88 Prof. A. J. Brown. Selective Permeability of the [Jan. 23, 



in a solution of mercuric chloride during several days and then dried were 

 steeped iu a normal solution of sulphuric acid, it was found that the 

 coverings still retained their original power of resisting the diffusion of 

 sulphuric acid while permitting the diffusion of water into the seed. 



Still more conclusive evidence of the possession by the seed-coverings of 

 a differentiating power was furnished by an experiment in which seeds were 

 steeped in a mixture of half a volume of a saturated solution of mercuric 

 chloride with half a volume of normal sulphuric acid. After three days' 

 steeping in this solution the mercuric salt was found to be diffused 

 throughout the contents of the seed ; even after five days' steeping, however, 

 no trace of sulphuric acid could be found within the seed-coverings. The 

 seeds of H. vulgare therefore possess the very remarkable property of 

 absorbing mercuric chloride and rejecting sulphuric acid when steeped in 

 a solution in which both are present. The exhibition of this property by 

 the seeds appears to be of very special interest from a physiological point 

 of view. 



To ascertain their behaviour towards mercury salts generally, seeds were 

 steeped in solutions of mercuric chloride, cyanide, nitrate and sulphate of 

 cquimolecular strength. The cyanide diffused as readily as the chloride 

 into the seeds, but after several days no trace of mercuric salt could be 

 recognised in those placed in the solutions of nitrate and sulphate. Moreover, 

 the amount of water absorbed by seeds from solutions of mercuric chloride 

 and cyanide did not differ from that taken up from water alone. 



It should be noticed iu passing that chloride and cyanide of mercury — 

 which diffuse through the seed-coverings — are commonly regarded as but 

 very slightly dissociated in aqueous solution, whilst mercuric sulphate and 

 nitrate — which cannot penetrate the membrane — are salts which are supposed 

 to be freely dissociated. 



Cadmium Salts. — On steeping seeds of IT. vulgare in volume normal 

 solutions of cadmium iodide, chloride, and sulphate until equilibrium was 

 vestablished, the following results were obtained : — 



Table IV. 



Solute. 



Percentage of water 

 absorbed. 



Cdl 2 



54 -2 

 46 -3 

 46 -0 

 39 -8 



CdCl 2 



CdS0 4 



NaCl (control) 



