228 



The Origin of Osmotic Effects. 



out into the liquid; if a substance which can penetrate into the leaf be 

 added to the water, as a rule, not only does the leaf change in appearance 

 but substances soon pass out from it into the surrounding liquid. In the 

 case of Aucuba, for example, an amount of reducing sugar equal to from 

 3 to 4 per cent, on the original weight of the leaf diffuses out into the 

 solution in the course of three or four days. 



If the hormone used be hydrogen cyanide, however, although changes take 

 place within the leaf, no reducing sugar passes out into the solution. It 

 suffices to use a solution containing only 0'2 per cent, of the cyanide. The 

 difference has been noticed in the case of a considerable variety of leaves, in 

 roots such as that of the radish and beet, in unripe fruits (cherry and 

 currant) and in unripe seed pods. 



Most leaves become coloured more or less distinctly brown, some even 

 black, on exposure in water saturated with either chloroform or toluene ; but 

 in a solution of hydrogen cyanide the colour change is far less marked, the 

 green colour being preserved often during a considerable period. The difference 

 is particularly noticeable in the case of leaves which blacken in chloroform, 

 such as those of Vicia faba, for example. The almost black colour assumed 

 by the Aucuba leaf in presence of chloroform is evidently due to several 

 superposed effects ; in the cyanide solution such leaves become highly 

 coloured but not nearly to the same extent as when they are exposed to the 

 action of other hormones. 



These differences would seem to be proof that differential septa which 

 break down under the influence of most hormones remain intact when 

 hydrogen cyanide is used, though hydrolytic changes take place within the 

 leaf under the influence of this latter agent.* 



Taking into account the manner in which leaves change in appearance 

 when exposed in water saturated with a substance such as toluene, there can 

 be little doubt that the coloration is at least mainly an oxidation effect ; and 

 bearing in mind what is known of the effect hydrogen cyanide has in 

 inhibiting oxidation, it appears probable that differential septa remain 

 intact because the " oxidase effect " is eliminated in presence of hydrogen 

 cyanide. 



It is well known that oxidation processes are at a maximum in plants 

 during the period when light is inactive and that growth takes place chiefly 

 during this period : the translocation of nutritive materials which necessarily 

 sets in during this period may well take place because the septa are broken 



* We have already called atteution in our previous communication to the production 

 of reducing sugars within the laurel leaf when it is exposed to the action of hydrogen 

 cyanide. 



