82 DISACCHARIDES, POLYSACCHARIDES AND GLUCOSIDES 



result (containing methyl-pyrrole groupings), may very possibly be 

 unobtainable by mammals in any other way. 



The higher polysaccharides are very imperfectly defined. We have 

 no reliable methods which are available for determining their molec- 

 ular weights, and we do not know, therefore, how many molecules of 

 sugar take part in their formation. The group is a very large one, and 

 the general formula (C 6 H 10 O 5 )n may be ascribed to the majority of its 

 best-known members, indicating that they are formed by the union 

 of an indefinite number, n, of hexose anhydrides. The following sub- 

 stances are important and typical members of the group: Starch, 

 Glycogen, Dextrins, Inulin, Pectin, Humin, Cellulose, Gums, and Vege- 

 table Mucilages. It is very important to recollect, however, that these 

 are merely arbitrary terms used to describe very ill-defined members 

 of the series. Thus we cannot be certain that there is only one chemical 

 individual Starch ; on the contrary, it appears probable that there may 

 be many starches, and starch is certainly known in two widely different 

 forms, to wit : a form insoluble in water and a form which is soluble in 

 water. On the other hand it should be recollected that the differences 

 which are observed between these forms of starch may possibly be 

 purely physical, and not chemical differences at all. We here encounter, 

 in fact, a problem which is presented generally by the colloids, and 

 which we shall meet with again in connection with the proteins. 



Starch, inulin, gums, mucilages and glycogen do not reduce metallic 

 oxides in alkaline solutions. They do not, therefore, contain potentially 

 active aldehyde-groups. Dextrins, on the contrary, do contain alde- 

 hyde-groups, for they reduce Fehling's solution. With the possible 

 exceptions of glycogen and inulin, the polysaccharides do not form 

 crystals, or at least, they have not as yet been prepared in crystalline 

 form. Water dissolves some of them, others only swell in cold water 

 and dissolve in hot water, others are unaffected by water. Solutions 

 of the polysaccharides do not taste sweet unless held in the mouth for 

 a sufficient period to enable the diastase (Ptyalin) in the saliva to 

 bring about hydrolysis. Solutions of the polysaccharides are optically 

 active. The higher polysaccharides do not diffuse through parchment 

 paper, thus behaving typically as colloids. They do not form com- 

 pounds with phenylhydrazine. 



The polysaccharides play a wide variety of parts in the vegetable 

 kingdom. In the first place, they serve as reserve materials, or stores 

 of sugar, laid up against a future time of need; such a part is that 

 played by starch (or vegetable glycogen, as it may be called in analogy 

 to animal glycogen, which plays a similar part in the animal economy) 

 and also by inulin. The gums and mucilages, on the contrary, serve, 

 in part at least, to close up injuries and protect them while healing. 

 The celluloses, again, have yet another function to perform. They, 

 or their derivatives, constitute the supporting tissues of plants, just as 

 bones or exoskeletons constitute the supporting tissues of animals. 



The Celluloses (the plural is employed because it appears highly 



