i^ATTER X. 

 THE CARBON FOOD OF PLANTS. 



140. We came to the conclusion in a former chapter that 

 some chemical change took place within the protoplasm of the 

 green cells of plants during the absorption of carbon dioxide 

 and the giving off of oxygen. We should examine some of the 

 green parts of those plants used in the experiments, or if they are 

 not at hand we should set up others in order to make this ex- 

 amination. 



141. Starch, formed as a result of carbon conversion. — We 

 may take spirogyra which has been standing in water in the 

 bright sunlight for several hours. A few of the threads should 

 be placed in alcohol for a short time to kill the protoplasm. 

 From the alcohol we transfer the threads to a solution of iodine 

 in potassium iodide. We find that at certain points in the 

 chlorophyll band a bluish tinge, or color, is imparted to the ring 

 or sphere which surrounds the pyrenoid. In our first study of 

 the spirogyra cell we noted this sphere as being composed of 

 numerous small grains of starch which surround the pyrenoid. 



142. Iodine used as a test for starch. — This color reaction 

 which we have obtained in treating the threads with iodine is 

 the well-known reaction, or test, for starch. We have demon- 

 strated then that starch is present in spirogyra threads which 

 have stood in the sunlight with free access to carbon dioxide. 



If we examine in the same way some threads which ha\e stood in 

 the dark for a day we obtain no reaction for starch, or at best 

 only a slight reaction. This gives us some evidence that a 

 chemical change does take place during this process (absorption 



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