INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



275 



of individual organs. The most common phenomenon to be noted in this con- 

 nection is the daily periodicity of growth. Plants grow more slowly by day 

 than by night, so that it appears that light exerts a retarding influence upon 

 growth. 1 The growth maximum occurs in the early morning hours and the 

 minimum occurs in the evening. The curve 3s of Fig. 125 shows the diurnal 

 march of the rate of plant growth, which is seen to increase gradually from about 

 6 p.m. to about 6 a.m., after which it gradually decreases, from morning until 

 evening. The accelerated growth of the night hours occurs in spite of the lower 

 night temperature, as may be seen from the figure just mentioned, where the 

 curve t° represents the diurnal march of temperature corresponding to that of 

 growth. This periodicity is mainly dependent upon light, although it continues 

 to be manifest — but with less regularity — when the plant is kept continuously 



6a, 



JZ.rv 6 m 12 1 6 a, 12n 6m 72t 



Fig. 125. — Graph showing daily periodicity of growth, the broken line 32. The full (t°) is 

 corresponding graph of temperature. (After Sachs.) 



in darkness. The latter fact has been explained as an induced rhythm; the 

 ancestors of the present plants have been exposed for countless generations to 

 the diurnal alteration of light and darkness, and the periodicity of growth ap- 

 pears to have become a habit (due to internal conditions), which is more or less 

 markedly inherited. 



One-sided illumination brings about a bending of plant organs, this response 

 being termed phototropism or heliotropism.' 1 When an organ bends toward the 

 more brightly lighted side it is said to be positively phototropic; when it bends 

 away from the more intense light it is negatively phototropic. Positive photo- 

 tropism is very common among plants and is usually observed when growing 

 stems are subjected to one-sided illumination. 



1 Baranetzky, J., Die tagliche Periodizitat im Langenwachstum der Stengel. (M6m. Acad. Imp. Sci. 

 St.-Petersbourg, VII, 2T- ■ 1-91. 1897. Godlewski, Emil, Studyja nad wzrostem roslin. Krakau, 1891.* 



2 Wiesner, Juluis, Die heliotropischen Erscheinungen im Pflanzenreiche. Eine physiologische Mono- 

 graphic I Theil. Denksch. K. Akad. Wiss. Wien 30^: 143-209. 1879. Idem, same title. II Theil. 

 Ibid. 43 7 : 1-92. 1882. Idem, Das Bewegungsvermogen der Pflanzen. Wien, 1881. P. 37-84. 



