1910.] 



The Origin of Osmotic Effects. 



591 



leaves are kept in contact with them during three or four days ; some 

 stronger sohitions and salts of a corrosive nature cause the destruction of the 

 membranes of the leaf and rapidly kill it. 



Only a few salts can permeate the differential septa of the leaf and function 

 as cyanogenetics ; these condition the escape not only of hydrogen cyanide 

 but also of reducing sugar, which passes out into the solution ; apparently the 

 membrane breaks down and ultimately the leaf turns brown. 



The majority of the experiments with solutions were niade at 37°; the 

 conditions under which the leaf was placed were therefore somewhat drastic, 

 so that even substances which did not at first penetrate the tissues sooner or 

 later determined its death by plasmolysis. Indeed, most salts of the heavy 

 metals gain access after 48 hours. In concentrated solutions, even inactive 

 substances cause the breakdown of the tissues ; a 50-per-cent. solution of 

 glycerol and a 10-per-cent. solution of sodium chloride, for example, 

 acted in this manner. The substances referred to above as active as vapours 

 are at least equally active in aqueous solution. Urea passes slowly into the 

 leaf from a weight-normal solution. 



The following salts have been found to function cyanogenetically : — 



Cadmium Iodide. — The activity of an M/20 solution* is apparent at the 

 end of about 10 hours; an M/15 solution is active within 6 hours. 



Sodium and Potassium Fhiorides. — In the case of M/20 solutions, the 

 effect is obvious after 24 hours ; M/2 solutions are active within about 

 6 hours. Apparently the potassium salt is the more active. The result is 

 of special interest, inasmuch as fluorides are used as disinfectants. 



Mercuric Chloride. — An M/20 solution is weakly active within 24 hours; 

 in M/5 solution the action is far more pronounced. 



Iodine dissolved in a 5-per-cent. solution of potassium iodide is slowly active. 



Ammonium Salts. — In M/20 solution the activity of the chloride is 

 apparent only after an interval of two days, in the case of the nitrate after 

 30 hours; both salts are active in M/2 solutions within 20 hours. 



Ammonia acts not only as vapour but also in solution (N/4, iSr/12 and 

 !N'/20), very rapidly causing the leaf to darken in colour ; the picrate paper 

 assumes a deep red colour within two hours. There can be little doubt that 

 ammonia acts as such, not as the hydroxide, as M/10 solutions of sodium 

 hydroxide and carbonate are without effect and an N/2 solution of the 

 former is active only after several hours, presumably because the membrane 

 is gradually destroyed. 



* The solutions were made of weight-molecular or weight-normal strength or fractions 

 thereof, i.e. the molecular or equivalent proportion in grammes of the substance was 

 dissolved in 1000 grammes of water. 



