318 SCIENCE PROGRESS 



extent, they may be said to be half right-hand and half 

 left-hand material. 



Two other hexose sugars isomeric with glucose occur 

 naturally — galactose and mannose ; but the three compounds all 

 belong to the one series and all may be said to be right-hand 

 material. 



Besides these three hexose sugars, plants also contain the 

 ketose, fructose, which is isomeric with glucose and differs from 

 it only in containing the CO group as the second instead of as 

 the terminal member in the chain of radicles composing the 

 molecule : 



CH,(OH) . CH(OH) . CH(OH) . CH(OH) . CO . CH, . OH. 



Fructose is convertible into glucose and vice versa. Natural 

 fructose and glucose are both right-hand material. Nature 

 apparently is single-handed and can make and wear only right- 

 hand gloves. 



It is possible to prepare such compounds in the laboratory 

 from the simplest materials, starting from carbonic acid — 

 CO(OH) 2 — the compound from which the plant derives carbon. 

 By reduction this is first converted into formaldehyde, COH 2 . 

 When digested with weak alkali, this aldehyde is in part 

 converted into fructose ; the fructose that is formed, however, is 

 not merely the form which is found in plants but a mixture of 

 this with an equal proportion of the left-hand form. When the 

 chemist makes gloves, he usually cannot help making them in 

 pairs for both hands. 



Some directive influence is clearly at work in the plant — 

 the formaldehyde molecules, which it undoubtedly makes use 

 of as primary building material, in some way become so ar- 

 ranged that when they interact they give only the right-hand 

 form of sugar ; there is reason to think, moreover, that the 

 action takes place only in this one direction — that the sugar is 

 the only product. My own belief is that the synthesis is 

 effected against a sugar template 1 just as a brick arch is built 

 upon a wooden template curved as the arch is to be curved. 



A similar argument 'is applicable to the albuminoid or 

 protein matters derived from animal and vegetable materials ; in 

 fact, to nearly all the natural optically active substances : these 

 are all formed under directive influences. It is not improbable 



1 Proceedings of the Royal Society, 1904, vol. 73, 541. 



