MAITUAL OF CATTLE-FEEDIlSp. 45 



Tlie close relationsliip between starch and eellnlose is 

 albo bliown by their beha% ioi* toward iodine. As we have 

 seen, starch is colored blue by this reagent, while cellulose 

 re(|iiires the addition of sulphuric acid (or one of several 

 other substances) to produce the blue color. 



It is only the graiiuhse of starch, however^ which gives 

 a blue with iodine, while the stareh-eelluhse behaves like 

 ordinary cellulose, and, on the other hand, J. Kuhn "^ has 

 shown that the cotyledons of the flax-seed contain a form 

 of cellulose which is colored blue by iodine alone. 



Indeed, the most recent investigations seem to show 

 that there is a numerous series of carbhydrates, vaiying 

 from the most insoluble and resistent to the most soluble 

 and easily attacked forms, and capable of mutual intereon- 

 version in the plant and, to a certain extent, out of it. 



The PEGTm Substances. — This ffroup includes a num- 

 ber of bodies of rather uncertain composition, which are 

 the characteristic ingredients of fi'uit- jellies. They are 

 found in ripe fruits, and, together with sugar, constitute 

 the larger part of the non-nitrogenous organic matter of 

 the common root crops. Uncooked fruits and roots are 

 supposed to contain a body called pectose, which, on boil- 

 ing with water or exposure to heat, is converted into 

 pectin, which is soft and soluble in water. It is this 

 change which takes place in the cooking of fruit. 



By further heating, the pectin is converted into pectic 

 and pectosic acids. These substances ai-e insoluble in cold 

 ■water, and constitute the essential part of fruit-jelly. Pec- 

 tosic acid is soluble in boiling water, and hence most jel- 

 lies become liquid on heating ; on cooling, its solution gela- 

 i inizes again. Pectic acid is insoluble, even in boiling water. 



* EmahruBg des Bindviehes, 6fcli ed. , p. 49. 



