MOVEMENTS. 113 



73. Is the growth of elongating structures, apart from re- 

 sponses to lateral external stimuli, exactly in the line of their axes, 

 or does it show lateral movements? 



Answer by Experiment 42. 



EXPERIMENT 42. This may be determined by placing a plant in 

 such a position that the chief external stimuli, light, gravitation, 

 heat, and moisture, cannot exert a lateral stimulus upon its elongating 

 stem, and by arranging a system of sighting along the axis of the 

 stem so that the smallest deviation from a straight line will be made 

 visible. The former maybe accomplished well enough for all prac- 

 tical purposes by placing the plant (one growing rapidly with a ver- 

 tical stem) on the table of a well-lighted greenhouse, and surround- 



FIG. 29. SIMPLE ARRANGEMENT FOR THE STUDY OF CTRCUMNUTATION. 



One-fourth the true size. 



ing it by a black-paper cylinder which rises three or four inches above 

 its top. To accomplish the latter, some method must be employed 

 which will both allow the exact direction of the axis to be sighted, 

 and also will magnify the slightest lateral movement, and permit it 



