178 PEPPER TREE AS A CAUSE OF HAY FEVER. 



Table XIV . 

 Meteorological Observatioxs : Bloemfonteix, 1920. 



tical purposes, to depend only on the size of the pollen grain, and 

 to vary directly as the square of the diameter of the grain. The 

 larger the pollen, the greater the velocity of fall and, therefore, 

 the shorter the distance to which it can be carried by the wind 

 before reaching the ground. The size of the pollen is, therefore, a 

 very important factor determining the area over which it is 

 distributed, and hence its power to produce hay fever. Thus, 

 the majority of plants that have been foimd to be serious causes 

 of hay fever have small pollen, that of the Giant and Common 

 Ragweeds {Anibrnsia irifi.da and -4. clatior, for instance, two of 

 the commonest hay fever plants in America, being respectively 

 20 and 15 microns in diameter. Compared with these pollens, 

 that of the pepper tree, which is usually some 33 microns long 

 by 17 microns broad, is very large and therefore less buoyant. 

 The area over which part of it is distributed must be still further 

 reduced by the circumstance that some of it leaves the anther 

 before it is quite dry, and whilst the grains are still partially 

 stuck together. This is apparent from the fact that the grains 

 of pepper tree pollen found on the pollen plates were frequently 

 in small grou])s. 



On the other hand, the following facts have to be remem- 

 bered in this connection : First : The plant producing this pollen 



