1910.] 



The Origin of Osmotic Effects. 



597 





Reducing sugar. 



Hydrogen cyanide 



Per cent. 

 5-13 

 4 -24 

 5-4 

 5-77 

 3 -8 



* The action of chloroform in causing the formation of reducing sugar has also been observed 

 in ivy leaves, -which contain neither cyanophoric glucoside nor j3-enzyme. Whereas freshly picked 

 leaves contained only 3"04 per cent., after treatment 6'24 per cent, of reducing sugar was found 

 to be present. The breakdown of the leaf is also attended by a marked development of the 

 characteristic odour faintlj" perceived in leaves in which no action has taken place. 



A similar development of aroma, accompanied by browning of the leaf, was observed when 

 leaves of the two varieties of laurel, Laurus nohilis and Cerasus lusitanica, which contain neither 

 glucoside nor (3-enzyme, were acted on by chloroform. The browning may be regarded as an 

 indication that even the oxydases are brought into action. 



To Study the manner in whicli substances gain entry, parallel experiments 

 were made in which the upper or lower surface was moistened with solutions 

 which it had been found were able to penetrate into the leaf ; such are 

 <;admium iodide, sodium fluoride, acetic acid and mercuric chloride. 



In every case, action was most pronounced and took place sooner when the 

 lower surface of the laurel leaf was treated but was also noticeable when the 

 upper surface alone was moistened. Owing to its waxy nature the upper 

 surface cannot well be wetted properly and liquid easily passes to the lower 

 surface ; recently we have overcome this difficulty by applying wetted strips 

 of bibulous paper to the leaf surface ; from the results obtained in this 

 manner we judge that the upper surface is impermeable. 



A macroscopic study of the leaf whilst change is taking place in a solution 

 -of carbon dioxide reveals at first the appearance of little dots of dark green on 

 the under surface of the leaf ; these areas next become brown ; subsequently 

 the brown spreads until it uniformly covers both sides of the leaf; at the 

 same time the veins become swollen and very conspicuous on the underside. 



If a leaf be immersed in a dilute solution of methylene blue, it becomes 

 ■stained locally, a series of small blue dots appearing on the under surface 

 precisely similar to the dark green dots mentioned above. Hydrogen cyanide 

 is not liberated in this case ; apparently the dye stuff is laid down entirely at 

 the surfaces of entry. 



The explanation we have advanced involves the assumption that in all cases 

 in which growth is promoted or a stimulative eifect exercised, even by a sub- 

 stance to which merely " mechanical " activity can be ascribed, the concen- 

 tration is lowered to a point which is either favourable to a change previously 

 ■only in the potential stage, or more favourable to one already in operation. 



2 Y 2 



