24 



LECTURE II. 



(especially the polysaccharides of fucose), which are found everywhere in 

 sea-moss. The fucose pentosans are furthermore found in gum-arabic, 

 cerasin, and gum-tragacanth, in the leaves of plane and linden trees, in pine 

 and beechwood, etc. The rhamnoses, first found in quercitrin, the coloring 

 principle contained in the bark of dyer's oak (Quercus tintoria), are also 

 widely distributed. Most pentosans, however, are derived from the 

 simple pentoses. The most important of these are Z-arabinose, which is 

 obtained from different gums; and /-xylose, also called wood sugar because 

 it is the most important mother substance of lignin (xylogen) . Xylogen 

 is also found in oat-, rye- and wheat-straw, etc. The pentosans in 

 general are by no means simple compounds, and yield on being sub- 

 jected to hydrolysis all sorts of different sugars of the five and six carbon 

 series. Doubtless there are a great many intermediary products lying 

 between the simpler and more involved complexes. As regards their 

 physiological function in the plant organism, our knowledge is still very 

 limited. We shall see later on that they are of importance for the 

 nourishment of animal organisms, particularly the herbivora. The fol- 

 lowing table will give some idea concerning the occurrence of pentosans, 

 the values being given in terms of pentose: 



As regards the formation of pentoses in plant organisms, we have no 

 experimental data. Chemically, we can, as has already been mentioned, 

 easily account for it in three ways. The simplest explanation is that of 

 the formation from formaldehyde, which is hypothetically the first assim- 

 ilation product of the carbonic acid in the air by the green leaves. Five 

 molecules of the aldehyde will condense to form one molecule of pentose 

 (5 X CH 2 O = C 5 HioO 5 ). It is also conceivable, that glycerose obtained 

 by the oxidation of glycerol is the starting-point, and from thence by the 

 third method of building up a sugar, namely, the addition of carbon 

 atoms, the pentoses may be formed. Finally it is possible that the 

 pentoses are formed by the breaking down of higher sugars. At all events 

 this relatively simple class of chemical compounds shows to what ex- 

 tremely diverse purposes the vegetable organism is capable of building 

 itself up. 



