CARBOHYDRATES 31 



octoses, and nonoses, respectively. Substances corresponding in 

 composition and properties with the artificial tetroses and one or 

 two derivatives of heptoses are occasionally found in plant tissues, 

 and a considerable number of pentoses and their condensation 

 products are common constituents of plant gums, etc. ; but the 

 great majority of the natural carbohydrates are hexoses and their 

 derivatives. 



GROUPS OF CARBOHYDRATES 



Since the simpler carbohydrates are sugars, i.e., they possess 

 the characteristic sweet taste, the name " saccharide " is used as a 

 basis for the .classification of the entire group. The simplest 

 natural sugars, the hexoses, CcH^Oe, are known as mono-sacchar- 

 ides. The group of next greater complexity, those which have the 

 formula Ci2H22On and may be regarded as derived from the 

 combination of two molecules of a hexose with the dropping out 

 of one molecule of water at the point of union, are known as 

 di-saccharides. Compounds having the formula CisH^Oie (i.e., 

 three molecules of CeH^Oe minus two molecules of H^O) are 

 tri-saccharides; and the still more complex groups, having the 

 general formula (CeHioC^n, are called the poly-saccharides. 

 The mono-, di-, and tri-saccharides are generally easily soluble m 

 water, have a more or less pronouncedly sweet taste, and are 

 known as the sugars; while the polysaccharides are generally 

 insoluble in water and of a neutral taste, and are called starches. 

 As will be seen later, there are many natural plant carbohydrates 

 belonging to each of these groups. 



In addition to these saccharide groups, there are other types, 

 or groups, of compounds which resemble the true carbohydrates in 

 their chemical composition and properties and are often considered 

 as a part of this general group. These are the pentoses, 

 and their condensation products, the pentosans 

 and their methyl derivatives, CeH^Os; certain polyhydric alco- 

 hols having the formula CeHgtOH^; pectose and its derivatives, 

 psctin and pectic acid; and lignose substances of complex compo- 

 sition. It is doubtful whether these compounds are actual 

 products of photosynthesis in plants, or have the same physiological 

 uses as the carbohydrates and it has seemed wise to consider 

 them in a separate and later chapter. 



