41 



NATURE 



{March 21, 1878 



means that an animal must be capable of being affected by 

 changes which, considered as mere physical agents, are insigni- 

 ficant. A fly living in the same room with an active-minded 

 boy will depend for its safety on its power of rapidly appreciat- 

 ing the approaching shadow of the boy's hand. Now the 

 changes produced in the arrangement of forces in the universe 

 are not perceptibly affected by this shadow — it is utterly insigni- 

 ficant — yet what a violent effect it has on the fly. It is because 

 the nervous system of the fly possesses the property of magni- 

 fying external changes so that apparently slight disturbance 

 causes large results. 



This power of being strongly affected by apparently slight 

 changes is a very important character of living matter. The 

 processes which occur within the fly have been likened to the 

 explosion of a pistol, the force used in moving the trigger being 

 utterly insignificant when compared with the result produced. 

 I do not mean that this exploding power is a distinguishing mark 

 of living matter, but it certainly is a well marked feature. 

 Besides the power of magnifying or intensifying external changes, 

 which we have described as the exploding power of irritable tissue, 

 there is another, the power possessed by nerves of transmitting a 

 stimulus wave from one part to another. We will first look for 

 this transmitting power as it exists in plants. 



The leaf of the sundew, or Drosera, consists of a shallow, 

 slightly saucer-shaped disc covered over with short glands, and 

 fringed all round with projecting tentacles which also terminate 

 in glands. The glands secrete a sticky fluid, which hangs in 

 drops on them, hence the name of sundew, because the leaves 

 seem to be covered with dew in sunshine, when other plants 

 are dry. Insects are caught by the sticky secretion, and are 

 also embraced and held fast by the outer tentacles, which possess 

 the power of moving. When the insect has been killed by being 

 drowned in the sticky secretion, it is digested by the acid juice 

 poured out by the glands and subsequently absorbed. 



The external or movable tentacles may be made to bend in- 

 wards, either by insects alighting on the centre of the disc of the 

 leaf, or on the sticky glands of the tentacles themselves. In the 

 first case, when an insect is caught on the middle of the leaf, 

 and the external tentacles bend in and surround it, we have a 

 true transmission of stimulus, a message sent, like a message is 

 sent along a nerve. The insect may be struggling to free itself, 

 and will probably succeed in doing so, unless the external 

 tentacles give their help. The external tentacles can be made 

 to bend not only by insects or other objects placed on the centre 

 of the leaf, but also by anything placed on the gland at the end 

 of the tentacle itself. In this case the meaning of the movement 

 is equally obvious. If a gnat or fly lights on one of the external 

 glands, it will probably escape, unless carried to the centre of the 

 leaf, where it will be also held by the small sticky glands. Here 

 also there is a true transmission of stimulus. The message has 

 to be sent from the gland at the top to the place where the 

 tentacle bends ; a message is sent from the gland to the bending 

 part of the tentacle, just as a message goes through nerve tissue 

 from our skins to our muscle. 



In this case the tentacle always carries the fly it has caught 

 to the actual centre of the leaf. But if a fly has been caught 

 by the disc of the leaf, and not quite in the centre, then the 

 messages are sent in accordance with the position of the 

 fly, and all those tentacles within reach move to the point 

 of irritation with marvellous precision. This transmission 

 of messages is all the more wonderful, because, as far as 

 our powers of observation go, there is no special structure to 

 convey the stimulus. It is true that waves of stimulation do 

 travel with special facility along the fibro-vascular bundles, or what 

 are usually called the veins of the leaf. But in this case, where 

 tentacles converge to a given point in the disc of the leaf, this 

 mode of transmission is impossible, because the veins are few in 

 number, and could not cause so nice an adaptation of move- 

 ments. Moreover, stimuli can travel across a leaf of Drosera 

 after the vascular bundles have been cut through.^ So that 

 we have the wonderful fact of a wave of stimulation travel- 

 ling with great accuracy transversely through a number of cells 

 with absolutely no structure like nerve-fibre to guide the course 

 in which the stimulus- wave shall flow. 



One other curious phenomenon may be alluded to as showing 

 the extraordinary power of stimulus-transmission. If a piece of 

 meat is placed on an external tentacle, the gland on which it 

 rests sends forth an acid secretion ; and if a piece of meat is 



I See Batalin, " Flora," 1877, who has correctly pointed out the import- 

 ance of the fibro-vascular bundles as goaveying stimulus-waves. 



placed on the centre of the leaf, the tentacles, as before said, 

 bend in and ultimately touch it ; but if the external glands are 

 tested with litmus paper before they reach the meat in the centre, 

 they will be found to be covered with acid secretion, proving 

 that not only had a message been sent to the moving part of the 

 tentacle, but also to the secreting cells in the gland. 



One might find a parallel to this in the action of the human 

 salivary glands. The gland nerves may be excited either by the 

 stimulus of food placed in the mouth, or by the voluntary action 

 of the muscles of mastication. Here the saliva is poured out, 

 although there is no food to act on, just as the Drosera-gland 

 secretes during the movement of the tentacle before there is any- 

 thing for its secretion to digest. 



Having briefly considered the transmission of stimulus-waves 

 as shown in Drosera I will pass on to consider what manifesta- 

 tions may be found of the other general property of nerve tissue, 

 the property which I have called exploding power. It is chiefly 

 manifested in Drosera by the extreme sensitiveness of the glands 

 on the external tentacles. It is found not to be necessary to 

 place meat or insects on the gland, but tkat bits of glass, wood, 

 paper, or anything will excite them. Smaller and smaller atoms 

 were tried and still the glands were found to be sensitive to their 

 presence.^ At last a minute piece of a human hair, about one- 

 hundredth of an inch in length, and weighing just over ^-1^^-^ of a 

 grain, was placed on the gland of a tentacle and it caused unmis- 

 takable movement. The case is yet more wonderful than it 

 sounds, because the piece of hair must be partly supported by the 

 thick drop of secretion on the gland, so that it is probably no 

 exaggeration to say that the gland can perceive a weight of one- 

 millionth of a grain. This degree of sensitiveness is truly 

 astonishing, it seems to us more like the sense of smell than that 

 of touch, for to our most delicate tactile organ, the tongue, such 

 atoms are quite imperceptible. 



The power which Drosera has of perceiving the presence of 

 ammonia is perhaps still more astonishing. A solution of phosphate 

 of ammonia in pure distilled water in the proportion of one part 

 to over two million of water, caused inflection of tentacles.^ One 

 may form an idea of this result by making a solution of a single 

 grain of the phosphate and thirty gallons of distilled water, and 

 then finding out that it is not pure water. Considering the 

 water-supply which we at present enjoy, we may well be grateful 

 that our senses are duller than those of a sundew. 



As examples of simple sensitiveness these facts are sufficiently 

 striking, but the powers of discriminating between differert kinds 

 of stimuli are equally curious. The tentacles having proved so 

 extraordinarily sensitive to light bodies resting on them, one 

 would expect that the slightest touch would make them bend. 

 But it is not so ; a single rapid touch, though it may be violent 

 enough to bend the whole tentacle, does not cause inflection. 

 The meaning of this is clear, for in windy weather the glands 

 must be often touched by waving blades of grass, and it would 

 be a useless labour to the plant if it had to bend and unbend its 

 tentacles every time it was touched. It is not excited except by 

 prolonged pressures or quickly repeated touches. This is also 

 quite intelligible, for when an insect is caught on the sticky 

 secretion of the gland it will give a somewhat prolonged pressure, 

 or a number of kicks to the sensitive gland, unless indeed it flies 

 away after a single struggle, and in that case the tentacle will 

 be also saved from uselessly bending. 



In another carnivorous plant, Dionsea, the specialisation of 

 sensitiveness is exactly the reverse ; thick and comparatively 

 heavy bits of hair can be cautiously placed on the sensitive organs 

 without causing any movement, but the delicate blow received 

 from a cotton thread swinging against the hair causes the leaf to 

 close.3 Dionasa catches its prey by snapping on it like a rat- 

 trap — there is no sticky secretion to retain the insect as in 

 Drosera till the slowly moving tentacles can close on it. Its 

 only chance of catching an insect is to close instantly on the 

 slightest touch. The specialisation of sensitiveness in Dioncea 

 is therefore just what it requires to perfect its method of capture. 



In describing the sensitiveness of Drosera and Dionaja I wish 



rather to insist on a wide and general similarity to the action of 



nerves. There may be said to be an analogy between the 



specialisation of extreme sensitiveness in Drosera and Dionaea and 



the nervous tissues of animals, because these properties play the 



same part in the economy of the plant that is supplied through 



some kind of nerve machinery in the higher animals. Closer 



analogies could be pointed out. There are, for instance, the 



I ' ' Insectivorous Plants," p. 32. 



» "Insectivorous Plants," p. 170. 



3 '< Insecfiyorous Plants," p. 289. 



