2 o8 TROPIC MOVEMENTS 



The Tact that geotropic excitation does not inhibit the heliotropic 

 irritability leaves it undetermined whether the two excitations fuse or 

 proceed to separate motory responses. Teleological considerations lead to 

 the conclusion that the excitations summate so that a single motory response 

 is produced, and positive evidence of this could readily be obtained if the 

 two stimulatory reactions had latent periods of response of very unequal 

 length. According to Miiller l the respiratory activity decreases during the 

 performance of a geotropic curvature, but the experiments are not altogether 

 satisfactory. The geotropic and heliotropic curvatures of growing organs 

 do, however, occur without any acceleration of the average rate of growth, 

 so that the respiratory activity need not increase. When, however, as in 

 the nodes of grasses, growth is induced by geotropic induction, not only 

 is the power of heliotropic curvature gained but also the respiratory activity 

 of the awakened nodal cells may be raised. 



The co-operation of geotropic and heliotropic stimuli in orienting an organ was 

 first observed by Dutrochet and by Mohl, and their interaction was studied in detail by 

 Miiller-Thurgau and by Wiesner 2 . Pfeffer s pointed out that during such co-operation 

 changes of tone might modify the results observed, and a variety of instances of such 

 action have been subsequently obtained. It is of course possible to invent special 

 terms to indicate the different ways in which changes of tone may be produced, but 

 such terms are quite unnecessary and afford no explanation of the phenomena 

 observed. This applies even to the term ' heterogeneous induction ' used by Noll *, 

 who has unfortunately failed to recognize the general importance of tone and of the 

 changes of tone due to internal and external factors B . The discussions of Herbst 

 and of Driesch as to whether special terms are needed when the change of tone is 

 not due to the external conditions, or when it is connected with special responses, are 

 without value. 



Exact determinations of the actual relationships are extremely difficult, and hence 

 it is not surprising that Czapek and Noll should have obtained opposite results with 

 seedlings 7 . According to Czapek, geotropic induction does not affect the heliotropic 



1 N. J. C. Miiller, Fiinfstiick's Beitr. z. wiss. Bot., 1898, Bd. II, p. 267; Arct. Fiinfstiick's Beitr. 

 z. wiss. Bot., 1903, Bd. v, p. 145. 



1 Dntrochet, Recherches anat. et physiol., 1824, p. 92 ; Mohl, Vegetabilische Zelle, 1851, p. 140; 

 Muller-Thurgau, Flora, 1876, p. 94; Wiesner, Die heliotropischen Erscheinungen im Pflanzenreich, 

 1878, I, pp. 55, 63. 



3 Pfeffer, Pflanzenphysiologie, i. Aufl., 1881, Bd. II, p. 338. 



4 Noll, Heterogene Induction, 1892. Cf. also Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, 

 p. 496. 



* Cf. Pfeffer, Die Reizbarkeit der Pflanzen, 1893, p. 22. 



6 Herbst, Biolog. Centralbl., 1894, Bd. xiv, p. 733; Driesch, Die organischen Regulationen. 

 1901, p. 19, footnote. 



7 Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, p. 372 : cf. also Czapek, Jahrb. f. 

 wiss. Bot., 1898, Bd. xxxn, p. 271 ; Noll, Heterogene Induction, 1892, p. 56; Jahrb. f. wiss. Bot., 

 1900, Bd. xxxiv, p. 494. 



