20 RESEARCHES ON FUNGI 



that they do, but so far I can produce no evidence in support of this 

 supposition. It is not unlikely that a hilum-drop is only excreted 

 when a minute portion of the wall of the hilum changes into 

 mucilage, and that the mechanism of excretion is therefore similar 

 to that for the excretion of drops on the pileal hairs of Coprinus 

 ephemerus, the cystidia of Inocybe trichospora, and the sporangio- 

 phore of Pilobolus. It is also possible that the drops in addition 

 to water and mucilage contain crystalloidal bodies such as calcium 

 oxalate, etc., but here again there is at present no evidence to 

 support such a supposition. We may conclude, therefore, that the 

 drops, consist chiefly of water but that, judging by analogy, it is 

 probable that they contain other substances in solution. 



Stokes' Law. In the first volume I gave an account of some 

 experiments upon falling spores which were made for the purpose 

 of testing Stokes' Law. 1 My calculations, which were based on 

 measurements of the density, radius, and rate of fall, showed that 

 the spores of Amanitopsis vaginata fall 46 per cent, more rapidly 

 than they should do according to Stokes' Law. The Law was 

 therefore not verified in detail. The observations on the rate of 

 fall were made upon spores which were just passing out from 

 between two gills in a small compressor cell, the air of which was 

 saturated with water vapour. At the time when my experiments 

 were made, I did not know, as I do now, that a tiny drop of water 

 is carried away upon the exterior of each spore. Now, according 

 to Stokes' Law, the rate of fall of a minute sphere varies directly 

 as the square of the radius. The addition of a water-drop to a 

 spore increases its effective radius and therefore also its rate of 

 fall. It seems to me, therefore, that the excess rate of fall which 

 I observed was, in part at least, due to the water-drop which I had 

 not perceived and which my calculations therefore did not take 

 into account. With a correction of the radius based on the size 

 of the drop carried away by each spore, my experimental results 

 could be brought into closer agreement with Stokes' Law. To 

 continue my investigations in this direction, however, has become 

 unnecessary on account of the fact that, since my observations 

 were published in 1909, Zeleny and M'Keehan, using a method far 

 1 Vol. i, 1909, p. 173. 



