1876.] 



on the Leaf of Dionsea muscipula. 



425 



Similarly in a vigorous leaf specially investigated with reference to the 

 internal and external surfaces of the midrib, the results were as follows : — 



imP— ^P=— 3-0, mP-j>P = 0, ^my=-3•0 to-3-5, emY=~b'b. 



In at least a dozen excitable leaves which we have investigated with 

 reference to the conditions of the external and internal surfaces, we have 

 met with no exception to the statement made above as to the predominance 

 of the external variation. But in two leaves which, though apparently 

 healthy, failed to close when excited, the external variation not only did 

 not exceed the internal, but was actually positive, ranging from O'^d to 

 1-Ocl, the internal variation being normal, i. e. negative, and ranging from 

 — 0*5 to —1*2. In both instances the potential of the outer surface was 

 unusually negative, both as compared with the petiole {eTP—pF=. — 4'5c?) 

 and as compared' with the opposite internal surface (i?P— jpP= — 3*5c?). 



These were the only instances in which the external surface exhibited 

 a positive variation. Other leaves, however, were observed in which the 

 same condition presented itself on the internal surface, and was again 

 associated with an extremely negative potential. Thus in leaf 51, in 

 which the internal variation was -\-l-2d and the external —3-5d, the 

 difference of potential was 3'Sd ; and in leaf 52, in which the internal 

 variation was +6'0c? and the external —3'5d, the difference of potential 

 was 4:-5d. 



Before stating in what way we think that these apparent anomaKes 

 may be explained, we must give an account of a phenomenon of the same 

 kind but of much more frequent occurrence. In perfectly fresh leaves, 

 i. e. in leaves which have not yet been touched, it sometimes happens that 

 when a single hair is excited at regular intervals, the first few excursions 

 obtained with the movable contact on the internal surface of the lobe 

 differ from the succeeding ones in being preceded by what may be pro- 

 visionally called a preexcursion in the opposite direction. Thus in leaf 

 22 the first effect was a mere shudder of the mercurial column, followed 

 by a very small negative excursion. The second was larger (?7V= — l'2d) 

 preceded by a very perceptible movement of the mercury in the opposite 

 direction. The next two were still larger, but the preexcursion was 

 still perceptible. After this the excursions were purely negative. Three 

 other instances were met with of the same kind. 



That any part of the surface of the leaf should exhibit a positive ex- 

 cursion seems, at first sight, inconsistent with the statement which has 

 been made that the variation both of the external and internal surface 

 of the leaf is negative. That it is not so, may, however, be readily under- 

 stood from the following considerations. 



We have already seen that the petiole or other part of the plant with 

 which the fixed electrode is directly or indirectly in contact is electrically 

 indifferent, or, in other words, serves the purpose of an ordinary moist 

 conductor, just as if it were part of the clay plug by which the contact 



