THE PRODUCTS OF PHOTOSYNTHESIS 189 



in ether. Apiose benzylphenylhydrazone, crystallized from benzol, melts 

 at 135°, and the p-bromphenylosazone, from alcohol, melts at 212°. 



c. Pentoses. 



Pentoses are very common components of leaves and other portions 

 of plants. In the combined form, as pentosans, they have been found 

 widely distributed, while free pentoses have been found only in small 

 quantities. In fact, the existence of the free sugars as components of 

 leaves has not been universally accepted. The pentoses usually occur 

 as components of complex substances such as glucosides. nucleinic acids 

 and most commonly as anhydride-like condensation products of unknown 

 molecular weight. In the latter form they are found as pentosans, hemi- 

 cellulose, gums and mucilage. The chemistry and structure of these 

 latter compounds has as yet been but imperfectly worked out. The 

 pentose and hexose sugars in the leaves differ in that the concentra- 

 tion of the latter in the free or uncondensed form is usually much higher 

 than the former. On the other hand, of the total amount of sugars 

 present not infrequently one-half are pentoses. While, then, of the re- 

 ducing sugars present in plants, the pentoses may constitute but a very 

 small portion or be entirely absent, of the total non-reducing sugars or 

 polvsaccharides of the plant, there is usually a considerable proportion 

 of pentoses. They are usually found in the anhydride form as compo- 

 nents of the cell walls and vessels in the structural elements of plants 

 and may constitute a portion of the protoplasm. Thus, they are found 

 in straw, wood, pith, seed hulls, the expressed juice, and in many gums. 



The amounts of pentosans in different plants have been determined by 

 ToUens '^ and his students. Alany of these determinations are. however, 

 not to be taken as precise values, as the analytical methods employed 

 have since been found not to yield very exact results. Fresh oak leaves 

 have been found to contain about 10 per cent of pentosans, pine needles 

 about 7 per cent, sphagnum 15 per cent, lichens 3 to 8 per cent, mosses 

 3 to 17 i^r cent, and the woody portion of American conifers 6 to 10 

 per cent, while American deciduous trees contain 10 to 24 i>er cent. 



In mangold leaves. Davis, Daish and Sawyer -° found the free pentoses 

 to vary from about 0.4 to 0.9 per cent according to the season and Spoehr ^^ 

 found slightly lower amounts in Opuntia phaeacantha. 



d. Methods of Analysis of Pentoses. 



Owing to the fact that in the analysis of plant material, as for ex- 

 ample, the carbohydrates of leaves, we have to deal with a very complex 



"Tollens Bcr. chcm. Gcs.. 2, 1751 (1890); 24. 694, 3575 (1891); 25, 2912 

 (1892) De Chalmot. Atu. Clicni. Jour.. 15, 21, 276 (1893) ; 16, 218 (1894). Stone, 

 /. Am. Chcm. Soc, 19, 183 (1897). Lippmann, "Chemie der Zuckerarten," I, 52 



(1904). 



^ Davis, Daish and Sawyer, Jour. Agri. Sci., 6. 406 (1914). 

 "Spoehr, Carnegie Inst, of Washington, Pub. 287, 51 (1919). 



