10 70 Proceedings of the Royal Irish Academy. 



than reduction in weight." In this connexion see the remarks of H. N". Ridley 

 quoted on p. 66. Very minute seeds provide the most effective of all 

 devices for plant-dispersal. It is in the spores of Cryptogams that we find 

 minuteness and lightness developed to the highest degree, rendering them 

 infinitely more fitted for wide dispersal than even the most complicated 

 and efficient of parachute-seeds. 



The behaviour of small particles falling in air differs from that of larger 

 bodies, inasmuch as, with continued reduction in size, the impelling force (the 

 action of gravity) becomes rapidly smaller in comparison with the decrease 

 of resistance offered by the air, so that very small velocities result. A 

 mathematical expression for the velocity of fall of microscopic spheres in a 

 viscous medium was deduced by Stokes, 1 and is known to physicists as Stokes's 

 Law : — 



2 p - (7 



9 fi 



where V = the terminal velocity, 



p = the density of the sphere, 

 a = the density of the medium, 

 g = the acceleration due to gravity, 

 a = the radius of the sphere, 

 p. = the viscosity of the medium. 



In recent years several series of experiments have been carried out with 

 a view of providing a practical verification of Stokes's Law. Zeleny and 

 McKeehan 2 have experimented with Lycoperdon, Polytrichum, and 

 Lycopodiurn spores. The velocities of fall which they observed were about 

 one-half of those given by the formula. It has been suggested by Buller, 3 

 and more recently by Miss Stoney, 4 that the shape of the spores of 

 Lycopodiurn, which are four-sided and have sculptured walls, may account 

 for their retardation. A. B. Basset 5 has suggested a mathematical explana- 

 tion of this want of agreement. Subsequently Zeleny and McKeehan have 

 announced 6 that further experiments with small spheres of paraffin wax, a 

 black wax, and mereuiy, give results which are in close agreement with 

 those obtained from Stokes's formula. Buller 7 experimented with the spores 



1 G. G. Stokes : On the Effect of Internal Friction of Fluids on the Motion of Pendulums. 

 Cambr. Phil. Trans., ix, part 2, pp. 8-106, 1856. Abstract in Phil. Mag. i, pp. 337-339. 1851. 



2 J. Zeleny and L. W. McKeehan ; An Experimental Determination of the Terminal Velocity of 

 Fall of Small Spheres in Air. Paper read before the Amer. Assoc, for the Advancement of Science. 

 Abstract in Science, N.S., xxix., p. 469, 19th March, 1909 ; and The Terminal Velocity of Fall of 

 Small Spheres in Air. British Association Report for 1909, pp. 407, 408. 1910. 



3 Nature, lxxx., pp. 186-187. 1909. i Ibid., lxxxii., p. 279. 1910. 



6 Hid., lxxxiii., p. 521. 1910. c Ibid., lxxxii., p. 158. 1909. 



7 A. H. R. Bcllek : Researches on Fungi, chap. xv. London, 1909. 



