EXERCISE 19 

 COMPOSITION OF SOME PLANT SUBSTANCES 



Materials. Dry starch ; bits of wood ; test tubes ; bunsen burner 

 or alcohol lamp. 



Directions for work. In the previous study it has been found 

 that plants take up from the soil both water and substances 

 dissolved in the water. Is the material of which the plant is 

 composed secured in this way ? Does starch, for instance, enter 

 the plant from the soil in solution in water? Is the wood 

 composed of materials which may have entered in this way ? 

 We may start an investigation of these matters by decompos- 

 ing plant materials to determine of what they are composed. 

 Proceed as follows: 



Place a small amount of dry starch in a test tube. Heat it 

 slowly over a flame. Does anything collect on the sides of the 

 tube in the upper and cooler part? So far as you can tell, 

 what is this substance ? Continue heating until the residue in 

 the bottom of the tube no longer changes in appearance. Note 

 the black color of this residue. It is carbon. Remove part of 

 it, if possible, and find out whether it will burn. 



When the tube has cooled, add water and find out whether 

 the carbon is soluble in water. Do you believe that the carbon 

 could have entered the plant in solution in water, through the 

 roots ? Is there, so far as you can learn, any large amount of 

 carbon in the soil? Is carbon commonly added to soils as a 

 fertilizer ? 



Repeat the experiment with chips of wood in another test 

 tube. Compare the results in this case with those noted above. 



On the basis of what you have learned in this experiment, 

 does it appear probable that all of the materials which enter 

 into the composition of a plant have been secured from the soil ? 



[22] 



