374 H. A. SPOEHR 



Such specific action of enzymes is one of the most complex and 

 subtle questions in chemistry. 



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, as has been indi- 

 cated above very little is known of the origin and physiological 

 role of these substances in the plant. 



In the mammalian body the pentoses have been found to be 

 about isodynamic in food value with the fats but are not pro- 

 tein sparers as the hexoses are. In carnivorous animals as high 

 as 50 to 60% 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 relatively large 

 quantities. ^^ 



Although physiological work with plants has some advantages 

 over the use of animals in metabolism studies, the former never- 

 theless is associated with obstacles which are exceedingly difficult 

 to overcome. The non-existence of great functional differentia- 

 tion, the impracticability of injecting substances, the lack of any 

 but gaseous excreta, and the high synthetic power of almost all 

 plant cells, make the study of the intermediate steps in plant 

 metabolism an exceedingly difficult experimental investigation. 



The fleshy joints of the cacti have been found to offer splendid 

 material for various phases of these studies. With the develop- 

 ment of new and special methods of analysis it has been possible 

 to gather data which it is believed may offer added light to 

 these important problems. In the experiments here recorded 

 Opuntia sp. was used exclusively. During March the new joints 

 develop, these grow quite rapidly, so that within about one 

 month the new ovoid joints have attained their full size of 100 

 to 125 cm. Apparently the young joints are autonomous very 

 early in their development; when cut from the plant with but a 

 very small portion of the parent joint, and placed with the 

 base in tap water, the young joints grow to full size and develop 



'5 Schirokich, P., Biochem. Zeit. 55: 370, 1913. 



Magnus-Levy, Oppenheimers Handbuch der Biochem, IV, 1: 399. 



