NYCTITROPIC MOVEMENTS 685 



alternately in the plant under alternating light and dark- 

 ness, thus leave a molecular impress, and this impression is 

 deepened by repetition, rinding subsequent expression even 

 when the primary alternating cause is absent. The length 

 of time during which such after-oscillation persists will 

 depend, amongst other things, on the depth of the molecular 

 impression. 



Physical analogue. — An analogous physical phenome- 

 non will make the point clearer. If a wire be taken and 

 subjected to alternating molecular strains, say by giving it 

 alternate positive and negative twists, the wire being held in 

 each of these twisted positions for a time, then, even after 

 stoppage of such alternate twisting movements, the released 

 wire will continue to vibrate to and fro in expression of the 

 release of impressed latent strains, consequent on previous 

 forced vibrations. 



That such forced vibrations may persist in a plant has 

 been shown by F. Darwin and D. Pertz, who subjected a 

 plant to alternating geotropic stimuli, by which a rhythmic 

 movement was found to persist for a time, even on the 

 stoppage of stimulation. Czapek and Wiesner obtained 

 similar after-vibrations with alternating phototropic stimuli. 



Impressed periodic vibrations in organ originally 

 radial. — I shall here give a very remarkable instance of such 

 forced periodic vibrations, as induced in an originally radial 

 organ. I had a row of sunflowers planted, in a line which 

 ran accurately east and west, at the season when the sun 

 moved daily in an almost vertical plane. The plants were 

 thus stimulated in the morning from the east, and in the 

 afternoon from the west. All these plants, then, by helio- 

 tropic action, followed the path of the sun during the course 

 of the day, and during the course of the night, again, there 

 was recovery. At first the diurnal swing east and west was 

 only through a small angle, but under the action of these 

 repeated periodic impulses it became larger and larger, like 

 that of a pendulum under regular and repeated blows. 

 When the plants had grown to a height of one metre, it was 



