396 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



may come into play. Haake l found that in general the warmer half 

 of a shoot or leaf experienced an increase of positive potential, which 

 caused either an increased or a decreased deflection according to whether 

 the previous difference of potential was increased or diminished thereby. 



Photosynthesis. The assimilation of carbon dioxide involves alterations 

 of potential, which hence become perceptible when a leaf is exposed to 

 changes of illumination, as was first shown by Haake and by Klein. Waller 

 and also Ouerton have shown that the current in the external circuit 

 moves from the shaded to the illuminated half of a leaf, but Tropaeolum 

 and Matthiola show exceptions to this rule. The most pronounced effect 

 is produced behind a solution of potassium bichromate which lets through 

 mainly the rays most effective in photosynthesis. 



Anaesthetics such as chloroform and ether 2 also produce changes of 

 potential, probably largely owing to their influence upon metabolism. 



The movements of water in the cell-membranes and in capillary 

 spaces exert a purely physical electromotive action in both living and 

 dead objects 3 , but in the living plant the electricity produced in this 

 way is but slight in amount. Haake 4 could detect no difference in the 

 galvanometric deflection obtained from a leaf or stem on permitting and 

 then preventing transpiration with its attendant movement of water. Even 

 when a flaccid plant was suddenly made turgid by forcing in water under 

 pressure, only a slight variation of the current in the external circuit was 

 shown. 



Since the normal differences of potential on plants are shown also in 

 air saturated with moisture, Kunkel's supposition 5 can hardly be correct, 

 for according to this author all the electrical currents in plants are derived 

 from the mechanical energy of the movements of water. Kunkel attaches 

 especial importance to the fact, corroborated by Haake, that the current 

 passing from the midrib to the mesophyll of a leaf undergoes a transitory 

 reversal when a drop of water is placed upon the mesophyll. The same 

 result is, however, produced when the leaf is saturated with water so that 

 no absorption occurs, and it remains an open question whether the varia- 

 tions in the electrical currents produced by rapidly bending a shoot are 

 due to movements of water as Kunkel supposes, or are produced in other 

 ways 6 . Since the effect is produced at once it cannot be the result 



1 Haake, Flora, 1892, p. 476. 



- Waller, Proc. of Royal Soc., 1900, p. 134; Querton, I.e., p. no. 



:1 Querton, I.e., 1902, p. 119; Haake, I.e., p. 480. For a few observations on the influence of 

 chemical agents upon the production of electricity by animals cf. Biedermann, 1. c., pp. 302, 408. 



1 On stream currents, and on electrical endosmosis, cf. Winkelmann, Handbuch d. Physik, 1893, 

 Bel. in, i, pp. 493, 504; G. Bredig, Zeitschr. f. Elektrochemie, 1903, Bd. IX, p. 738. 



5 Knnkel, I.e., 1878, 1881 ; cf. Haake, I.e., p. 457. 



' Whether changes of concentration or of resistance take part in the phenomenon is uncertain. 

 Protoplasmic streaming may be inhibited without the production of electricity being appreciably 



