93 



An Instructive Modification op an Old Experiment. 



Howard J. Banker. 



We linve lonji; been familiar witli the experiment in photdsyntliesis ol' 

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

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

 off the action of the sunlight. The experiment, further, has excited nt, 

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

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

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



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

 both hght and air; — 6. 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 oi'lginal experiment. So 

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

 apparatus has completely obscured the significance of the experiment. We 

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

 upon the plant instead of uiion the apparatus. 



