12 NUTRITION OF FARM ANIMALS 



through a wide range, but the molecular weights of the in- 

 dividual polysaccharids have not been finally determined. 



The polysaccharids are tasteless and usually amorphous sub- 

 stances which, with the exception of cellulose, are more or less 

 soluble in water. They are optically active but in general are 

 not diffusible through membranes. They are hydrolyzed easily, 

 especially by the action of heat and acids and by enzyms, yield- 

 ing ultimately monosaccharids. 



In addition to their common names, they are designated by 

 terms derived from the monosaccharids out of which they are 

 built up. Thus starch, which is an anhydrid of dextrose and 

 yields only this sugar upon hydrolysis, is a dextran. Similarly, 

 there are levulans, galactans, mannans, arabans, xylans, etc., 

 yielding the corresponding sugars when hydrolyzed. In the 

 same manner, it is customary to distinguish between the 

 hexosans, derived from the hexoses, and the pentosans, the 

 anhydrids of the pentoses. 



16. The hexosans. This group of carbohydrates includes 

 those which are most abundant in the vegetable kingdom and 

 of the greatest significance as sources of nutriment for man and 

 animals, viz., starch, the dextrins and gums, and cellulose and 

 its various derivatives. It will be convenient to consider the 

 more important hexosans somewhat in the order of their re- 

 sistance to solvents. 



17. Cellulose. Cellulose constitutes the basis of the cell 

 walls of plants and is also found in certain lower animals (tuni- 

 cates). Clean cotton consists of nearly pure cellulose, each 

 fiber being a single cell from which the contents (protoplasm) 

 have nearly disappeared. Linen and the best qualities of paper 

 are other examples of nearly pure cellulose. A crystalline 

 form has also been described. 



Cellulose is insoluble in water and comparatively resistant 

 to reagents in general. Plants, however, contain enzyms 

 (cytases) which are able to bring it into solution in the processes 

 of plant growth, and apparently these enzyms play some part 

 in its digestion by animals. It is also attacked and dissolved 

 by some species of bacteria. Concentrated sulphuric acid dis- 

 solves it, and the solution, on dilution and boiling, undergoes 

 hydrolysis, yielding dextrose. Cellulose is therefore a dextran. 

 Its molecular weight is unknown. 



