THE RATE OF FALL OF SPORES 177 



steep corkscrew-like paths. Special attention was paid to the 

 fall of the spores of Polyporus squamosus, which are nearly 

 three times as long as they are wide. It was found that on 

 emerging from the hymenial tubes many of them have their 

 long axes nearly vertical, and that whilst falling they often 

 appear to turn over and over on themselves or to rock from side 

 to side. However, by following the spores individually with the 

 help of a mechanical stage, it was plainly seen that after falling 

 about 5 mm. they were almost without exception nearly or quite 

 horizontal, and that they then rotated in a horizontal plane very 

 slowly or not at all. The final position which the spores took up 

 in still air was therefore such that the greatest surface was 

 presented to the resistance of the air. We may conclude, there- 

 fore, that long spores tend to fall in a similar manner to that 

 assumed for the simple, prismatic ice-crystals which cause the 

 phenomena of sun-dogs, &c., in northern regions. 



APPENDIX 



The compressor-cell method of measuring the rate of fall of spores was 

 devised in 1905. I then came to the conclusion that the spores of Hymenomy- 

 cetes fall at a rate which is roughly in accordance with Stokes' formula, 

 and this fact was announced by A. J. Ewart in his translation of PfefFer's 

 Physiology of Plants. 1 During the summer of 1906, I carried out a large 

 number of measurements of the size, specific gravity, and terminal velocity 

 of spores, and in 1907 Chapters XIII., XIV., and XV. were communicated to 

 the Royal Society as sections of a paper which I subsequently withdrew. 2 



Recently Zeleny and M'Keehan 3 of the University of Minnesota have 

 announced that they have made a direct test of Stokes' formula by using 

 lycopodium power. Their method of measuring terminal velocity consisted 

 in allowing the powder to fall in wide tubes and noting the rate of movement 



1 Vol. iii., 1906, p. 416. 



2 The paper called " The Production, Liberation, and Dispersion of the Spores 

 of Hymenomycetes " was accepted for publication in the Philosophical Transactions 

 of the Royal Society, but on conditions which I was unable to accept. 



3 John Zeleny and L. W. M'Keehan, "An Experimental Determination of the 

 Terminal Velocity of Fall of Small Spheres in Air." A paper read at the meeting 

 of the American Association for the Advancement of Science, held December 1908. 

 Abstract in Science, March 19, 1909. 



M 



