282 CHAP. XXVI. GEOTROPISM 



In order to clear away obscurities connected with 

 geotropism, it will be necessary to elucidate the following 



points : 



1. The determination of the effective direction of stimulus 

 of gravity. This will be demonstrated by two independent 

 means of inquiry: (i) by the method of algebraical sum- 

 mation of the reaction to geotropic with that to photic 

 stimulation, and (2) by the method of geotropic torsion. 



2. The sign of excitation is, as previously explained, 

 a contraction and concomitant galvanometric negativity. 

 Does the stimulus of gravity, like stimulus in general, induce 

 this characteristic excitatory reaction ? 



3. What is the law relating to the ' directive angle ' 

 and the resulting geotropic curvature ? By the directive 

 angle (sometimes referred to as the angle of inclination) is 

 meant, as previously explained, the angle which the direc- 

 tion* of the stimulus makes with the responding surface. 



4. Finally, it is necessary to investigate whether the 

 assumption of opposite irritabilities of the root and the 

 shoot is really justified. If not, the opposite curvatures 

 exhibited by the two organs have to be explained. 



I propose in this and in the following chapters to describe 

 the investigations sketched above, employing two inde- 

 pendent methods of inquiry — namely, those of mechanical 

 and of electric response. I describe first the automatic 

 method that has been devised for an accurate and magnified 

 record of geotropic movement and its time-relations. 



The Geotropic Recorder 



The Recorder shown (fig. 165) is very convenient for 

 the study of geotropic movement. The apparatus is four- 

 sided, and it is thus possible to obtain four simultaneous 

 records with different specimens under identical conditions. 

 The recording levers are free from friction with the recording 

 surface. By an appropriate clockwork mechanism the 

 levers are pressed for a fraction of a second against the 



