D. R. HOAGLAND 



the immense problem of the origin of polysaccharides other than cellu- 

 lose and starch, which comprise a large fraction of many tissues of 

 higher plants, such as hexosans, galactans, pentosans, and related com- 

 pounds. There is need for more information about the synthesis of the 

 important pectin compounds. 



As a brief digression from the main theme, it is of general bio- 

 chemical interest to refer to the specificity of the enzyme system from 

 Pseudomonas saccharophila which catalyzes the synthesis of sucrose. 

 This enzyme system was not restricted in its catalytic potentiality to 

 the synthesis of sucrose. It was also efTective in bringing about the 

 synthesis of two new disaccharides. One was synthesized in vitro 

 from glucose- 1 -phosphate and /-sorbose, the other from the glucose 

 ester and a ketoxylose (3). The versatility of enzyme systems of this 

 type was thereby demonstrated. 



To the student of practical plant nutrition interested in the 

 application of fertilizers to soil, the utilization of simple nitrogen com- 

 pounds by crop plants is always a topic of dominant interest. This 

 interest is accentuated today because of the enormous expansion of 

 industries for the fixation of atmospheric nitrogen. Greatly increased 

 quantities of fi.xed nitrogen could be made available after the war for 

 agriculture. Field and pot experiments on the effects of nitrogen 

 fertilizers on crop growth are of course legion, but this type of investi- 

 gation discloses little of the biochemical processes by which the nitrate 

 or ammonia absorbed by the plant is elaborated into organic com- 

 pounds such as the amino acids. 



Fortunately, this subject has attracted the efTorts of a number 

 of able investigators whose primary concern has been that of biochem- 

 istry; for reviews on the history of research on nitrogen metabolism of 

 plants and recent trends, the monograph by Chibnall (2) and re- 

 ports by Vickery ^< a/. (15-18) may be consulted. In the considera- 

 tion of this aspect of plant biochemistry, it is apparent once more that 

 guidance in the interpretation of data and in the design of experiments 

 is greatly influenced by previous research concerned with the bio- 

 chemistiy of muscle and other animal tissues. 



The importance of organic acids and their cycles of metabolism, 

 including the tricarboxylic acid cycle, have been stressed by some 

 investigators. Prominent among the organic constituents of plants are 

 malic and citric acids. Oxalic acid also occurs very frequently, and 



72 



