PART II 



EXPERIMENTAL OBSERVATIONS AND DISCUS- 

 SIONS BEARING 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 Nasturtium ( Tropaeolum) 



a. Introduction. — It is well known that many plant 

 structures have a sensitive zone which may be separated 

 by some distance from the motory zone and that impulses 

 are transmitted from the one to the other. Darwin (1880), 

 Pfeffer (1894), Czapek (1900), Pollock (1900), Haberlandt 

 (1904) and others demonstrated this for leaves and plumules 

 stimulated by light and for radicles stimulated by gravita- 

 tion and injury (cauterization). Newcombe (1902, p. 346) 

 also proved that impulses due to stimulation by water 

 currents are transmitted in radicles. In radicles the dis- 

 tance of transmission of impulses is frequently over 10 mm., 

 while in leaves it is often several centimeters. 



Just how the external agent produces the stimulus is not 

 known, although it is generally supposed that it is by caus- 

 ing chemical changes. With regard to light it has been a 

 question as to whether the orienting stimulation is depend- 



59 



