Sept, 14, 1882] 



NATURE 



485 



ir. view is to demonstrate the contrast between the motion of the 

 leaf and muscular contraction. A muscle in contracting acts as 

 one organ— at once. The motion of the leaf is the result of the 

 action of many hundred independent cells, all of which may act 

 together, but may not. In either case they take a great deal 

 longer to think about it ; for during a period after excitation, 

 which amounts at ordinary summer temperature to about a 

 second, the leaf remains absolutely motionless. 



And now we have to inquire what happens during this period 

 of delay. There are two things which we may assume as certain 

 without further proof, namely, first that something happens ; 

 for when I see a certain movement followed after a time in- 

 variably by another, I am quite sure that the chain between 

 cause and effect is a continuous one, although the links may be 

 invisible ; and secondly, that this invisible change has its seat in 

 the protoplasm of each of the excitable cells. 



We have already seen that in muscle this latent state of excita- 

 tion is not withhout its concomitant sign — the excitatory electri- 

 cal disturbance, and I have now to show you that this, which is 

 the sole physical characteristic of the excitatory process in animal 

 tissues, manifests itself with equal constancy and under the same 

 conditions in plants. 



It will be unnecessary for my present purpose to enter into 

 any details as to the nature of the electrical change; it will be 

 sufficient to demonstrate with respect to it, first, that when 

 observed under normal physiological conditions, its phenomena 

 are always conformable to certain easily defined characters ; 

 secondly, that it culminates before any mechanical effect of 

 excitation is observable, and consequently occupies, for the most 



part, the period of latent excitation already referred to ; and 

 thirdly, that it is transmitted with extreme rapidity from one 

 lobe of the leaf to the opposite. Of these three propositions, it 

 will be convenient to begin with the second. On the left-hand 

 screen is projected the mercurial column of the capillary electro- 

 meter of Lippmann. The instrument which we use this evening 

 is one of great sensibility, given me by my friend Prof. Loven of 

 Stockholm. The capillary electrometer possesses a property 

 which for our purpose is invaluable — that of responding instan- 

 taneously to electrical changes of extremely short duration. We 

 cannot better illustrate this than by connecting the wires of the 

 telephone with its terminals. If I press in the telephone plate 

 I produce an instantaneous difference between the terminals in 

 one direction, and in the opposite when I remove the pressure. 

 You see how beautifully the mercurial column responds. 



We now proceed to connect the terminals with the opposite 

 sides of a leaf, so that by means of the mirror we can observe 

 the moments at which the leaf begins to close and the first move- 

 ment of the mercurial column, both being projected on the same 

 screen. We shall see that the mercurial column responds (so to 

 speak) long before the mirror. The difference of time will be 

 about a second. 



We now take another leaf which, with the plants of which it 

 forms part, is contained in this little stove, at a temperature of 

 about 32° C. Our object being to subject the leaf to a succes- 

 sion of excitations, the effects of which would of course be to 

 determine its closure, we prevent this by placing a little beam of 

 dry wood across it, and fixing the ends of the beam with plaster 

 of Paris to the marginal hairs of each lobe. At the same time, 



Fig. 10.— Copy of photograph of the excursions of the capillary electrometer as pri 



The fcur " excitatory variations " shown were due to as many touches of a sensitive h 

 were connected with the terminals of the instrument. 



ensitive plate moving ;.t the rate of i centimetre per second. 

 r of the lobe opposite to that of which the opposite surfaces 



u edges of plaster are introduced in the gap between the lobes at 

 either end of the midrib. [The leaf so fixed was projected on 

 the screen (Fig. 7).] This having been done, we can excite the 

 leif any number of times without its moving ; and we know 

 that we actually excite it by observing the same electrical effect 

 which, in the first leaf experimented on, preceded the movement 

 of the lobe. 



And now I beg you to notice what the nature of the experiment 

 is. The diagram (Fig. 8) shows the position of the electrodes by 

 which the opposite surfaces are connected with the terminals of 

 the electrometer. You will notice that they are applied to 

 opposite points of the internal and external surfaces of the 

 right lobe, and that the left lobe is excited. The experiment 

 consists in this . By the electrodes near r, an induction shock 

 passes through the right lobe. Apparently at the same moment 

 the electrometer, which is in relation with the opposite lobe, 

 responds. I say apparently, because in reality we know that 

 the response does not begin until about 3-looth of a second 

 later. We prove this by a mode of experimentation which is of 

 too delicate a nature to be repeated here. I will explain the mode 

 of action of the instrument used by a diagram (Fig. 9) which 

 represents a pendulum in the act of swinging from left to right. 

 As it does so, it opens in succession three keys, of which the 

 first is interpolated in the primary circuit of the induction appa- 

 ratus which serves to excite the leaf; the second breaks a deri- 

 vation wire which short-circuits the electrodes, so that, so long 

 as it is closed, no current passes to the galvanometer, which in 

 this experiment takes the place of the electrometer, while the 

 third breaks thegalvanometric circuit. Consequently the opposite 

 surfaces of the leaf are in communication with the galvanometer 



only between the opening of the second and third keys. These 

 three keys can be placed at any desired distance from one 

 another. If they are >o placed that the galvanometer circuit is 

 closed 1 -icoth of a second after excitation, and opened 3-iooth 

 of a second, and it is found that there is no effect, it is certain 

 that the electrical disturbance does not begin at the part of the 

 leaf which is interpolated between the galvanometer electrodes 

 until at least 3-iooth of a second after the excitation. If, on 

 extending the period of closure to 4-iooth of a second, the effect 

 becomes observable, you are certain that the disturbance begins 

 between three and four hundredths of a second after excitation. 



By this method we have learnt, first, that even when the seat 

 of excitation is as near as possible to the led off spot, there is a 

 measurable delay, and secondly, that its duration varies with 

 the distance which the excitation effect has to travel so as to 

 indicate that, in a warm stove, the rate of transmission is some- 

 thing like 200 millimeters per second. It is, consequently, 

 comparable with the rate of transmission of the excitatory 

 electrical disturbance in the heart of the frog. 



And now I come to my last point, namely, that the electrical 

 chance has always the same character under the same conditions. 

 You have already seen that when the method used is that which I 

 have indicated, the electrical effect consists of two phases, in the 

 first of which the external surface of the leaf becomes negative to 

 the internal. I will now exhibit this in another way. Many present 

 have probably seen in a recent number of Nature reproduc- 

 tions of photographs recently taken by M. Marey, of the phases 

 of the flight of birds. If the movement of a bird'» wing 

 can be photographed you will easily imagine that we can also 

 obtain light-pictures of such a movement as that of the electro- 



