694 SCIENCE PROGRESS 



indirectly deprived of its oxygen : it seems to me probable 

 that water undergoes electrolysis under the influence of light 

 and chlorophyll and that whilst oxygen is evolved, hydrogen 

 becomes attached to the chlorophyll, subsequently acting on 

 carbonic acid as a reducing agent and converting it into 

 formic acid and then into formaldehyde. The evidence that 

 this aldehyde is the primary material with which the plant 

 works is almost overwhelming. 



From this point onwards, change undoubtedly proceeds to 

 some extent as in the laboratory but mainly along lines which 

 require us to assume that directive forces are at work : the 

 chief substances elaborated, the carbohydrates (sugars, starch, 

 etc.) and the albuminoids or proteins, also the essential oils 

 and alkaloids, are all optically active substances. 



The significance of this circumstance can be appreciated 

 only when certain geometrical considerations are understood. 

 The carbon atom has the power of combining with four other 

 atoms or groups of atoms ; a compound in which four different 

 systems are associated with a carbon atom may be symbolised 

 by a four-sided prism or tetrahedron. Two such models may 

 be constructed — you can make them easily from cardboard — in 

 which the systems are arranged in a different order, so that the 

 one is to the other what an object is to its reflected image or a 

 right-hand to a left-hand glove — such models will not fit over 

 one another and are said to be without a plane of symmetry, as 

 they cannot be divided into halves. A carbon atom in such a 

 condition is termed asymmetric. 



All asymmetric compounds have the remarkable property 

 of deflecting polarised light ; hence they are said to be optically 

 active. Whenever substances containing asymmetric carbon 

 are produced artificially from inactive material, the two forms 

 of opposite but equal activity are always produced in equal 

 proportions — the one form turning polarised rays to the 

 right, the other turning them to the left, to equal extents 

 although in opposite directions : the product is therefore 

 optically inert, as the equal opposite activities of the two 

 forms neutralise one another. If change took place in the 

 plant in the same way as in the laboratory, it is to be expected 

 that mixtures in equal proportions of the two optically active 

 forms of each particular compound would be produced. As 

 a matter of fact, the carbohydrates are of one series only — the 



