38o 



NATURE 



l^Marck I, 1877 



" excursion " indicating a change of potential in a negative 

 direction at the movable contact amounting to 3 '5 to 5'0 ^ 

 / , _ t De La Rue element \ 

 \ ~" 100 /" 



If a similar plug is applied to the internal surface, so as to 

 cover the whole of it, the result is the same, but the extent of 

 the excursion is somewhat less. Hence it may be generally 

 stated that during the electrical disturbance ike surface of the 

 leaf becomes more negative ^ as compared with any other surface 

 of which the potential is constant, and that on the external sur- 

 face the change is greater than on the internal. This electrical 

 disturbance is Imiited to the leaf and ceases at the point dividing 

 the petiole from the isthmus or bridge, by which it is united with 

 the leaf ; on the petiole side of this point no sign of electrical 

 disturbance is shown by the electrometer. 



For various reasons the authors determined to direct their 

 attention to the middle third of the leaf. The following were 

 selected as representative points of contact : — (l) a point {t I) on 

 the internal sur.''ace of the leaf equidistant from the three sensi- 

 tive hairs ; (2) a point on the external surface (el) exactly oppo- 

 site to i I ; (3) and (4) points on the internal (i m) and external 

 {em) surfaces of the midrib, where the line joining the points iJ 

 on either lobe cuts the midrib ; (5) the petiole {/) ; and (6) the 

 bridge or isthmus (i>) already mentioned. The letter P denotes 

 the potential at any point, and V the variation of the potential 

 during the electrical disturbance. 



In four leaves the potentials and variations of the external 

 surfaces of the midrib and lobe were severally in hundredths of 

 a De La Rue cell : — 



emF as compared with pF ... o 

 elP „ pP .. 16 

 emV - 5 'O 



elV ... ... ... ...-20 



The external variation is usually greater than the internal of 

 a corresponding point, and the variation at em is usually greater 

 than that at any other point ; thus in six leaves — 



and 



elY= -yd -4-0 

 i/V = -1-5 -17 



4'2 -4"o 

 1-6 -1-8 



-4-0 -4-5 

 -22 -22 



tm V = -3"o 

 emV = -5-5 



3'S 



When a leaf is excited at intervals of a minute or ofiener by 

 single shocks from a du Bois-Reymond's induction coil,'^ which 

 are of just sufficient intensity to produce a response, it invariably 

 happens that after a time the electrical variation ceases. The 

 variation can be reproduced either by (i) shifting the needle- 

 points to a fresh spot, (2) by increasing the strength of the in- 

 duction-current, or (3) by allowing the leaf to rest for a longer 

 interval. With relation to electrical stimuli, it is shown that the 

 excitability of the leaf resembles thai; of the terminal organs of 

 the higher animals, in this respect, viz,, that relatively feeble 

 stimuli, if applied at very short intervals and repeatedly, are 

 competent to elicit a response. 



If a leaf be excited at short intervals by faradisation, the ex- 

 citations (makes and breaks) being con'inued each time until an 

 excursion is produced, the combined effects of summation and 

 gradually increasing exhaustion can be readily observed. At 

 first the leaf responds after eight to ten excitations, but 

 gradually the number of excitations required to awaken the 

 tissues to action increases, the effect being postponed for longer 

 and longer periods, until it finally fails to occur. When a leaf is 

 excited at regular intervals by single shocks of such intensity as 

 to be just beyond the limit of adequacy, the effects sometimes 

 become rhythmical. 



The time which intervenes between an excitation and the 

 beginning of the electrical disturbance varies in different leaves, 

 and is very much affected by variati\)ns of temperature. This 

 time the authors have called the period of electrical delay. 



As a mean of many experiments it was found that when the 

 fixed electrode was on the petiole and the movable electrode 

 on ^w, the a^f/a'j' was o"295 second. If the movable electrode 

 was at f/ or it, the delay varied according to the proximity of 

 the sensitive hair touched to the point of application of the 

 movable electrode. Thus if the movable electrode was at el and a 



' It is interesting to note that the surface of a frog muscle, during the 

 electrical disturbance which precedes contraction, becomes positive. 



Two steel needles she-iihed in gla^s, and bound together, were used as 

 excitm? electrodes, the points of tlie needles being thiust through the epi- 

 dermis of the leaf. 



sensitive hair on the same lobe was touched, the delay was 0'23i 

 sec. ; but if a hair on the opposite lobe was touched the delay 

 was 0*403 sec, the disturbance having to make its way from the 

 sensitive hair on the opposite lobe through and across the midrib 

 and up to the electrode. It is obvious that by measuring the 

 diitance between the hair touched and the electrode we can 

 ascertain, more or less exactly, the rate of the trans rnission of 

 what may be called the " wave of negative variation " through the 

 leaf. From many experiments, the stimulation being sometimes 

 mechanical and sometimes electrical, it was found that the wave 

 traversed a distance of about 8 mm. in 0"i8 sec, or at a rate of 

 about 4 "4 centims. per second. When the period of delay at e I 

 was compared with that at i I, it was found that it was shorter at 

 e/ than il; e.g., in some experiments (the excitation being 

 weak faradisation and the excursions being taken from eland i I 

 alternately), the following numbers were obtained : — 



Finally, if either el, il, em, or im be compared with the bridge b, 

 it will be found that the period of delay at b will be much greater 

 than that at any of the other points :, 



In normally active leaves, in which the disturbance is first seen 

 about asixth of a second after mechanical stimulation, the excursion 

 attains its maximum in about one second, and the whole disturb- 

 ance is over in about two seconds after the excitation, so that 

 the electrical disturbance is entirely over before the mechanical 

 effect begins, and consequently occurs in a period whic^in muscle 

 is called the period of latent stimulation. 



All these periods are, however, very much modified by tem- 

 perature, bemg shortened if the temperature is raised (within 

 certain limits), and lengthened if the temperature falls. 



The following is one of several tables given in the paper, 

 illustrating the effect of temperature o\. the periods of delay, 

 maximum and total duration of the electrical disturbance : — 



To beginning of excursion 

 £ .^ i ^ ^ o maximum 



f^ g S I To end 



fn leaf at 

 ordinary 

 tempera- 

 ture. 



023 

 1-46 

 2 2 



fn warm 

 chamber 

 at 45* C. 



Cooled by 



proximity 



of a block 



of ice. 



079 

 '•37 



0-44 



1-68 

 2 "94 



THE SPONTANEOUS GENERATION 

 QUESTION' 



A T the meeting of the French Academy of Sciences on 

 •^ January 29, M. Pasteur read the following reply to Dr. 

 Bastian : — 



Dr. Bastian, in reply to the communication which I made on 

 January 8, along with M. Joubert, addressed to the Academy 

 last Monday a long note, in which he still contrives, I think, to 

 elude the main point of the debate. In our communication of 

 January 8 there was one word of prime significance, pure potash ; 

 but, what is surprising, in the reply of three pages of Dr. 

 Bastian there is not even allusion made to that condition ofj 

 purity, which was everything. 



I shall make a new attempt to recall the English savant to the 

 criterion, from which he cannot escape, do what he will. 



The discussion was raised by his statement, that a solution of| 

 boiled potash caused bacteria to appear in sterile urine at 50°, 1^ 

 after it had been added to ihe latter in quantity sufficient for-j 

 exact neutralisation. Dr. Bastian concluded that he had thuSji 

 discovered the physico-chemical conditions of the spontaneous! 

 generation of certain bacteria. 



This is my reply to the learned London professor of patho- 

 logical anatomy : — 



I defy Dr. Bastian to obtain, in presence of competent judges,* 

 the result to which I have referred, with sterile urine, on thesolej 

 condition that the solution of potash which he employs be purCjl 

 i.e. made with pure water and pure potash, both free from organic 



' Continued from p. 314. 



