OUTLINES OF THE PHYSIOLOGY OF NUTRITION 215 



perhaps the most important advance which organic 

 chemistry has made in our time in a physiological 

 direction. It is an interesting fact that bodies 

 of the nature of aldehydes are important links in 

 the chain of reactions by which sugars have been 

 artificially produced. 



It is, however, perfectly possible that the carbon 

 monoxide and nascent hydrogen may first be taken up 

 by the proteids which constitute the protoplasm, and 

 that the sugars may be formed by a subsequent 

 breaking down of the highly complex proteid sub- 

 stances. Our actual knowledge of what takes place in 

 a plant does not go beyond this : that the first demon- 

 strable product of carbon-assimilation is some form of 

 sugar, which is produced, however indirectly, as the 

 result of the decomposition of carbon dioxide and water. 



A certain part of the sugar, which is formed as an 

 ultimate result of assimilation, is in most plants 

 employed by the protoplasm for the manufacture 

 of starch ; the remainder is probably conveyed away 

 at once to other parts of the plant as some form 

 of sugar, without necessarily passing through the 

 intermediate form of starch. Starch, in fact, is 

 nothing more than an insoluble form of carbohydrate, 

 which is convenient for storing purposes. Both in 

 the chloroplastid and in the leucoplastid starch is 

 produced at the expense of sugar. The difference 

 between them consists in the origin of the sugar, 

 which in the former case is the result of the assimila- 

 tion of inorganic compounds, while in the latter it is 

 derived from previously existing carbohydrates. 



