40 MANUAL OF GENERAL AGRICULTURE. 



37. TEST FOR THE PRINCIPAL CLASSES OF 

 PLANT COMPOUNDS. 



Materials: Knife, test tube, iodine solution*, Fehl- 

 ing's solutionf, 10% solution copper sulfate, 10% solu- 

 tion potassium hydroxid, evaporating dish. 



(a) Carbohydrates. Starch. Cut a small potato in 

 half. Peel and cut into small slices or rub on an ordinary 

 grater a small portion and collect the pieces or gratings 

 in a small dish of cool water. Boil and allow the solution 

 to cool. Add a drop of iodine. A deep blue color proves 

 the presence of starch. To another piece of potato add 

 a drop of iodine. Eesult? 



Sugar. Cut another piece of potato into very thin 

 slices and place in a test tube. Test for grape sugar with 

 Fehling's solution as follows: Measure out 2 c.c. of solu- 

 tion 1, and add to it 5 c.c. of solution 2, and 3 c.c. of 

 water. Add this to the test-tube containing the potato 

 and boil two or three minutes. A red precipitate (sedi- 

 ment) indicates the presence of grape sugar. If the red 

 precipitate does not appear soon, allow the boiled solu- 

 tion to stand until next laboratory period. 



There is no elementary test for cane sugar. 



(b) Proteids. Cut cross section of beans and potato 

 and carefully touch the cuts with a glass rod that has 

 been dipped in nitric acid. A yellow color should appear 

 which will become more intensely yellow if ammonia is 

 applied. Try it. This coloration is due to the action of 

 the chemicals on the proteids in the substances tested. 



Optional test. Pour a small quantity of the white 

 of an egg, which is a good example of protein, in an 

 evaporating dish, and barely cover with a 10% solution 



*Iodin solution is prepared by dissolving potassium iodide in 

 water (about one part to seventy-five of water) and adding iodine 

 crystals until the solution becomes dark brown in color. 



tFehling's solution is made by dissolving 34.65 grams of 

 copper sulfate in 200 c.c. of water to make solution 1. To make 

 2, dissolve 173 grams of sodium potassium tartrate (Rochelle Salt) 

 in 480 c.c. of a ten per cent solution of sodium hydroxid. Use as 

 directed in the experiment, making up the reagent fresh when- 

 ever needed. 



