52 PLANT COMPOUNDS 



or monosaccharides, contain from two to nine carbon atoms, 

 and are named according to the number of carbon atoms in 

 the molecule: Dioses, trioses, tetroses, etc. Each molecule 

 consists of a single "sugar" group of atoms. The only 

 important monosaccharides are the hexoses, or sugars con- 

 taining six carbon atoms, C 6 Hi 2 6 . Dextrose and levulose 

 are the best examples of hexoses. The disaccharides are 

 formed by the condensation of two molecules of a monosac- 

 charide with the elimination of one molecule of water. Each 

 molecule of a disaccharide, then, consists of two single 

 "sugar" groups of atoms. Sucrose and maltose are the 

 principle disaccharides found in plants. 

 52. Dextrose, Glucose, Grape Sugar. — CeH^Oe, graphically: 



H 



H— c— o— H 



H— C— O— H 

 H— C— O— H 

 H— C— O— H 



H— C— O— H 



I 

 H— C=0 



calculation the percentage composition of sugar or other substance can be 

 determined in a solution. The instrument is called a polariscope, or, since 

 it is used mostly for the determination of sugar, a saccharimeter. It con- 

 sists essentially of a tube containing at one end a polarizing prism which 

 polarizes a ray of light from some source, and an analyzing prism at the 

 other end mounted on a revolving disk graduated into degrees. Between 

 the two prisms is a trough in which may be placed a tube, closed at both 

 ends with glass, containing the solution to be examined. The polarized 

 light if undisturbed passes through the analyzer and illuminates the field 

 of vision through an eye piece. If the ray is rotated by an optically active 

 substance, the light does not pass through the analyzer and the field of 

 vision is darkened. If the analyzer is now rotated to the right or left, as 

 the case may be, just as much as the substance rotates the polarized ray, 

 the field of vision will again be brightly illuminated. There are many 

 modifications of this polariscope tending to increase the accuracy of obser- 

 vation, but the principle is the same in all of the instruments. For com- 

 paring the rotatory power of different substances, there is used the term 

 specific rotation which is the amount of dextro- or levorotation of plane 

 polarized sodium light caused by a solution 10 cm. long, each cubic centi- 

 meter of which contains one gram of substance, at a temperature of 20° C. 



