OUTDOOR AEROBIOLOGY — GREGORY 447 



pollen may overshadow bacteria and fungi for a time in mass, but 

 even then not usually in number of particles. About 90 percent of 

 the species of flowering plants are usually insect-pollinated, but only 

 about 10 percent are adapted for wind pollination and habitually shed 

 their pollen into the air. However, these wind-pollinated species are 

 numerically exceedingly abundant and in the aggregate shed large 

 quantities of pollen, with the result that the unfortunate sufferer from 

 hay fever who is allergic to certain species of pollen finds his respira- 

 tory tract a reliable indicator of flowering dates. In temperate coun- 

 tries there are three main seasons for airborne pollen. The "tree 

 pollens" in spring begin with the opening of the catkins of deciduous 

 trees and end with the conifers; fortunately, sensitivity to tree pollen, 

 and especially to pine, is rare. In early summer the grass-pollen 

 season brings the greatest number of hay-fever victims. Late sum- 

 mer brings a mixture generally grouped as "weed pollens." These 

 include nettle in Europe and the highly potent pollen of ragweed 

 {Amhrosia spp.) in North America; freedom from airborne ragweed 

 pollen may be as valuable to an American health resort as a high 

 figure for sunshine is in Britain. 



Airborne bacteria can be enumerated only by cultural methods, and 

 because of the technical problems of culture we have no idea how many 

 such bacteria elude detection. It is therefore impossible accurately 

 to compare total numbers of bacteria and fungi in the air. However, 

 it is clear that the numbers of cultivable molds usually much exceed 

 the numbers of bacteria, and Miquel was clearly embarrassed by the 

 immense numbers of airborne molds. His early work suggested 700 

 bacteria and 30,000 mold spores per cubic meter; his long-term aver- 

 ages of about 300 bacteria and 200 mold spores per cubic meter at the 

 Observatoire Montsouris were obtained only after he changed over to 

 using sugar-free culture media so as to discourage mold growth, a 

 practice that has been followed by many later workers. The bacteria 

 of the air include many micrococci and bacilli, but also a surprisingly 

 large proportion of kinds that do not form spores. 



Visual examination of the fungus spores deposited on a microscope 

 slide during continuous sampling with the Hirst trap in an arable field 

 at Kothamsted Experimental Station [5] shows that the predominant 

 organisms in outdoor air during the day in the warmer months are 

 spores of CladosporiuTn^ a genus of saprophytic molds found on 

 decaying vegetation ; the average was 5,800 per cubic meter of air near 

 ground level during June to October 1952. This dominance of CJado- 

 sporium is also true of many other parts of the world, and it is fully 

 confirmed by cultural methods and examination of dust deposits. 

 More study is needed to find out how Cladosporium becomes airborne. 

 Second most abundant in the air-spora at Rothamsted were spores of 

 the type known as ballistospores. The sources of these include the 



625325—62 30 



