PLANT PHYSIOLOGY 243 



tinual transformation from one to the other. The chemical 

 reactions involved require in most cases simply the loss or 

 addition of the elements of water. These reactions are peculiar, 

 also, in being brought about through the agency of enzymes 

 which in many instances are known to be capable of producing 

 a reversible reaction. In plants, water and enzymes are both 

 abundant, so that there is nothing strange or improbable in the 

 belief that here the various members of the carbohydrate group 

 are constantly undergoing transformations back and forth 

 into each other. For example, glucose and fructose may yield 

 sucrose, glucose may yield maltose and then starch or cellulose, 

 and vice versa in each case. Not only are such transformations 

 possible theoretically, but direct experiment has proved that in 

 many instances they actually do occur. If then glucose is the 

 first essential constituent that is formed as a product of photo- 

 synthesis, all of the other carbohydrates may be derived from it by 

 metabolic changes. 



Sugars. — In all plant sap and juices, sugars are present and 

 oftentimes several of them, not only monosaccharoses but also 

 disaccharoses. Thus in fruit juices glucose, fructose and sucrose 

 are found, while in plant sap sucrose is the sugar usually present. 



Translocation MateriaL — As sugars are the soluble car- 

 bohydrates, all of the insoluble carbohydrates, starch in par- 

 ticular, must be converted into one of the sugars in order to 

 be possible of translocation from one part of the plant to another. 

 Also, as all carbohydrates, used as building material or as cell 

 food, must be translocated throughout the plant organism to 

 all the various cells, the sugars are the particular form of car- 

 bohydrates thus used. 



Cell Food. — In plants, as in animals, glucose is the form in 

 which all carbohydrate food, and perhaps all food, is finally 

 oxidized to yield energy. In plants, however, the production of 

 energy is of much less relative magnitude than in animals, and 

 not all of the glucose and other soluble sugars are required to 

 be used as energy food. The greater part of these compounds 

 is used in the plant as intermediate translocation material for 



