THE DAILY PERIODICITY OF GROWTH 201 



effect of the lower temperature. During cold, clear nights, however, the 

 reverse may be the case. 



Duchartre l found that the growth in length of various shoots was greater during 

 the night than during an equal period of daylight, and was in some cases two or 

 three times as rapid. An increased rate of growth in thickness of trees was observed 

 by Friedrich 2 at night-time and by Kraus 3 in fruits and fungi. 



Rauwenhoff 4 , however, found that from June to October, about one-third more 

 growth took place during the twelve hours between 6 a.m. and 6 p.m., than during 

 the following twelve night hours. Similarly in alpine regions the nights are so cold 

 that growth is much retarded. On the contrary, submerged plants, and also 

 subterranean organs, are subject to a lesser fall of temperature at night-time. 

 Hence the different organs of the same plant may exhibit dissimilar periodicities, 

 while reference to Fig. 30 will show that a slight fall of temperature at evening 

 is insufficient to counteract the accelerating action on growth of the increased 

 turgidity and diminished illumination. 



The course of the periodic curve not only shows variations due to the 

 external factors, but may also be modified by the reactive power of the 

 plant, by the after-effect of the previous conditions, and also by autonomic 

 oscillations. The latter may still occur when all the external factors 

 excepting the illumination are kept constant, and hence although the curves 

 obtained by Sachs 5 and by Baranetzky 6 show a general resemblance they 

 differ in detail. In many plants, such as DaJdia variabilis, the maximum 

 elongation occurs in the early morning hours, the minimum in the evening, 

 whereas in other plants these cardinal points are displaced until in extreme 

 cases the maximum occurs at midday, the miminum after midnight. 



The secondary maxima and minima due to autonomic oscillations are 

 never entirely absent, although in some plants Baranetzky found they 

 were more conspicuous than in Dahlia variabilis, in others less so 7 . In the 

 curve given, however, the oscillations were partly due to variations in the 

 external conditions, which were not kept as constant in Sachs' experiments 

 as in those by Baranetzky. In the curve constructed from the averages 

 for every three hours, the secondary oscillations are less pronounced. 



The daily periodicity of growth is apparently produced in the same 



1 Duchartre, Compt. rend., 1866, 6 avril, p. 815. G. Kraus (Ann. d. Jard. Bot. d. Buitenzorg, 

 1895, T. xn, p. 203) observed the same on plants of Dendrocalamus growing in Java. Cf. also 

 Kirchner, Bot. Ztg., 1878, p. 28. 



3 Friedrich, Bot. Ztg., 1897, p. 369. 



3 G. Kraus, Sitzungsb. d. naturf. Ges. z. Halle, 1880, p. 94; Schmitz, Linnaea, 1843, 

 Bd. xvn, p. 464. 



4 Cf. Sachs, Arb. d. Bot. Inst. in \Viirzburg, 1872, Bd. I, p. 190. 



5 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1872, Bd. I, p. 99. 



6 Baranetzky, Die tagl. Periodicitat im Langenwachsthum, 1879 ( re P r - from Mem. d. 1'Acad. 

 d. St.-Pe"tersbourg, 7 se"r., Bd. xxn). 



7 Cf. also G. Kraus, Ann. d. Jard. Bot. d. Buitenzorg, 1895, T. XII, p. 203. 



