18 JOURNAL OF THE HOYAL HORTICULTURAL SOClfiTi^. 



The liberation of hydrogen cyanide is due in both cases to the resolu- 

 tion of a gliicoside (prunasin) present in the leaf into the sugar, 

 glucose, benzaldehyde, and hydrogen cyanide — or, expressed in 

 chemical symbols : 



CN.CH(C6H5).O.C6Hn05 + OH, = CNH + C«H,.CHO + C6Hi206 



Pruuasin Water Hydrogen Benzaldehyde Glucose 



cyanide 



The resolution of the glucoside is effected by an enzyme (prunase) 

 which acts by determining the addition of the elements of a molecule 

 of water to the glucoside. Probably, under normal conditions, the 

 glucoside which is in solution in the cell sap is separated from the 

 enzyme by a membrane through which it cannot pass, and one effect 

 produced by the stimulant " is to render this membrane permeable 

 by the glucoside. 



The common spotted Japanese laurel, Aucuha japonica, behaves 

 in somewhat similar manner. All who have observed this shrub must 

 have noticed the way in which the leaves blacken when injured. If the 

 Aucuba leaf be exposed to the vapour of chloroform, it soon acquires 

 a rich brown-black colour. Again, in this case, the change is primarily 

 due to the resolution of a glucoside (aucuhin) by the agency of an 

 enzyme. 



Very many other substances besides chloroform will provoke the 

 changes described — the vapours of almost all volatile organic sub- 

 stances, in fact, hydrocarbons, alcohols, phenols, ethers, essential 

 oils, &c., such substances acting very frequently the more readily 

 the more volatile they are. But volatility is not the real criterion of 

 activity ; if substances such as the alcohols and organic acids be com- 

 pared, they are found to be more and more active the less soluble they : 

 are in water, so that, speaking generally, it may be said that the sub- 

 stances which penetrate most readily into the leaf tissues and pro- 

 duce effects such as have been described are those which are least 

 readily soluble in water ; thus common alcohol is but slightly active 

 in comparison with amylic alcohol (fusel oil) and acetic but slightly ' 

 active in comparison with butyric acid. 



A very striking series of observations on the permeability of plant 

 tissues has been carried out by Professor A. J. Brown"^ with the aid 

 of common barley. Most samples of barley contain corns which 

 have a distinct blue colour, and in some cases — in Chilian barleys 

 especially — a large proportion of the corns are blue. The colour is 

 due to the presence of a *' pigment " in the layer just below the 

 outer skin. In the accompanying microphotograph (fig. 6) of a 

 section across a barley grain, this pigmented layer stands out very 

 clearly. The colouring matter resembles blue litmus in becoming 

 red when acid is brought into contact with it. 



Professor Brown finds that when sound blue barley grains are 

 immersed in solutions of the strong acids, only water is absorbed, 

 the blue layer remaining unchanged in colour. The same is true 

 * Proceedings of the lloyal Society (1909), B, vol. 81, p. 82. 



