338 Mr Wilson, On the Action of Uranium rays, etc. [Oct. 25, 



degrees of supersaturation necessary to make condensation take 

 place upon them. Whatever view may be held as to the nature 

 of the nuclei introduced by X-rays and Uranium rays, they are 

 shown in this way to be identical with one another, as well as with 

 those existing in very small numbers in ordinary moist, dust- 

 free air. 



Monday, 8 November, 1897. 



Mr F. Darwin, President, in the Chair. 



The following Communications were made to the Society: — 



(1) On Farmer's method of demonstrating assimilation. By 

 Francis Darwin, F.R.S., President of the Society. 



The method here given is merely a simplification of the in- 

 teresting experiment described by Farmer in the Annals of 

 Botany \ He there pointed out that the protoplasmic circulation 

 ceases in an Elodea leaf kept in the dark, in a current of hydrogen : 

 and that the circulation recommences if the preparation is illu- 

 minated. The cessation of the movement is due to the absorption 

 of the free oxygen, its reappearance to the oxygen produced by 

 the assimilatory activity of the chloroplasts. Farmer's positive 

 results should in any case I think outweigh Pringsheim's 2 failure 

 to observe a revival of circulation in illuminated specimens of 

 Chara &c, and the results which I have obtained fully confirm 

 Farmer's conclusions. 



My object in modifying Farmer's method was to place an 

 experiment of great educational value within the reach of those 

 who possess no apparatus beyond a microscope. This can be done 

 by mounting an Elodea leaf 3 in water and sealing the preparation 

 by careful ringing it with melted " wax-mixture." 4 The prepara- 

 tion is then placed in the dark, and the observer waits until the 

 available oxygen has been absorbed and the circulation has come 

 to rest, which takes place in from 3 to 6 hours. 



i Vol. x. p. 284. 



2 Sitzb. k. Preuss. Akad. 1887. See also Vines' interesting discussion Annals of 

 Botany, Vol. i. p. 371. 



3 For reasons given below it is better to mount two or even three leaves under 

 a single cover-glass. 



4 Resin 15 parts, vaseline 50 parts, beeswax 35 parts. 



