1 



NATURE 



[March 7, 19 18 



with which he makes recondite matters plain and 

 the living- atmosphere with which he surrounds his 

 subject can scarcely fail to excite a response in 

 everyone who is not utterly dead to intellectual 

 stimulus. 



The volume is essentially one 

 to possess, for every page can 

 be read with interest, whilst its 

 graceful style will surely com- 

 mend it to a wide circle of 

 friends. J. B. F. 



Photographic Determination 

 Aurora of December 16, 



of the 

 1917, in 



Altitude of 

 Christiania. 



the 



LETTERS TO THE EDITOR. 



[The Editor docs not hold hini.scll 

 responsible for opiiiio)is expressed 

 hv Ills eorrespondenls. Xeilher cati 

 he imdcrtake to reliir)i, or io eor- 

 respond -with the writers of, re- 

 jected manuscripts intended for 

 this or any Oither part of Xati'rk. 

 Xo notice is taken of anonyntoiis 

 eoi)n)iii)iications.\ 



The Stimulation of Plant-growth by 

 Electric Fields. 



The experiments on the electric- 

 stimulation of vegetable growth, 

 initiated in this country by Sir 

 Oliver Lodge and others, are gener- 

 ally held to have given a substantial 

 result. But there does not seem 

 to be any definite view among botanists and experi- 

 menters as to the way in which it is brought about. 



It appears that present practice is to fix a horizontal 

 network of wires ten yards apart at a height of five 

 yards above the ground, and to maintain it at an 

 electric potential of 120,000 volts. This gives a vertical 

 field of about 200 volts per centimetre, which is not far 

 from a. hundred times the order of magnitude of the 

 natural atmospheric gradient. The procedure suggests 

 that it is the field of force that Is expected to produce 

 the stimulation. The comparatively trifling amount 

 of electricity that leaks from the wires into the atmo- 

 sphere could scarcely produce directly any sensible 

 effect. ' ' 



It has, of course, been surmised for a long time 

 that one function of spicules and edges and hairs 

 on vegetation may be to promote discharge into the 

 atmospheric electric field. Although electrostatic dis- 

 charge is a surface phenomenon, the growing points 

 may thus be stimulated by the electric field, there very 

 highly concentrated. A positive gradient might con- 

 ceivably have a different effect from a negative one. 

 The discharge would go on at the enhanced rate due 

 to the increased field, even if the wire grating were 

 protected entirely from leakage. In that case no 

 motive power would be requited to maintain its poten- 

 tial, notwithstanding the current that is produced. \o 

 paradox is thereby involved. If the atmosphere were 

 absolutelv still, the current would pass from the earth 

 to the wires, and leakage would be an essential part 

 of it. But actually the electric discharge from the 

 spicules of the vegetation is mainly borne away on 

 the breeze, and whatever power is needed to sustain 

 the action is contributed from the energv of the wind. 

 It would appear that effective observations might go 

 on even in the limited space of an ordinary green- 

 house, using a grating attached to a static source of 

 potential. J. L. 



Cambridge, February 14. 



NO. 2523, VOL. lOl] 



I.— Aurora rays, December i6, 1917, 2ih. 9m. G.M.T., photographed simultaneously from 

 Christiania (left) and Aas (right). The rays reached, down to a height of 100 kilometres above 

 the earth. Stars of the Great hear constellation are seen in the photograph. 



two statH)n> ( h'nstiani; 



metres from eaeh other. 



The measur('> yivc ah 



(I A; 



itudes of the sanie order 



Fig. 2.— Corona, December 16, 1917, 2ih. 10m. G.M.T. in Christiania. 

 Kxposure two seconds. Stars ot the constellation Auriga are seen 

 to the light. 



those obtained in Bossekop in 1913 — that is, about 

 100 km. for the lower border of rays and curtains. 

 This aijrees with other measurements of the aurorr^ 



