1876.] 



on the Leaf of Dionsea muscipula. 



423 



abundant stratum stretches from one lobe to the other across the midrib. 

 This being so, we have a region roughly mapped out for us comprising 

 so much of the internal surface of the leaf as lies within and around the 

 sensitive hairs on either side along with the whole of the space which 

 lies between these parts. If, now, we proceed to contemplate the region 

 thus indicated, in relation to its structure, and limit it by two parallels 

 crossing the midrib and following the direction of the veining, we have 

 the whole leaf divided into three structurally analogous parts, of which 

 the middle part may, with reference to the excitatory changes of form, 

 and therefore, it may be presumed, wdth reference to the excitatory 

 changes of potential, be regarded as representative of the other two. 

 That we are justified in thus regarding the leaf is rendered strikingly 

 evident by the result of an experiment in which a leaf is mutilated by 

 first cutting away entirely the further third in the line c (fig. 6), and 

 then removing all of the nearer zone, with the exception of the midrib, 

 so that the leaf is reduced to the form exhibited in fig. 5. In such a 



mutilated leaf the electrical phenomena of the whole leaf are unaltered, 

 there being at first no difference between it and the entire leaf, either as 

 regards the potentials of the surfaces or the changes which are produced 

 by excitation. 



It having, on these grounds, been determined that we should direct 

 our attention to the middle third of the leaf, the next step was to fix on 

 contact points for electrical investigation. On the internal surface of 

 each lobe we selected the point (evidently the most important) which is 

 equidistant from the three sensitive hairs, on the external surface the 

 point opposite and corresponding to the first, and similarly two points 

 on the external and internal surface respectively of the midrib, situated 

 in the line of junction of the before-mentioned points on the lobes. "We 

 now proceed to state what results are obtained when these several points 

 are investigated, it being understood that each of them exhibits a negative 

 variation when it is compared with some other part of the plant lying 

 outside of the area of excitation. We shall employ the following abbre- 

 viations — viz. the letters I and m to denote the points already indicated 



