An Instructive Modification of an Old Experiment. 



Howard J. Banker. 



We have long been familiar with the experiment in photosynthesis ol' 

 pinning a cork on opposite sides of a green leaf and exposing to the light 

 in order to show tliat no starch is formed in the leaf where the cork shuts 

 off the action of the sunlight. The experiment, further, lias excited no 

 little comment and criticism because of the fact tliat the cork, tinfoil, or 

 other opaque screen as usually applied not only shuts off the light but 

 also interferes with the interchange of gases through the stomata. This 



Starch Distribution in Geranium Leaf — a. Opaquo disks in close contact with leaf, shutting off 

 both hght and air; — b. Opaque disks with air space shutting off light but not air; — c. Glass 

 disks in close contact with leaf shutting off air but not Light. 



consideration has led to various modifications of the experiment and to the 

 development of some highly accurate and scientifically constructed pieces 

 of normal apparatus to correct tlie eiTor of the original experiment. So 

 far is this sometimes carried that the construction and adjustment of the 

 apparatus has completely obscured the siguificauce of the experiment. We 

 need, therefore, to get back to first principles, that is, to experimenting 

 upon the plant instead of upon the ai)paratus. 



