176 GLUCOSIDES 



that in the former plant the localization of the glucoside is 

 not so clearly defined as in the latter ; also this substance dis- 

 appears from the leaves of the cherry laurel, when kept in the 

 dark, much more slowly than does the glucoside in Pangium 

 on similar treatment This indicates that these two gluco- 

 sides have a different chemical constitution, and analysis has 

 shown this to be the case. In Pangium edule, and also in Linum 

 and other plants, the glucoside has an acetone cyanhydrin 

 residue, while in the case of Prunus the residue is benzalde- 

 hyde cyanhydrin. The former glucosides are less stable than 

 the latter. 



With regard to the stages which lead up to the formation 

 of prussic acid and its compounds, Gautier has put forward 

 the supposition that it may possibly be formed by the action 

 of formaldehyde on nitrates, and this view is not inconsistent 

 with the distribution of nitrates in the leaves of some plants, 

 but nothing definite is known. 



Reactions, Microchemical and Otherwise. 



1. The presence of cyanogenetic glucosides or of free 

 hydrocyanic acid can generally be detected by chewing a small 

 piece of the material. 



2. Thoroughly crush the part it is desired to examine 

 under water and set it aside for some time, then filter and add 

 to the filtrate a little silver nitrate ; a white precipitate indi- 

 cates hydrocyanic acid, but this test must be used with 

 caution as many other substances give a white precipitate with 

 silver nitrate. 



If the amount of enzyme present in the tissue be very small, 

 the maceration must be allowed to proceed for some time, or 

 emulsin may be added to hasten the decomposition. 



3. Cut a thick section of the fresh tissue to be examined 

 and place it in a 5 per cent alcoholic solution of potash for 

 about a minute; transfer to a solution containing 2-5 per cent 

 ferrous sulphate and I per cent ferric chloride and keep at 

 about 60 C. for ten minutes. Place the preparation in a 

 dilute solution of hydrochloric acid one part of strong acid 

 to six parts of water for five to fifteen minutes. The presence 

 of hydrocyanic acid is indicated by the formation of Prussian 

 blue. 



