1876.] 



on the Leaf of Dionsea muscipula. 



427 



but it Mali be readily understood that, in some instances, the conditions 

 are so complicated that we can scarcely hope to explain their operation 

 satisfactorily or completely *. 



b. Electrical Stimulation. — In the experiments of which the results 

 were communicated to the E-oyal Society in 1873 it was found that the 

 passage of induction-shocks in rapid succession (faradization) through the 

 tissue of the leaf in the immediate neighbourhood of the external surface 

 of a lobe produced effects which resembled those which follow mechanical 

 excitation. It also appeared that this mode of excitation was exhausting ; 

 for although a leaf could be excited at 2 minutes' intervals for an hour or 

 two without any appreciable impairment of its excitability, it soon ceased 

 to respond when excited very frequently. 



During the last summer we have confirmed these observations, using 

 not only faradization but single induction-shocks. For both purposes we 

 have found it convenient to employ as exciting electrodes steel needles 

 sheathed in glass and bound together so that their exposed points were 

 about a millimeter apart. These points were thrust through the epi- 

 dermis of the leaf either on the internal or external surface. 



I^or the production of single closing shocks the primary circuit of 

 Du Bois's induction-apparatus (that of a single Daniell) was closed at 

 regular intervals by a Bonder's fall-apparatus (a modification of Pfl tiger's), 

 the opening shocks being suppressed by a suitable contrivance. The ex- 

 periment was begun with a fresh leaf and with the secondary coil at a 

 distance of from 15 to 20 centims. The electrometer indicating that there 

 was no effect, the distance was gradually shortened until the electrical 

 disturbance took place. The excitation was then repeated at one minute 

 intervals until the leaf ceased to respond. It was always found that it 

 was necessary to push up the secondary coil to within a couple of inches 

 of the primary — in fact, to use induction-currents of such strength as 

 would be sufficient to awaken reflex action from the cutaneous surface of 

 the frog. There was an appreciable delay between the passage of the 

 induction-shock and the electrical disturbance which resulted from it, 

 a delay of which the duration varied according to conditions to be dis- 

 cussed in the next section. 



When a leaf is excited at intervals of a minute, or oftener, by closing 

 shocks which are of just sufficient intensity to produce a response, it in- 

 variably happens that after a time the effect ceases, i. e. that the mercurial 

 column remains motionless. The effects can, however, be reproduced 

 either by shifting the needle-points to a new spot or by diminishing the 

 distance of the secondary coil. The same result can be attained T\dthout 

 interfering either with the coil or the electrodes, by allowing the leaf to 

 rest for a longer interval. 



* Compare Du Bois-E,eymond, " Ueber die neg. Schwankung des Muskel-Stromes " 

 (Arch. f. Anat. u. Physiol. 1873, p. 549). In this paper is described an analogous effect 

 (doppelsinnige Schwankung) observed in the gastrocnemius and triceps of the frog. 



VOL. XXY. 2 H 



