast. Spruce pollen is carried from southern to northern Sweden. 

 Peat samples taken in Greenland contained pollen of Picea mari- 

 ana and Pinus banksiana, which must have originated on Labrador 

 or southwestward. One of the most interesting studies of pollen 

 transport was made by Erdtman as he crossed the Atlantic from 

 Gothenburg to New York. Using a vacuum cleaner equipped with 

 filters, he obtained a more or less continuous quantitative record 

 of pollen in the air on the entire trip. Numbers of grains decreased 

 with distance from land, but at no time did sampling fail to show 



