June 1 8, 1 8 74 J 



NA TURE 



examined, and by at the same time maintaining a bril- 

 liantly illuminated patch in an adjoining part of the field 

 of view. If therefore we use the combination N, O, P, Pj 

 (/.(•. if we remove the second quartz plate), we can, by 

 turning the prism round, diminish to any required extent 

 the intensity of the light in one pair of the complementary 

 images, and at the same time increase that in the other 

 pair. This is equivalent to the conditions of Helmholtz's 

 experiments ; and the tints in question will be found to 

 be produced. 



W. Spottiswoode 



VENUS'S FLY-TRAP {Dionaa mtisciptcla) * 

 II. 



Contractility of Dioncra. — I have given you a gene- 

 ral view of our plant and of its behaviour. We next 

 proceed to examine more particularly that property of 

 contracting when irritated to which the plant owes its 

 faculty of catching insects, and to which my own investi- 

 gations have been directed. Before beginning the ex- 

 perimental demonstration of the facts, I wish to lay before 

 you some considerations relating to the nature of this 

 prcpei'ty as it manifests itself in living beings belonging 

 to both kingdoms. 



We have to do here not merely with contractility but 

 with irrito-contractility. The fact that the property 

 requires two words to express it implies that there are 

 two things to express, viz. (i) that contraction takes 

 place, and (2) that it takes place in answer to irritation. 

 As this is the case not only here, but in all other in- 

 stances of animal or vegetable active motion, we recog- 

 nise in physiology these 'two properties as fundamental : 

 irritability, or excitability, zwA. contractility, the former 

 designating the property possessed by every living struc- 

 ture whatsoever of being excited to action {i.e. of having 

 its stored-up force discharged) by some motion or dis- 

 turbance from outside ; the latter, that kind of discharge 

 or action which results in change of form, and usually 

 declares itself in the doing of mechanical work. This 

 property of excitability, which, let me repeat, is com- 

 mon to all living structures, is, as we have seen, com- 

 parable in its simplest manifestations to that possessed 

 by many chemical compounds (of explosiveness) and 

 many mechanical contrivances (of going off or dis- 

 charging when meddled with, as in the case of the rat- 

 trap already referred to). 



Ill physiology, as in the other sciences of observa- 

 tion, the process of investigation is, throughout, one of 

 comparison. Not only do we proceed from first to 

 last from the known towards the unknown, but what we 

 speak of as our knowledge or understanding of any new 

 fact consists simply in our being able to bring it into 

 relation with other facts previously well ascertained and 

 familiar, just as the geographer determines the position 

 of a new locality by ascertaming its topographical rela- 

 tion to others already on the chart. 



The comparison we have to make this evening is be- 

 tween the contractility displayed by the leaf of Dionrea 

 and the contractility of muscle. I choose muscle as the 

 standard of comparison, not merely because it is best 

 known and has been investigated by the best physicists of 

 our time, but because its properties are easily illustrated 

 and understood. I shall be able to show that the resem- 

 blance between the contraction of muscle and that of 

 the leaf is so wonderfully complete that the further we 

 pursue the inquiry the more striking does it appear. 

 Whether we bring the microscope to bear on the struc- 

 tural changes which accompany contraction, or em- 

 ploy the still more delicate instruments of research 

 which you have before you this evening, in order to deter- 

 mine and measure the electrical changes ■which take 



* Continued from p. 107. 



place in connection with it, we find that the two processes 

 correspond in every essential particular so closely, that 

 we can have no doubt of their identity. 



Muscle, like every other living tissue, is the seat, so 

 long as it lives, of chemical changes, which, if the tissue 

 IS mature, consist entirely in the disintegration of chemical 

 compounds and the dissipation of the force stored up in 

 these compounds, in the form of heat or some other kind 

 of motion. This happens when the muscle is at rest, but 

 niuch more actively when it is contracting, in which con- 

 dition it not only produces more heat than it produces at 

 other times, bat also may do— and, under ordinary cir- 

 cumstances, does — mechanical work ; these effects of 

 contraction of muscle are, of course, dependent in quan- 

 tity on the chemical disintegration which goes on in its 

 interior. 



Again, muscle so long as it is in the living state is 

 electromotive. This property it probably possesses in 

 common with other living tissues, for it is very likely that 

 every vital act is connected with electrical change in the 

 living part. But in muscle, as well as other irritable 

 and contractile tissues in animals, the manifestation of 

 electromotive force is inseparably connected with the 

 special function of the tissue, i.e. with contraction, the con- 

 nection being of such a nature that the electromotive force 

 expresses, not the work actually done at any given moment, 

 but the capacity for work. Thus, so long as the muscle lives, 

 its electromotive force is found to be on the whole pro- 

 portional to its vigour. As it gradually loses its vitality, its 

 power of contracting and its electromotive force disap- 

 pear /(zr/'/(7«/c. When it contracts, the manifestation of 

 electromotive force diminishes in proportion to the degree 

 of contraction. But it is to be borne in mind that, al- 

 though when the muscle or the leaf contracts electro- 

 motive force disappears and work is done, there is no 

 reason for supposing that there is any conversion of the 

 one effect into the other, or that the source of the force 

 exercised by the organ in contracting is electrical. 



The lecturer then proceeded to demonstrate the corre- 

 spondence between the electrical phenomena which 

 accompany muscular contraction, and those which are 

 associated with the closing of the Dionsa leaf, by a series 

 of experiments.* 



I. The form of the gastrocnemius muscle of the frog, 

 in the uncontracted state, was projected on the screen 

 with the aid of the electric light. A contraction was then 

 determined by passing through it a single opening induc- 

 tion shock. It was seen to shorten and to become propor- 

 tionately broader. 



[In contraction, the bulk of a muscle remains unaltered. 

 Further, the change of shape of the whole muscle de- 

 pends on an exactly similar change of shape of every 

 particle of which it is composed. This might be inferred 

 from the consideration that a muscle is not an apparatus 

 made up of parts dilTering from each other in structure, 

 but a mass of substance equally instinct with life in every 

 part. We know it to be the case by direct observation, 

 for if we observe living muscle in the act of contraction 

 under the microscope (as can easily be done in the muscles 

 of insects), f we see that each minutest fibre participates 

 in the change of form. The same holds good as regards 

 the plant. The agent in the contraction is, without doubt, 

 the protoplasm of the cells of the contractile organs. In 

 Dion;ea this has not as yet been sufficiently investigated, 

 but in DroseraMr. Darwin has shown that when the hairs 

 which project from the upper surface of the leaf, become 

 " incurved " under the exciting influence of appropriate 

 stimuli, the contents of the cells undergo a most peculiar 



* Tile statements contained in the first part of lliis lecture, especially those 

 relating to the mechanism of the closure of the Diosxa leaf and its digestion , 

 are founded almost entirely on information which I owe to Mr. Dai^in. The 

 experiments which led to the discovery of the *' leaf current " and its " nega- 

 tive variation " were made last autumn, Mr. Darwin having kindly furnished 

 me with plants for the purpose. — J. IJ. S. 



t See Schiifer "On the Contiaction"of the Muscles of the Water-beetle, 

 rhil. Trans., vol. clxiii. p. 429, 187J. 



