THE DAILY PERIODICITY OF GROWTH 203 



growth, for Sachs l and especially Baranetzky found that it persisted for 

 a time in darkness. Baranetzky observed that the after-effect disappeared 

 in a few days in shoots of Gesneria tubiflora^ whereas in those of Helianthus 

 tuber ostis a daily periodicity was still perceptible after 14 days in dark- 

 ness. The same author could detect no daily periodicity in shoots of 

 Helianthus grown from the tubers in darkness, and Godlewski 2 made 

 similar observations on seedlings. Autonomic variations are shown, how- 

 ever, and when these have a long period, they are not easy to distinguish 

 from the daily variations in the growth-curve. Baranetzky in fact observed 

 that certain, though not all, of the shoots produced in darkness from a tuber 

 of Brassica Rapa exhibited a periodicity of growth resembling the daily one, 

 an observation which seemed to point, in the absence of the above 

 explanation, to a transference of a daily periodicity from the tuber to the 

 shoots. 



Neither the daily periodicity of growth in length nor that of nyctitropic 

 movement is therefore hereditary in character, but simply due to the 

 cumulative effect of regularly repeated stimuli upon organisms capable of 

 response. The periodicity induced by this cumulative after-effect probably 

 extends itself to a certain extent to all processes related to growth, including 

 the tissue-strains and the opening and closing of the stomata. Many 

 questions in regard to the origin of daily periodicity still require solution 

 by experiment. It is, for example, uncertain whether rhythmic changes of 

 temperature or of turgor under constant illumination could produce by their 

 after-effect a periodicity similar to that induced by the alternation of night 

 and day, and also whether the progress of the after-effect is determined by 

 the variations of growth actually attained, or by a combination of factors. In 

 the latter case the curve in Fig. 30 would show, as an after-effect of the 

 previous exposure, a minimum growth at night-time instead of during the 

 day. Only direct experiment can answer this question, and determine whether 

 the after-effect follows the same course as the original curve, or whether the 

 after-effect of the changes of illumination predominates. 



Historical. The daily periodicity of growth as shown in darkness and under 

 constant external conditions was first closely studied by Sachs 3 , who also criticized 

 the earlier observations of Meyer, Mulder, Harting, Caspary, and Rauwenhoff, in 

 which few or no precautions were taken to maintain constant conditions as regards 

 temperature, moisture, &c. Baranetzky 4 , like Sachs, employed a self-registering 



1 Sachs (Arb. d. Bot. Inst. in WUrzburg, 1872, Bd. I, p. 167) inclined to regard the after-effect 

 as being due to the entry of light, whereas Baranetzky found that it is still shown when a little light 

 is admitted so as to act against the periodic acceleration. 



2 Godlewski, Anzeiger d. Akad. d. Wiss. zu Krakau, 6. Juni 1889. 

 8 Sachs, 1. c., p. 99. 



* Baranetzky, Die tagl. Periodicitat im Langenwachsthum, 1879 (repr. from Mem. d. 1'Acad. d. 

 St.-Petersbourg, Bd. xxvn); Vorlauf. Mittheil., Bot. Ztg., 1877, P- 6 39- Dde (Die Biologic 

 von Monotropa, 1873, p. 58) made some observations on Monotropa. 



