LEAF FUNCTIONS 103 



Photosynthesis. To show that Green Plants produce Starch. 

 Leaves can be taken from active green plants, scalded to kill the 

 protoplasm and release the chlorophyll, and soaked in alcohol 

 to remove the green color. Then, if tested with iodine, a dark blue 

 color is produced, showing that starch was present. The chlo- 

 rophyll had to be removed so that this blue could be seen. This 

 proves that starch was in the leaf. To prove that it is made there, by 

 the action of light on the chlorophyll, requires further experiment. 



To show that chlorophyll is necessary, a leaf from a green and 

 white-leaved geranium may be used, as above, when it will be found 

 that little starch is revealed in the white portions. 



To show that light is necessary, parts of an active leaf are cov- 

 ered with corks, pinned through, on both sides. After a few days 

 the covered portions will not yield, the starch test, while the ex- 

 posed parts will still do so. Another proof of the same thing is 

 to keep a plant entirely in the dark, as a check experiment, and 

 when it has become pale, test for starch, which will be found 

 lacking. Of course the same kind of plant, under the same con- 

 ditions, except the light, should be used in this and in the experi- 

 ment to be compared with it. 



To show that Green Plants produce Oxygen. Oxygen is the 

 waste product of photosynthesis; it is thrown off when starch is 

 made. It is easier to collect a gas over water, hence a water plant 

 is used for this experiment, but all green plants carry on the same 

 process. 



The water plant is submerged in a glass jar under a glass funnel, 

 whose stem is covered by a small test tube, filled with water and 

 inverted. The apparatus is set in the sun and soon bubbles of gas 

 will rise in the funnel and be collected in the tube. These, when 

 tested, prove to be oxygen. If carbon dioxide be dissolved in the 

 water, the process will go on faster, as carbon dioxide is one of 

 the materials used in photosynthesis, and that in the jar of water 

 is soon exhausted. 



Another similar experiment ought to be set up in the dark, so as 

 to prove, again, that light is the source of energy for this very 

 important process. 



