THE LEAF AND ITS FUNCTIONS 173 



insight into the general nature of the changes that occur by considering 

 a few of the simpler and more important cases. 



As we have already seen, glucose (dextrose, grape sugar) is one of the 

 simple sugars, or monosaccharides, with the chemical formula C6Hi 2 6 . 

 Plants also make another simple sugar, fructose (levulose, fruit sugar), 

 which has the same chemical formula as glucose but somewhat different 

 properties. Fructose, which is perhaps formed directly by photosynthesis 

 like glucose, is still sweeter than the latter and is especially abundant in 

 sweet fruits. It appears to be important for tissue formation, whereas 

 glucose is primarily an energy food. 



Most of the sugar stored in plants is in the form of disaccharides, or 

 double sugars, which are made by the union of two monosaccharide mole- 

 cules with the loss of one molecule of water, as shown by the following 

 equation : 



Glucose or -+- Glucose or + Energy (yields) A disaccharide + Water 

 fructose fructose 



C 6 H 12 6 + C 6 H 12 6 + Energy -* C 12 H 22 O u + H 2 



The two commonest disaccharides in plants are sucrose (cane sugar), 

 composed of one molecule of glucose and one of fructose, and maltose 

 (malt sugar), composed of two molecules of glucose. Sucrose is much 

 the sweeter of the two and is the ordinary sugar that we use on the table. 

 It is one of the commoner substances used by plants for food storage, and 

 in plants like sugar cane and sugar beet, it almost wholly replaces starch 

 in this role. Maltose is of interest, since it appears to be an intermediate 

 step in the formation of the important material starch. 



Starch is one of the chief food-storage materials; it is especially abun- 

 dant in tubers, grains, and certain fruits and constitutes 30 to 70 per 

 cent of the dry weight of our own food. Chemically, starch is a poly- 

 saccharide, i.e., a carbohydrate built up of many monosaccharide units. 

 It is related to maltose in somewhat the same way that maltose is related 

 to glucose, since for each molecule of maltose that enters into the starch, 

 one molecule of water is lost. The chemical formula of starch is 

 n(C 6 Hi O 5 ), which, translated, means that the starch molecule is of no 

 fixed size but contains an indefinite number (n) of the C 6 Hio0 5 units; its 

 structure is thought to be that of an indefinitely long chain of linked 

 molecules, and the molecular weight is usually very high. Starch occurs 

 in the form of granules; it is an excellent storage material, both because 

 it is relatively insoluble, yet easily reconverted into soluble sugar for 

 transportation, and because it contains more energy per unit of weight 

 than do the sugars. During the day granules of starch form in the chloro- 

 plasts where glucose is being made, the excess sugar being thus removed 



