io6 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



to obtain, were produced owing to their using more concentrated rays, 

 or owing to some accidental external conditions. It is also doubtful 

 whether the Eecquerel rays l , or other forms of radiant energy which are 

 not directly visible, produce any physiological effects on plants. 



PART IV 

 THE INFLUENCE OF MAGNETISM AND ELECTRICITY ON GROWTH 



SECTION 28. 



Weak electrical currents continually circulate in .plants, and these are 

 produced by differences of potential between the earth and the atmosphere, 

 as well as by those created and maintained by chemical and physical 

 agencies at work in the plant itself. It is possible that these currents may 

 influence metabolism, and various tropistic actions of electrical currents 

 have already been discovered. No satisfactory proof has however been 

 given that weak electrical currents directly influence growth, when the 

 currents are led through the soil, and precautions are taken to avoid 

 secondary effects 2 . The accelerations and retardations observed by certain 

 authors were due to electrolytic actions upon the constituents of the soil, 

 or to other accessory circumstances. Similarly Muller-Hetlingen 3 found 

 that the growth of a seedling was not affected when weak currents were led 

 longitudinally through it. The retardation observed by Elfving 4 was 



1 Cf. de Haen, Ann. d. Phys. u. Chem., 1899, Bd. Lxvm, p. 902; Curie, Compt. rend., 



1899, T. CXXIX, p. 823; Le Bon, ibid., 1899, T. cxxvm, p. 174; Giesel, Ber. d. Chem. Ges., 



1900, p. 3569. A summary of the literature for 1899 is given in Beibl. zu Ann. d. Phys. u. Chem., 

 1899, Bd. xxili, p. lix. 



3 Wollny, Forschung. a. d. Gebiete der Agriculturphysiol., iSSS, Bd. XI, p. 88 ; 1893, Bd. xvi, 

 p. 243, and the literature there given. A few other works by Chodat, Leod, &c., are quoted in the 

 Bot. Jahresb., 1893, p. 36; 1894, p. 232; Ahlfvengren, Bot. Centralbl., 1899, Bd. LXXIX, p. 53; 

 Euler, Biol. Centralbl., 1901, Bd. XXI, p. i. Cf. also Solvay, Du role d. 1'electricite d. 1. phenom. 

 d. 1. vie, 1894. [For the most part these researches, and later ones also, lack the requisite 

 physical precision, and it is highly advisable that the vague use of the term 'weak* electrical 

 currents should be replaced by definite quantitative statements in recognized physical units, the 

 coulombs and current-density being given in all cases.] 



3 Pfliiger's Archiv f. Physiologic, 1883, Bd. XXXI, p. 212. 



4 Elfving, Bot. Ztg., 1882, p. 257. [Since protoplasm conducts as an electrolyte (cf. Ewart, 

 On Protoplasmic Streaming, 1903, Clar. Press, pp. 96, 123), no current below the potential required 

 to produce permanent dissociation is able to traverse it. A ' weak ' current produces no effect simply 

 because it does not pass through the protoplasm at all ; a strong one of sufficient potential unavoidably 

 exercises an electrolytic effect, and this is not confined to the points of exit and entry, but takes place 

 along the path of the current in every cell which contains special substances not present elsewhere, 

 or present in very small amount only. Moreover the resistance offered by plants to the passage of 

 a current is extremely great, and in the absence of other conducting channels, differences of electrical 

 potential between the earth and the atmosphere can only produce electrical currents through an 

 intervening electrolyte when the difference of potential is greater than the back potential due to the 

 products of electrolysis. In the case of bacteria, the electrolytic products formed at the poles 

 naturally influence the composition of the nutrient medium, and hence the development of the 

 organisms, often to a fatal extent. Cf. Cohn and Mendelssohn, Cohn's Beitrage z. Biolog., 1879, 



