PEPPER TPiEE AS A CAUSE OF HAY FEVER. 179 



is a tree, and that the pollen therefore commences its flight at 

 a considerable distance above the ground. The middle of the 

 crown of a medium-sized pepper tree in Bloemfontein is probably 

 some 25 feet high ; and pollen starting at this level would be 

 carried ten times as far as from flowers 2^ feet high, and twenty- 

 five times as far as from flowers only a ifoot high. Most hay 

 fever pollens are produced within two or three feet of the ground. 

 The distance to which small particles such as pollen grains 

 are caiTied by wind can be calculated from Stokes' Law. This 

 has been done in the case of pepper tree pollen and the results 

 are shown below. In making the calculation it has been assumed 

 that the density of the pollen is 1, that of the air negligible, and 

 that the grains, which are elongated in form and about 33 microns 

 long by 17 microns broad, would fall with the velocity of spheres 

 of 25 microns diameter, this being the mean of their two 

 dimensions. Under these circumstances the velocity of fall 

 would be 0'25 feet per second, and the distance to which they 

 would be caiTied when blown from a height of 25 feet would be 



291 feet in a wind of 2 miles j^er hour 



1,455 ,, ,, 10 



2,909 ,, ,, 20 



3,637 ,, ,, 25 



4,365 ,, ,,30 



The corresponding distance for smooth spheres of 35 microns 

 diameter are 149, 748, 1,497, 1,871 and 2,245 feet respectively. 

 The calculations have been made by Miss D. M. Gemmell from a 

 fonnula supplied by Prof. W. H. Logeman; and the distances 

 stated are in general agreement with those given by Dr. Scheppe- 

 grell on p. 16 of " Hay Fever and Hay Fever Pollens " (1917). 



The figures it should be stated apply to level ground, and, in 

 view of the uncertainty regarding the assumptions made, are to 

 be regarded only as rough approximations. 



A rough idea of the extent to which the height at which 

 pepper tree pollen is produced compensates for its large size 

 may be given by making a comparison with a well-known hay 

 fever plant, having very small pollen. Assuming that the 

 elongated grains of pepper tree pollen fall with the velocity of 

 sinooth spheres of even 35 microns diameter, the result of start- 

 ing from a height of 25 feet would be that they can be carried 

 by the same wind approximately twice as far as pollen of 15 

 microns (the size of the pollen of the Common Ragweed) starting 

 from a height of 2i feet. 



Secondly : In Bloemfontein during November and December, 

 when the epidemic is most severe, the ground is dry and bare, so 

 that pollen dropped by the wand can be picked up again by a 

 subsequent gust. In this respect the conditions are vitally 

 different from those in a moist climate, where a damp soil and 

 covering of grass and weeds would prevent fallen i)ollen from 



