4 THE CARBOHYDRATES. 



positively known, and therefore is represented by x. They 

 have probably a much higher molecular weight than the 

 other classes. Their constitution is not known. They are 

 mostly amorphous, insoluble in water, and, consequently, 

 tasteless. Ferments and acids convert them into the mono- 

 saccharids. They do not have reducing power. 



STARCH. 



Starch occurs in the cells of the plant. It is in the 

 form of grains or granules, which vary in size in different 

 plants from about 0.002 of a millimeter to ten times that. 

 The granules are composed of two parts: an inner, soluble 

 one, called granulose, and an outer one, called cellulose. 

 This latter is insoluble in water and protects the starch 

 from the action of many of the weaker ferments. When 

 boiled or acted upon by alkalies it is broken, allowing the 

 granulose to escape and forming starch-paste, or soluble 

 starch. The shape and size of the granules differ so much 

 in the different plants that the source can often be deter- 

 mined by its microscopic appearance. Those of the potato 

 have a shape somewhat similar to that of a clam-shell, those 

 of wheat are round and smaller, and those of buckwheat 

 more irregular. (Plate I, 1 and 2.) 



Starch can be obtained from the parts of the plant 

 where it is stored up, like the tuber of the potato or the 

 kernel of grain, by macerating it, then washing out the 

 starch with cold water. 



Starch is a colloid. A colloid is a substance which 

 when dissolved will not pass through an animal membrane 

 or parchment. They are the opposite of crystalloids, which 

 are usually crystalline and which will diffuse through such 

 membranes. This process of diffusion or separation of 

 colloid from crystalloid substances is called dialysis. As 



