THE ORIGIN AND ROLE OF PENTOSE SUGARS. 77 



The high nutritive value of 1-xylose is quite evident, while on the other 

 hand it is well known that the yeasts are quite incapable of utilizing any 

 of the pentose sugars. At the same time it must be remembered that no 

 one of the sugars is universally good nutrient material. Little is known of 

 the role of the pentoses in the metabolism of the higher plants. The 

 pentosans in the walls and vessels have been very extensively investigated, 

 but very little is known of the origin and physiological role of these sub- 

 stances in the plant. 



In the mammalian body the pentoses have been found to be about iso- 

 dynamic in food value with the fats, but are not protein sparers as the 

 hexoses are. 1 In carnivorous animals as high as 50 to 60 per cent of the 

 amount fed has been observed excreted in the urine ; in omnivorous animals 

 the percentage is less, while the herbivorous are capable of utilizing rela- 

 tively large quantities. 



That the pentose sugars are consumed at a very appreciable rate in the 

 course of catabolism of the cacti was shown in the starvation experiments 

 previously discussed. It was also demonstrated that when the cacti are 

 deprived of water there takes place, along with increase in the amount of 

 polysaccharides, a very decided increase in pentose sugars. As yet we have 

 very little definite knowledge of a coordination between results obtained on 

 the rate of respiration and the carbohydrate economy as influenced by con- 

 ditions such as water-balance and temperature. No simple or constant rela- 

 tion seems to exist between the rate of carbon-dioxid emission and the supply 

 of carbohydrates. It is most probable that in the protein-carbohydrate 

 complex the clavis to this relationship must be sought. Furthermore, the 

 rate of respiration can not be constantly or definitely associated with any 

 one group of sugars. It has been shown that the hexose monosaccharides, 

 which have been commonly assumed to be most directly involved in the 

 respiratory process, are at times present in only exceedingly small amounts. 

 This is particularly the case under conditions of low water-content and of 

 high temperatures. In spite of the fact that the monosaccharide-content 

 has been so greatly reduced, the rate of respiration does not show any marked 

 diminution. It would seem, therefore, that under stress the plant possesses 

 the power of utilizing the polysaccharides and aplastic material. An indi- 

 cation of this has already been obtained by MacDougal, Long, and Brown, 

 in the study already referred to, on cacti which had been starved for a long 

 time. 



The formation of pentosans is intimately associated with this condition. 

 As has been stated, the simpler sugars decrease in amount as the water- 

 content is reduced, and vice versa, an increase in water-supply results in an 

 increase in these sugars. Furthermore, pentosan formation is also depen- 

 dent upon the water-content of the plant. With continued low water-supply 

 the pentosans increase decidedly. 



1 SCHEBOKITCH, P. Beitrag zur Bedeutung der Pentosen als Energte quelle im tieri- 

 schem Organisms. Biochem. Zeitschr., 55, 370-392, 1913. 



