366 H. A. SPOEHR 



quality is undoubtedly due to the ease and multifarious ways in 

 which the sugars are dissociated as well as to their high free 

 energy. Thus, under aerobic conditions the sugars are easily oxi- 

 dized, and yield a large amount of energy, while under anaerobic 

 conditions also', thanks to their great dissociation, they may un- 

 dergo chemical changes liberating much energy.. 



In this respect neither the fats nor proteins can be compared 

 with the sugars as to usefulness for the organism. It is evident 

 therefore that the starting point for a rational study of the 

 chemistry of plant metabolism, as well as of the organic constit- 

 uents of protoplasm, is the investigation of carbohydrate synthe- 

 sis, rearrangement and disintegration. 



The carbohydrates found in plants are in general of two kinds: 

 those in which there are five carbon atoms, the pentoses, and 

 those containing six carbon atoms, the hexoses. Members of the 

 tetrose series, four carbon atom sugars, are exceedingly rare. 

 The pentoses and hexoses appear to some extent in the mono- 

 molecular form, but especially as di- and polysaccharides in 

 which a number of pentose or hexose molecules are condensed 

 to form a large molecule as cane sugar, starch and cellulose. 

 The pentoses are most familiar as pentosans as components of 

 the cell walls and vessels of the plants, and as found in various 

 gums in the form of xylan and araban. 



While widespread in the vegetable kingdom, they have but 

 recently been regarded in their important bearing, and, in fact, 

 were for a long time not recognized as belonging to a separate 

 group. These sugars occur mainly in the condensed form as 

 pentosans. In fact, the presence of monosaccharide pentoses 

 has been but very recently established. The main point how- 

 ever, is that the five carbon atom sugars have been found in 

 one form or another as a component of almost all plants. 



On account of their great similarity in chemical composition 

 and reaction, the quantitative separation and individual deter- 

 mination of the hexose and pentose sugars is associated with 

 considerable difficulty, and only by the exercise of great care 

 can reliable data be obtained. It has been found that pentoses 



