MONOSACCHARIDES 89 



recorded in the leaves of carrot, mangold, potato, sunflower, 

 TropcEolum and turnip,* and also in Opuntia phcsacantha.'\ 

 Pentosanes, however, which may be regarded as polymerized 

 anhydrides of pentose are very widely distributed in the 

 vegetable kingdom, forming cell wall constituents and entering 

 into the composition of various gums, mucilages, and pectins. 

 With regard to their physiological significance, it is im- 

 possible to say whether they are direct products of photo- 

 synthesis ; if, as Spoehr points out, the formation of sugar in 

 a green leaf is a series of additions of molecules of formalde- 

 hyde, the presence of pentose is to be expected. There is, 

 however, no evidence that this occurs in the green plant. 

 On the other hand, they may have their origin in the 

 oxidation of hexose. The facts that in the germination of 

 seeds, the amount of total pentoses falls as development 

 proceeds and that in some instances the amount is high at 

 certain phases, thus in Parthenium argentatum a high percen- 

 tage of pentose coincides with the period of growth during which 

 the production of rubber is at its highest, suggest that pentoses 

 are definite stages in the elaboration of other substances. As 

 a food material the value of pentoses is variable ; whilst xylose 

 has a high nutritive value for Aspergillus, it, together with 

 other pentoses, is not utilized by Saccharomyces. In the 

 higher plants, Spoehr % has shown that the respiration of 

 Cactacece is not depressed when the hexoses are insignificant 

 in amount, § and that the formation of pentosanes is bound up 

 with certain conditions, especially water content and tempera- 

 ture. Thus in the Cactaceae, a low water content coupled with 

 a high temperature results in a decrease in the amount of 

 monosaccharides and an increase in polysaccharides and pen- 

 tosanes. On the other hand, a high water content and a 

 low temperature are associated with an increase of mono- 

 saccharides and a decrease of pentosanes and polysaccharides. 



* Davis and Sawyer : " J. Agr. Sci.," 1914. 6, 406. 



t Spoehr : " Carnegie Inst. Pub.," 1919, No. 287 ; " Plant World." 

 1917, 20, 365. 



X spoehr : loc. cit. 



§ It is not uncommonly assumed that in the respiratory activity of 

 higher plants, hexoses are the significant fuel. 



