94 Agricultural Chemistry. 



saccharides, we have a di-saccharide. An addition of another 

 simple sugar produces a tri-saccharide. Further increments re- 

 sult in dextrines of increasing complexity and decreasing solu- 

 bility until we have as a product, starch. This is a substance 

 insoluble in cold water and decomposes with some difficulty. 



By some internal re-arrangement of the chemical elements in- 

 volved in the carbohydrate molecule, we may have cellulose pro- 

 duced instead of starch. This is an extremely resistant and 

 comparatively permanent compound in which apparently the 

 stability of the carbohydrates has reached a maximum. These 

 constructive processes take place only in the plant. We can fol- 

 low them in the chemical laboratory only in a reversed order, 

 proceeding from the complex to the simple. Our knowledge is 

 therefore concerned with the general relations of these com- 

 pounds, rather than with the actual changes by which they are 

 successively produced in the plant. 



The pectin substances and pentosans should be classed under 

 the general head of carbohydrates. 



Pectins are insoluble bodies which occur in the flesh of most 

 unripe fruits. Upon boiling with water they yield various poor- 

 ly defined compounds of gelatinous nature, sometimes referred 

 to as pectoses or pectic acids. It is to these bodies that the " set- 

 ting" of fruit jellies is due. On treatment with weak acids or 

 alkalies, they yield simple sugars, thereby disclosing their carbo- 

 hydrate nature. Besides dextrose, they yield a class of sugars 

 containing five parts of carbon and hence designated as pentoses. 

 The mucilaginous substances of flaxseed, quince fruit and parts 

 of many other plants, are of pectin nature. 



Pentosans are present in considerable amounts in certain 

 gummy exudations of plants, such as cherry gum, which oozes 

 from wounds on trees of the prunus genus, and gum arabic of 

 tropical Acacias, a genus of leguminous plants. The pentosans 

 of gum arabic yield on hydrolysis a pentose sugar called arab- 

 inose. Xylose is a pentose sugar obtained from the so-called 

 wood gums, or pentosans which are abundant in straws and some 



