186 THE CONSTITUENTS OF PLANTS. IX. 



atoms, and many of them exhibit optical activity, i.e., they 

 rotate the plane of polarised light to the right (-(-) or to the 

 left(-). 



Many members of the group of carbohydrates have been 

 prepared. They may be conveniently divided into the follow- 

 ing classes : 



A. The Amyloses or Polysaccharides. The general for- 

 mula for these bodies is (C,;H 10 O 5 ) n . They differ from the 

 sugars in heing less soluble in water, generally non-crys- 

 talline, and incapable of reducing alkaline copper solutions. 

 As a rule they are readily converted, by heating with dilute 

 acids, into sugars of the glucose class. 



The most important members of this class are 

 Starch 

 Dextrin 

 Glycogen 

 Inulin 

 Levulin 

 Gums 

 Cellulose. 



B. The Sugars. These substances are soluble, generally 

 crystalline, reduce alkaline copper and silver solutions, and 

 for the most part are fermentable under the influence of yeast, 

 yielding alcohol and carbon dioxide as the chief products. 



They may be divided into the following groups : 

 (a)* The Glucoses or Hcxo-monoscs, CH 12 O,,: 



* In recent years substances possessing all the essential characters of sugars have 

 been prepared artificially, containing from 2 to 9 atoms of carbon. These are to be 

 regarded as belonging to the class of the mono-saccharoses or monoses, and have been 

 distinguished by the names bioses, trioses, tetroses, pentoses, hexoses, heptoses, 

 octoses, and nonoses 



The monoses are found to combine the properties of alcohols and aldehydes, or 

 alcohols and ketones. and are sometimes divided into aldosex anil kftoges. Thus 

 Glucose or dextrose is an aliioxf. Lectdose or fructoie is a ketoxe. 

 CHoOH CHoOH 



I I 



CHOH CHOH 



I I 



CHOH CHOH 



I I 



CHOH CHOH 



I I 



CHOH CO 



I I 



HCO CH. 2 ()H 



The group HCO being characteristic of aldehydes, as CO is of ketones. 



