The Plant. 95 



grains. The pentosans are intimately associated with the cellu- 

 lose of plant tissue. They differ from their corresponding sugars, 

 the pentoses, by the equivalent of one part less of water. Upon 

 boiling with dilute mineral acids each of these compounds takes 

 on one part of water. Araban yields arabinose readily, while 

 xylan yields xylose only gradually under these conditions. This 

 behaviour demonstrates the carbohydrate nature of the bodies 

 under consideration. The following percentages of pentosans 

 have been found in some plant materials: 



Hays 20 per cent 



Gluten feed 17 " 



Linseed meal . .- 13 " 



Brewers' grains 24 " ' 



Wheat Bran 24 



From 60 to 90 per cent of these compounds in feeding stuffs 

 disappears from the digestive tract of herbivora. This may be 

 partly due to bacterial fermentation. Since pentosans, when 

 assimilated by the animal, appear to have a value similar to that 

 of starch, it is evident that in some cases they may be of con- 

 siderable importance as constituents of the carbohydrate material^/ 

 of feeding stuffs. 



Fats are uniform in their general composition, consisting of 

 one part of glycerine combined with three parts of fatty acid. 

 The latter constituent controls the nomenclature of the fats. 

 Thus, for example, the fat containing three parts of stearic acid 

 is known as ' ' tri-stearin, " or more commonly as "stearin." Fats 

 which contain two or three different fatty acids in combination 

 with the same part of glycerine are called "mixed glycerides." 

 Acetic acid, which causes the sour taste in vinegar, is a typical 

 example of the fatty acids, the simpler members of this group 

 of compounds being volatile liquids of characteristic, pungent 

 odor similar to that of the acid cited. The higher members of 

 the acetic acid series are solid substances ; and the fats in which 

 they occur are also solid, in distinction from liquid fats or oils 



