INTRODUCTIOX 



The researches recorded in Part I. were undertaken with the object 

 of throwing light upon the production, hberation, and dispersion 

 of the spores of Hymenomycetes. More especially, an effort has 

 been made to lind out how the spores manage to escape from the 

 hymenial surfaces where they have been produced, and how they 

 find their way between gills, down tubes, &c., to the exterior of the 

 fruit-bodies. By using appropriate optical methods, it has been 

 attempted to follow the spores individually from the moment they 

 leave the basidia, to determine their paths through the air, and to 

 measure by accurate means their rate of fall. This part of the 

 research has led me to the border-land where botany passes into 

 pure physics. Hitherto, it appears that physicists have never yet 

 determined directly by experiment the rate of fall of individual 

 microscopic spheres with a diameter of 3-10 jx through air.^ There- 

 fore, by means of observations on the fall of spores, I have en- 

 deavoured to test the well-known and often assumed Stokes' Law. 

 In studying the effect of external conditions upon the liberation 

 of spores, and in determining the length of the spore-fall period, 

 the work has been much simplitied by two discoveries. The first 

 is that spore-clouds, and even individual spores, can be seen falling 

 beneath a fruit-body without magnification when illuminated with 

 a concentrated beam of light. Whether or not spores are falling 

 from a fruit-body can thus be ascertained in a few seconds. The 

 second discovery is that practically all the leathery or corky fruit- 

 bodies to be found on logs, i.e. those belonging to the genera Lenzites, 

 Polystictus, Da3dalea, Stereum, &c., retain their vitality on desicca- 

 tion for months or years, and that, when they are subsequently 

 placed under moist conditions, the liberation of spores begins once 



1 Cf. the Appendix to Chap. XV. 



