1876.] 



on the Leaf of Dionsea muscipula. 



417 



experiment was quite conclusive. Each time, without exception, that the 

 leaf was excited the weight (one gramme at a distance of 10 centims.) 

 was slightly lifted, the extent of movement of the lever varying from one 

 to three degrees. 



As regards the interval between excitation and the resulting movement, 

 our observations show that it varies from 2| seconds (the shortest 

 observed after numerous excitations) to 10 seconds. The last estimate, 

 however, is probably exaggerated ; for in the early stages, when the mo- 

 tion is of extremely small extent, it is not possible to determine exactly 

 when it commences. In the later stages this source of inaccuracy does 

 not exist, so that we may confidently take two seconds as the inner limit 

 of the period in question. 



Part n. — Electeical DisTrKBAircE or the ISToemax Leaf. 

 Section 1. — Electrical Condition of the Leaf in the unexcited State. 



The electrical condition of the leaf of DioncBa in the unexcited state 

 has very recently been made the subject of a minute investigation by 

 Prof. Munk, of Berlin. He has found (1) that if we conceive the 

 external surface of the leaf divided into strips by parallel lines crossing 

 the midrib nearly at right angles, and coinciding in their direction with 

 the veining, the external surface of each lobe is negative to the midrib ; 

 (2) that in comparing different points of the midrib with each other 

 there is one, of which the position is two thirds of the distance from the 

 near to the far ends of the midrib, which is positive to the rest. He 

 has further (3) stated that the potential of any point on the internal 

 surface of the lobe is exactly equal to that of the corresponding and 

 opposite point on the external surface. 



These three statements may be generalized as follows : — On the 

 external surface of the leaf three lines may be distinguished — one which 

 may be called the positive line, which corresponds to the midrib ; and 

 two others, the lines of greatest negativity, which lie between midrib 

 and margin and are nearly parallel to the first. Although it is not the 

 special purpose of this paper to investigate this part of the subject, we 

 may state generally that we have found the first two propositions above 

 enumerated to be true in all normal leaves with the following exceptions, 

 namely : — first, that although the central part of the midrib is positive to 

 either end (more positive to the near than to the far end), the position of 

 the point of greatest positivity is not so definite as Dr. Munk states, 

 but differs in different leaves ; secondly, that the different points in his 

 isoelectrical negative line are never found to be absolutely identical. 

 As regards the third proposition we are compelled to say, generally, that 

 it is without foundation. We have found that so far from its being 

 generally true that opposite and corresponding points of the two sur- 

 faces exhibit the same potential, such identity occurs so rarely and 

 exceptionally, that it may be regarded as abnormal. In a future paper 



