430 Prof. Burdon-Sanderson and Mr. F. J. M. Page [Dec. 14, 



Eeymond has fitly termed tlie " period of latent excitation." In the 

 plant the time occupied by this preparatory and invisible change is, as 

 might be expected, many times as great as it is in animal muscle ; and 

 in consequence of this greater prolongation the electrical phenomena, 

 the only ones with which we are as yet acquainted, which accompany it 

 can be studied with much greater completeness. In muscle the electrical 

 disturbance begins, according to the researches of Bernstein, confirmed 

 by Du Bois-Seymond, about 0"*005 after electrical excitation of the 

 nerve*. In the plant this period, which may be called the period 

 of electrical delay, is always of perceptible duration, and may last over a 

 second. To measure it we have employed the apparatus previously 

 described as used for investigating the time of commencement of the 

 first mechanical effects, with the exception that the signalling-key is so 

 modified that the same act which closes the signal-circuit excites the 

 leaf. This is effected by fixing to the spring of the closing-key a lead 

 wire which carries at its end a fine camel-hair pencil, so that when the 

 spring is depressed contact is made, and the hairs of the leaf are touched 

 at the same moment. 



The time which intervenes between excitation and the beginning of 

 the electrical disturbance varies in different leaves according to their 

 vigour, but is very much affected by variations of temperature. In sum- 

 mer weather and with normal leaves the variation on the external sur- 

 face at the midrib, or at the outer surface of the lobe, when the sensitive 

 hairs on the same side are touched, is found to begin about one eighth of 

 a second after excitation. When the opposite hairs are excited, the 

 period is increased to a quarter of a second. In six fairly normal leaves 

 under various conditions, in which the delay was measured with the 

 fixed contact on the petiole, and the movable one on the outer surface 

 of one lobe, the results given in the following Table were obtained. 



Table II. 



No. of 

 Leaf. 



Date. 



Me an 

 tempera- 

 ture of 

 day. 



Hairs of same side excited. 



Hairs of opposite side excited. 



Mean delay 

 in seconds. 



Number of 

 observations. 



Mean delay 

 in seconds. 



Number of 

 observations. 



a 



Aug. 10 



64-8 F. 



0-13 



4 



0-24 



3 



b 



„ 11 



62-4 



0-09 



4 



0-23 



2 



c 



„ 15 



76-2 



0-17 



4 



0-23 



3 



d 





76-2 



0-34 



4 



0-51 



3 



e 



21 



64-7 



0-23 



14 



0-56 



6 



f 



Oct. 23 





0-43 



3 



0-65 



3 



In six other leaves in which the delay was measured at the outer sur- 

 face of the midrib, the mean results were as follows : — 



* Du Bois-Eeymond, loc. cit. p. 575. 



