i86 



NATURE 



[April 15, 1909 



Texas, Arizona and elsewhere, including also two 

 specimens from Alaska. The great majority of the 

 woods have proved to be Angiospermous, and two 

 new genera, with many new species, are attributed to 

 the famihes Quercinese, Simarubaceae, Araliaceae and 

 Platanacese among others. 



Several Coniferous trunks of the Pityoxylon, 

 Cupressinoxylon and other types are also described, 

 and the author contributes some interesting diagnostic 

 conclusions respecting a comparison of the structure 

 of the wood of the recent Sequoia and Taxodium 

 with the fossil stems known as Cupressinoxylon. 

 Certain pathological features observed in some of 

 the Coniferous woods, and in one case the presence 

 of a parasitic fungal mycelium, are also noted. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinipm 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous commuitications.] 



The Rate of Fall of Fungus Spores in Air. 



In the year 1905 I made what I believe was the first 

 direct test of Stokes's formula for the fall of small spheres 

 in air by using spores liberated spontaneously from the 

 pilei of the mushroom and of allied fungi. The conclusion 

 to which I then came was that the spores of these fungi 

 fall at a rate which is roughly in accordance with Stokes's 

 formula, and this fact was announced by Prof. A. J. 

 Ewart in his translation of Pfeffer's " Physiology of 

 Plants " (vol. iii., 1905, p. 416). The results of further 

 observation were communicated to the Royal Society in 

 1907 in a paper which I subsequently withdrew.' 



Recently, Messrs. Zeleny and McKeehan," of the Uni- 

 versity of Minnesota, have announced that they have made 

 a direct test of Stokes's formula by using lycopodium 

 powder. Their inethod of measuring terminal velocity 

 consisted in allowing the powder to fall in wide tubes and 

 noting the rate of movement of the centre of the cloud. 

 They came to the conclusion that, for lycopodium spores, 

 the formula gives velocities 50 per cent, in excess of those 

 observed. 



In view of the fact that a correct determination of the 

 rate of fall of small spheres in air has now become of 

 considerable importance in connection with the cloud 

 method used by Sir J. J. Thomson and Dr. C. T. R. 

 Wilson for investigations upon the electronic charge, and 

 also because the full details of my experiments will not 

 be published for some months, 1 have thought it advisable 

 to make a preliminary statement with regard to my 

 methods and results. 



The following equation represents what is known as 

 Stokes's law for the fall of small spheres in a viscous 

 medium : — • 



where V = the terminal velocity, p the density of the fall- 

 ing sphere, o- the density of the medium, g the accelera- 

 tion due to gravity, a the radius of the falling sphere, 

 and /n the viscosity of the medium. The new data which 

 were required for testing the law for the fall of small 

 spheres in air by my method were the terminal velocity, 

 the density, and the radius of the fungus spores. 



After a considerable amount of preliminary experimenta- 

 tion, the spores of Amanitopsis vaginata were chosen for 

 a critical test of Stokes's law, for the following reasons : — 



1 The paper, which wai partly hotanicTl and parity physical in character, 

 was accepted for publication in ihe Philowphical Traneaclions of the Royal 

 Society on condition; which I was unable to accept. This paper, together 

 with other researches, is in course of pnblicat'On in a book called 

 " Researches on Kungi. The Producion, Liheralion. and Dispersion of 

 the Spores of Hymenomycetes treated Bolanically and Physically, &c." 

 (Loncnians. Green and Co ). 



- " An Experimental Determination of Ihe Terminal Velocity of Fall of 

 Small Spheres in Air." A paper read before the American Associatit n for 

 the Advancement of Science. Abstract in Science, March 19. 



(i) they are spherical, except for a tiny "tail," and 

 smooth-coated ; (2) they are sufficiently large, so that one 

 can measure their diameters, which are about 10 11, very 

 accurately ; (3) their density is almost that of water, and 

 can be measured within i per cent, of accuracy ; (4) they 

 can easily be procured. 



The average diameter of the spores was obtained by 

 measurements made with the Poynting plate micrometer 

 as applied to the microscope. The density of the spores 

 was determined by the heavy fluid method. Drops con- 

 taining the spores were placed in the tiny chamber of an 

 apparatus used for counting blood corpuscles, and observa- 

 tions were made as to whether the spores rose or sank 

 in the fluid. The terminal velocity of fall was found in 

 the following manner. A small piece of a pileus of 

 Amanitopsis vaginata, including portions of three gills, 

 was placed in a compressor cell in the position shown at 

 p in the accompanying figure. To prevent the falling 

 spores from drying, two pieces of soaked blotting-paper or 

 cotton wool, b, and a drop of water, to, were then added. 

 Upon the cap being adjusted, the piece of fungus became 

 fixed by slight compression and hermetically sealed in the 

 disc-shaped chamber, of which the base and top were of 

 glass (g). The compressor cell was then placed in a 

 vertical position, so that the gills came to look downwards 

 in the natural manner. Thus enclosed in the chamber, 

 the gills continued to rain down spores for some hours. 

 With a horizontal microscope having a magnification of 

 about 25 diameters, a field was focussed just beneath the 

 gills, and the spores were observed crossing the eye-piece 



Plan and Section of the Compressor Cell. 



lines. In the figure the field is shown by the dotted 

 ring, and the course of three falling spores by arrows. 



On viewing the field just below the gills, spores can be 

 seen as distinct, but only just visible, minute, dark objects 

 steadily crossing the field in a vertical direction. Every 

 spore so falling is not in focus, but when the fungus 

 material is in good condition spores in focus come into 

 view at least every five seconds. Convection currents in 

 the tiny chamber are reduced to a minimum, and produce 

 no disturbing effect on one's observations. Even with the 

 minute spores of CoUybia dryophila, which take about 

 eleven seconds to cross a field 455 mm. wide, the direc- 

 tion of fall is vertical, and there is practically no swerving 

 from the course. The records of the velocity of fall of the 

 spores were made with the aid of a large drum, which 

 was driven by electricity, and was provided with a delicate 

 regulator. To the recording fountain pen was attached 

 an electric tapping key, by the depression of which with 

 the finger the passage of each spore across the field of 

 view bec.tme recorded on the drum paper. The drum 

 records of the fall of 100 spores served to give the average 

 time taken by the spores in falling a distance of 4-55 mm. 



The following table gives a summary of the data obtained 

 in testing Stokes's law. The velocities were the average 

 velocities of 200 spores in Specimen I., of 100 in Speci- 

 men II., and of 50 in Specimen III. The densities are 

 doubtless correct to within i per cent. The diameters are 

 the average diameters for at least fifty spores. The 



NO. 2059, VOL. So] 



