STARCH: PHOTOSYNTHESIS. 65 



at hand we should set up others in order to make this examina- 

 tion. 



135. Starch formed as a result of this process. We may take 

 spirogyra which has been standing in water in the bright sun- 

 light 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 potas- 

 sium 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, 



136. 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 have stood 

 in the dark for a few days 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 

 of CO 2 and giving off of oxygen), and that starch is a product of 

 that chemical change. 



137. Schimper's method of testing for the presence of starch. 

 Another convenient and quick method of testing for the pres- 

 ence of starch is what is known as Schimper's method. A 

 strong solution of chloral hydrate is made by taking 8 grams of 

 chloral hydrate for every $cc of water. To this solution is added 

 a little of an alcoholic tincture of iodine. The threads of spi- 

 rogyra may be placed directly in this solution, and in a few 

 moments mounted in water on the glass slip and examined with 

 the microscope. The reaction is strong and easily seen. 



We should also examine the leaves of elodea, or one of 

 the higher green plants which has been for some time in the 

 sunlight. We may use here Schimper's method by placing the 

 leaves directly in the solution of chloral hydrate and iodine. 



