O^ 



NATURE 



[August io, 1882 



out-of-the-way plant. It has an organisation which is not re- 

 presented in the European flora. The family of thistles, and 

 their allies the knapweeds (represented in our gardens by the 

 ladies blue bottle), all of which are common wayside plants, 

 exhibit excitable movements which, although of a very different 

 kind from those we have just described, have, like them, to do 

 with the visits of insects for the purpose of fertilisation. We will 

 now throw on the screen a single fertile floret of Centaurea Cyanus 

 (Fig. 5). The large diagram shows the same floret deprived of 

 its corolla. Its axis is occupied by the style, surrounded by iis 

 tube of anthers. Below, the anther-filaments expand into a kind 

 of cage, and again approach one another, when they are united 

 with the tube of the corolla. At the moment that the anthers 

 arrive at maturity these filaments are very excitable. When one 

 of them is touched, it contracts and draws the style towards 

 itself. Immediately afterwards the excitatory effect spreads to 

 the others, all five arches becoming straight, and applying them- 

 selves closely to the style. A similar effect is produced by an 

 induction shock. [The structure described was projected on the 

 screen ; on passing an induction current through it, the mode of 

 contraction of the filaments was seen.] 



The mechanism of Centaurea has been studied by many plant 

 physiologists, particularly by Frof. Ferdinand Cohn of Breslau, 

 and nmre recently with great completeness by Prof. Pfeffer. It 

 has in this respect a greater interest than any other — that the 

 shortening of these filaments in response to excitation strikingly 

 resembles muscular contraction. You hive here a structure in 

 the form of a flattened cylinder which resembles many muscles 

 in form, the length of which is diminished by about a sixth on 

 excitation. This superficial resemblance between the two actions 

 makes it the more easy to appreciate the differences. 



Let me draw your attention to the diagram of an experiment 

 made last year, which was intended to illustrate the nature of 

 muscular contraction, and particularly to show that when a 

 muscle contracts, it does not diminish in volume. The first 

 difference between muscle and plant is a difference in the degree 

 of shortening. A muscle shortens by something like a third of 

 its length, the anther filament only by a sixth. But it is much 

 more important to notice .that in contracting, the filaments do 

 not relain their volume. In shortening they broaden, but the 

 broadening is scarcely measurable ; hence they must necessarily 

 diminish in bulk, and this shrinkage takes place, as Pfeffer has 

 shown, exactly in the same manner as that in which the excitable 

 cushion of Mimosa shrinks, namely by the discharge of liquid 

 from its cells. 



We are now in a position to study more closely the question 

 to which I referred a few minutes ago — How do the cells dis- 

 charge their contents ? The structure of the filament of 

 Centaurea, from its extreme simplicity, is a better subject of 

 investigation with reference to this question, than any other. 

 Each filament is a ribbon consisting of (1) a single fibre-vascular 

 bundle, (2) delicate cells of regular cylindrical form, (3) an 

 epidermis of somewhat thick walled cells. [Microscopical pre- 

 parations were shown.] In Mimosa we saw that the epidermis 

 and vascular bundle took only a passive part in the production 

 of the motion. Here, the part they play is even less important. 

 Everything depends on the parenchyma, which, when excited, 

 shrinks by discharging its water. Pfeffer proved this by cutting 

 off the anther tube from the filaments, and then observing that 

 on excitation a drop collected on the cut surface, which was 

 reabsorbed as the filament again became arched. It is obvious 

 that if the whole parenchyma discharges its liquid, each cell 

 must do the same, for it is made up entirely of cells. To un- 

 derstand how each cell acts, we have only to consider its struc- 

 ture. Each consists of two parts — an external sac or vesicle, 

 which is of cellulose, and, so long as the cell is in the natural or 

 unexcited state, over-distended, so that, by virtue of its elasticity, 

 it presses on the contents with considerable force ; and secondly, 

 of an internal more actively living membrane of protoplasm, 

 of which the mechanical function is, so long as it is in its 

 active condition, to charge itself fuller and fuller with liquid — 

 the limit to further distension being the elastic envelope in which 

 it is inclosed. In this way the two (the elastic envelope and the 

 protoplasmic lining) are constantly in antagonism, the tendency 

 of the former being towards discharge, that of the latter towards 

 charge. This being so, our explanation of the effect of excita- 

 tion on the individual cell amounts to this — that the envelope 

 undergoes no change whatever, but that the protoplasm lining sud- 

 denly loses its water-absorbing power, so that the elastic force of 

 the envelope at once comes into play and squeezes out the cell- 

 contents. Consequently, although here, as everywhere, the 



protoplasm is the sea: of the primary change, the mechanica' 

 agent of the motion is not the protoplasm, but the elastic 

 envelope in which it is inclosed. 



( To be ctntinued. ) 



ELECTRIC LIGHTING BY INCANDESCENCE* 

 CPEAKINO in this place on electric light, I can neither 

 '-' forget nor forbear to mention, as inseparably associated 

 with the subject and with the Royal Institution, the familiar, 

 illustrious, names of Pavy and Faraday. It was in connection 

 with this institution that, eighty years ago, the first electric light 

 experiments were made by I)avy, and it was also in connection 

 with this Institution that, forty years later, the foundations of 

 the methods, by means of which electric lighting has been made 

 useful, were strongly laid by Faraday. 



I do not propi se to describe at any length the method of 

 Davy, I must, however, describe it slightly, if only to make 

 clear the difference between it and the newer method which I 

 wish more particularly to bring under your notice. 



The method of 1 avy consists, as almost all of you know, in 

 producing electrically a stream of white-hot gas between two 

 pieces of carbon. 



When electr c light is produced in this manner, the conditions 

 which surround the process are such as render it impossible tn 

 obtain a small light with proportionally small expenditure of 

 power. In order to sustain the arc in a state approaching 

 stability, a high electromotive force and a strong current are 

 necessary ; in fact, such electromotive force and such current as 

 correspond to the production of a luminous centre of at least 

 several hundred candle-power. When an attempt is made to 

 produce a smaller centre of light by the employment of a pro- 

 portionally small amount of electrical energy, the mechanical 

 difficulties of maintaining a stable arc, and the diminution in the 

 amount of light (far beyond the diminished power employed), 

 puts a stop to reduction at a point at which much too large a 

 light is produced for common purposes. 



The often-repeated question, " Will electricity supersede 

 gas?" could be promptly answered if we were confined to this 

 method of producing electric light ; and for the simple reason 

 that it is impossible, by this method, to produce individual lights 

 of moderate power. 



The electric arc does very w ell for street lighting, as you all 

 know from what is to be seen in the City. It also does very 

 well for the illumination of such large inclosed spaces as railway 

 stations ; but it is totally unsuited for domestic lighting, and for 

 nine-tenths of the other purposes for which artificial light is 

 required. If electricity is to compete successfully with gas in 

 the general field of artificial lighting, it is necessary to find some 

 other means of obtaining light through its agency than that with 

 which we have hitherto been familiar. Our hope centres in the 

 method — I will not say, the ihw method — but the method which 

 until within the last few years has not been applied with entire 

 success, but which, within a recent period, has been rendered 

 perfectly practicable — I mean the method of producing light by 

 electrical incandescence. 



The fate of electricity as an agent for the production of arti- 

 ficial light in substitution for gas, depends greatly onihe succe-s 

 or non-success of this method ; for it is the only one yet dis- 

 covered which adapts itself with anything like completeness to 

 all the purposes for which artificial lighting is required. 



If we are able to produce light economically through the 

 medium of electrical incandescence, in small quantities, or in 

 large quantities, as it may be required, and at a cost not exceed- 

 ing the cost of the same amount of gas-light, then there can be 

 little doubt— there can, I think, be no doubt— that in such a 

 form, electric light has a great future before it. I propose, 

 therefore, to explain the principle of this method of fighting by 

 incandescence to show how it can be app'ied, and to discuss the 

 question of its cost. 



When an electrical current traverses a conducting wire, a 

 certain amount of resistance is opposed to the passage of the 

 current. One of the effects of this conflict of forces is the 

 development of heat. The amount of heat st developed de- 

 pends on the nature of the wire — on its length and thickness, 

 and on the strength of the current which it carries. If the wire 

 be thin and the current strong, the heat developed in it may be 

 so great as to raise it to a white heat. 



■ Lecture delivered at the Royal Institution of Great Britain, March 10. 

 1882. by Joseph W. Swan. Sir Frederick Bramwell, F.R.S., vice-president, 



