6 INTRODUCTION [CH. 



Cane-sugar, starch, cellulose, etc. Some of these products, such as the 

 disaccharides, form true solutions and may be present in the cell-sap ; 

 others, such as cellulose and starch, are present in the solid state, though 

 they contain considerable quantities of water. Others, again, such as 

 dextrin and gum, are present in the colloidal state. Thus, given an 

 initial carbohydrate and a source of energy, we may proceed to indicate 

 the other main lines of syntheses in the plant. 



The next most important line of syntheses is probably that which 

 gives rise to the nitrogen-containing constituents of the plant. Nitrogen 

 is absorbed by the green plant in the form of nitrates and ammonium 

 salts, but the processes which lead to the synthesis of some of the 

 V j simplest nitrogen- containing compounds, such as the amino-acids, are still 

 very obscure. Aliphatic and aromatic acids of various kinds are 

 abundantly present in the tissues, but the reactions by which the NH 2 

 groups are introduced are by no means clear. There is little doubt, 

 however, that once the amino-acids are formed, condensation takes place 

 as already indicated, and more complex molecules, termed polypeptides, 

 arise. Such polypeptides have now been synthesized artificially by the 

 condensation of amino-acids. From the polypeptides, by further stages 

 of condensation, the albumoses, peptones, and finally proteins are 

 produced. 



Another line of syntheses is that which leads to the production of 

 the fats and allied substances. The fats are mainly glycerides of acids 

 of the methane and olefine series, such as butyric, palmitic and oleic 

 acids. Like ail other plant products the fa,ts must either directly or 

 indirectly arise from the carbohydrates. There is evidence that the 

 origin is fairly direct, as, for instance, in fatty seeds when the fats take 

 the place of sugars in ripening. The sugars, as we know, are aldehydes 

 of the polyhydric alcohols of the methane series. It has been suggested, 

 though the actual stages have not been ascertained, that by various 

 oxidation and reduction processes, the sugars yield fatty acid residues 

 which then condense to form the fatty acids of high molecular weights 

 present in fats. By a converse process, the fats, especially when they 

 are stored as reserve materials in seeds, are broken up, and sugars are 

 again formed which pass to other parts of the germinating seedling, and 

 are there used in other synthetic processes. 



A third main line of syntheses is that which gives rise to the 

 aromatics of the plant. Since no ring compound is absorbed by the green 

 plant, it follows that by some process the aliphatic structure must be 

 transformed into the aromatic. Thus, for instance, the trihydric phenol, 



