MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 305 



set up by inherent causes. Bararietzk}^ ^""^ concluded that the rhythm 

 exhibited by plants in a dark room was an after-effect due to the 

 lasting influence of alternating exposures to daylight and darkness, 

 and pointed out that in continued confinement the diurnal periodicity 

 was lost and the variations no longer occurred with sufficient regu- 

 larity to constitute a rhythm. Sachs seems to have been the first to 

 formulate the opinion that light retards growth and his position with 

 regard to minor periodicities may be best given in his own words : 

 *' I, on the contrary, am of the opinion that in the plant, or at any 

 rate in its growing parts, periodic variations occur in some way quite 

 independent of variations of temperature and light ; and these, as I 

 conclude from Baranetsky's observations, may continue for periods of 

 very different lengths. If now the plant is subjected to the regular 

 alternation of day and night, and the variations of temperature are 

 very small, the above-mentioned influences on growth make their 

 appearance, by which its maximum is transferred to the morning 

 hours, and its minimum to the evening, the above-mentioned period- 

 icity arising from purely internal causes being concerned as the 

 weaker factor in a definite daily period of time." 



Sachs ^"'' did not agree with Baranetzky however in the assertion 

 that the daily periodicity of plants in darkness was an after-effect of 

 light or temperature, and Vines took the position that the coincidence 

 of the variations with those of normally illuminated plants was prob- 

 ably accidental, although he conceded that the daily periodicity of a 

 plant continued for several days after it had been confined in a dark 

 chamber. Both Sachs and Vines ^°^ held that it was improbable that 

 the periodicity of fully etiolated plants was due to after-effects ; 

 indeed Sachs adduces the fact of periodicity in such a plant as a 

 refutation of the theory of after-effects in the matter, and likens it 

 to the starting of pendulum spontaneously after it had come to rest. 



It is to be seen however, that geotropic and other stimulatory 

 effects may be hindered and the response delivered long afterward, 

 and Darwin and Pertz have shown by a beautiful series of experi- 

 ments that geotropic rhythms may be induced in stems, which are 

 maintained after the stimulus is withdrawn. The position taken by 



^°^ Baranetzky, J. Die tagliche Periodicitiit im Langenwachstum der Stengel. 

 Mem", d. I'Acad. Imp. d. Sc. de St. Petersbourg. Ser. 7. 1879. 

 20' Sachs. Physiology of Plants. Eng. Ed., p. 560. 1887. 

 208 Vines, S. H. Physiology of Plants, p. 403. 1886. 



