THE PLANT 85 



(1) The Starches. — These have the composition expressed by 

 the formula CgHjyO^,but tlieir molecules aremuch more complex 

 than the formula indicates ; (CcHj^jOJ^, where w is a large 

 number, more correctly expresses their constitution. Among 

 the most important members of this group are : 



Starch. Glycogen. 



Dextrin. Cellulose. 



Starch is vei*y abundant in vegetable products, acting as a 

 reserve material for the nourishment of the growing portions. 

 It possesses ari organised structure, being in the form of 

 granules, which possess difterent forms and sizes in different 

 plants. It is insoluble in cold water, but when heated to about 

 C0° or 70° C. with water the granules burst, and their contents 

 form with the water a viscid, translucent liquid known as 

 starch paste. Starch is coloured intensely blue by free iodine. 



Glycogen has the same composition as starch, and occurs in 

 animals, especially in their livers. It is therefore sometimes 

 known as animal starch. It gives a red colour with iodine. 

 It is a white solid, soluble in water. 



Dextrin is formed when starch is heated to about 220° C. 

 It is easily soluble in water, and gives no blue with iodine. It 

 is made commercially for adhesive purposes, and is sometimes 

 known as " British gum." 



Cellulose is very abundant in the leaves, stems, and roots of 

 plants. It is mingled with other substances, from which it can 

 usually be separated because of its resistance to most reagents. 

 By treating the tissues of a plant successively with chlorine, 

 caustic alkalies, dilute acid, water, alcohol, and ether a residue 

 of almost pure cellulose is left. It is a white substance, in- 

 soluble in most solvents, but soluble in zinc chloride or 

 ammoniacal copper oxide solutions. By the action of nitric 

 acid cellulose is convertei into nitro-celluloses — e.g.., gun- 

 cotton, C8H,(N03)30„ collodion, CeH,(N03),0,. 



