50 HOW CROPS GROW. 



starch-grains of the parsnip are represented at /, and 

 those of the beet at g. 



The grains of potato-starch are among the largest, be- 

 ing often 3 ^ of an inch in diameter ; wheat-starch 

 grains are about TtsW f an inch ; those of rice, -5^^ i 

 an inch, while those of the beet-root are still smaller. 



The starch-grains have an organized structure, plainly 

 seen in those from the potato, which are marked with 

 curved lines or ridges surrounding a point or eye ; , Fig. 

 12. When a starch-grain is heated cautiously, it swells 

 and exfoliates into a series of more or less distinct layers. 



Starch, when air-dry, contains a considerable amount of 

 water, which may range from 12 to 23 per cent. Most of 

 this water escapes readily when starch is dried at 212, 

 but a temperature of 230 F. is needful to expel it com- 

 pletely. Starch, thus dried, has the same composition 

 ID 100 parts as cellulose, viz. : 



Carbon 44.44 



Hydrogen 6.17 



Oxygen 49.39 



100.00 



Starch-grains are unacted upon by cold water, unless 

 broken (see Exp. 26), and quickly settle from suspension 

 in it, having a specific gravity of 1.5. 



Iodine-Test for Starch. The chemist is usually able to 

 recognize starch with the greatest ease and certainty by 

 its peculiar deportment towards iodine, which, when dis- 

 solved in water or alcohol and brought in contact with 

 starch-grains, most commonly gives them a beautiful 

 blue or violet color. This test may be used even in 

 microscopic observations with the utmost facility. Some 

 kinds of starch-grains are, however, colored red, some 

 yellow, and a few brown, probably because of the pres- 

 ence of other substances. 



EXP. 26. Shake together in a test-tube 30 c. c. of water and starch 

 of the bulk of a kernel of niai/e. Add solution of iodine drop by drop, 

 agitating until a faint purplish color appears. Pour off half the liquid 



