EXERCISES, COMPOSITION OF PLANT TISSUES 109 



ous poisons. Among the most familiar are quinine, strychnine, 

 cocaine and morphine. In this connection, it is well to remem- 

 ber that about 90 per cent of our medicines are of plant origin 

 and that much of the early work in botany was for the discovery 

 of new drug plants with which to relieve the sufferings of 

 mankind. 



EXERCISES SHOWING THE COMPOSITION OF PLANT TISSUES 



1. Ash in Leaves. Weigh a bunch of green cabbage or lettuce leaves. 

 Dry them in an oven. Weigh again and compute percentage of water. 

 Burn the leaves, weigh and compute the percentage of ash. 



2. Water in Seeds. Put a few seeds in a test tube, plug with cotton 

 and heat gently so as not to scorch the seeds. Hold the tube as nearly 

 horizontal as possible while beating. Do you see signs of water on the 

 glass? 



3. Ash in Wood. Put a piece of wood in a test tube and heat until 

 a piece of charcoal (carbon) is formed. Burn the charcoal to ashes. 

 Shake the ashes in water. Do they dissolve? 



4. Tests for Starch and Sugar. Put a little commercial starch and 

 a little sugar in separate test tubes of water, and shake. Do they both dis- 

 solve? Add a few drops of iodine to the starch and note the change in 

 color. This is the test for starch. Add a small amountl of Fehling's fluid, 

 a few drops at a time; a brick red color indicates the presence of sugar. 

 This test will not work with cane sugar unless the solution has been 

 boiled with dilute acid before adding the Fehling's fluid. 



Fehling's solution can be made in the laboratory. It consists of two 

 solutions: (A) 36.G4 grams of pure powdered copper sulphate in 200 c.c. 

 of water. (B) 150 grams of Rochelle salt and 50 grams caustic soda in 

 500 c.c. of water. 



Keep the two solutions separate and mix before using as follows: 

 2 parts of solution A. 

 5 parts of solution B. 

 10 part's of water. 



5. Carbon in Sugar. Put a little siugar in a test tube and heat 

 slowly until you have a black mass of carbon. Note what occurs during 

 the process and explain. 



6. Testing Seeds and Fruits. Test fruits, tubers, fleshy roots and 

 various other parts of plants by applying the starch and sugar tests to 

 freshly cut surfaces. 



7. Examining Potatoes and Apples. Cut very small, thin slices of 



