1910.] 



The Origin of Osmotic Effects. 



595 



of the fluids within the leaf, tliese alterations being of such nature 

 that the glucoside is thereby brought effectively into contact with the 

 enzyme under conditions which favour hydrolysis. All substances which 

 gain access to the cell must have this effect in some degree. 



Initially, such changes need not cause any breakdown of the leaf system 

 and may be merely stimulative, or in some cases, perhaps, inhibitive ; they 

 cannot continue long, however, without damage to the protoplasmic mechanism 

 and in isolated tissues must soon pas& beyond the stage when recovery is pos- 

 sible — the more so because the effect is cumulative, owing to the influence 

 exercised by the products of change in changing the osmotic state. 



From this point of view, it is highly significant that two of the products 

 into which the glucoside prulaurasin is resolved, hydrogen cyanide and 

 benzaldehyde, which pass into the leaf as vapours when used as anaesthetics, 

 both condition the breakdown ; obviously, if this were due primarily to some 

 chemical interaction, it would scarcely be initiated by the products of change. 



Benzaldehyde, being only slightly volatile, is only slowly active but when 

 a laurel leaf is exposed in a moist atmosphere of hydrogen cyanide, it becomes 

 olive brown within 24 hours. 



It is even more striking that the effect is produced by carbon dioxide ; 

 when leaves are set aside under precisely similar conditions in corked tubes 

 containing air and carbon dioxide (carefully purified by passage through 

 water and through tightly packed moist cotton-wool scrubbers) respectively, 

 in every case those in the latter gas are the first to turn brown ; hydrogen 

 cyanide is given off but the change is manifest only after three or four days, 

 or even longer. An aqueous solution of the gas is somewhat more active. 



As freshly picked leaves are found to contain as much as 60 per cent, of 

 water, there is no difficulty in accounting for the presence in leaves of 

 sufficient water to pass from one region to another. The fact that the 

 region in which the enzyme is present is a favoured region, in that the 

 solution flows towards it, is perhaps a consequence of the position occupied 

 by the enzyme close to the exterior surface at which the hormone enters 

 and towards which, therefore, the flux of water is necessarily determined. 



It is questionable whether when water enters alone it be active in deter- 

 mining change ; in any case, the mechanical action of the entering substance is 

 probably not confined to the water in the cell ; the issue thus raised, however, 

 is one which we do not propose to consider until we have completed 

 experiments bearing on the problem. 



In the case of substauces which enter from solution, water passes into the 

 leaf together with the substance which was dissolved in it. To determine 

 the extent to which water is absorbed, leaves which had been soaked in 



VOL. LX.XXII.— B. . 2 Y 



