Vi TABLE OF CONTENTS 



2. More Thorough Experimental Analysis showing the Relative page 



Importance of Internal and External Factors in Behavior — 

 Jennings, Holmes and Others 44 



3. Summary of Historical Review 51 



4. Various Definitions of Tropisms 53 



5. Statement of Important Problems in the Study of Reactions 



to Light 57 



PART II 



EXPERIMENTAL OBSERVATIONS AND DISCUSSIONS BEAR- 

 ING ON THE QUESTION AS TO HOW ORGANISMS 

 {ESPECIALLY THOSE WITHOUT EYES) BEND OR 

 TURN AND MOVE TOWARD OR FROM A 

 SOURCE OF STIMULATION 



CHAPTER IV 



Processes Involved in the Bending of Different Parts of Higher 

 Plants toward the Source of Light 



I. Observations on Plumules of Indian Corn (Zea mays) and 



Leaves of Tropaeolum 59 



a. Introduction; b. Apparatus; c. Experiments; d. Re- 

 sults; e. Discussion. 



CHAPTER V 



Observations on Unicellular Forms in the Process of Attaining 

 and Retaining a Definite Axial Position with Reference to the 

 Source of Light 



1. Myxomycetes and Rhizopods 74 



2. Euglena 80 



a. Description; b. Historical account; c. Orientation in 

 light from two sources; d. Material; e. Method of loco- 

 motion; /. Accuracy of orientation; g. Mechanics of 

 orientation in Euglena x in the crawling state; h. Dis- 

 cussion; i. Orientation of Euglena in the swimming 

 state; j. Threshold or sensitiveness when different sur- 

 faces are exposed to light; k. Function of the eye-spot. 



3. Summary no 



