ARE PLANTS NERVOUS ?
A oaFAT step towards unity of outlook was marked by Claude Bernard's Lecons sur les Phenomenes de la Vie communs aux Animaux et aux Vegetaux (1879). It became clear that plants and animals share a common life, though their--detailed expressions of it are very ' different. The beech-tree feeds and grows, digests and breathes as really as does the squirrel on its branches. In regard to none of the main functions except excretion is there any essential difference. Moreover, many simple plants swim about actively ; growing shoots and roots have gently-swaying tips ; leaveS_ rise and _fall; flowers open and close, with the waxing and: waning light; of day. The tendrils of climbers, the leaves of the Sensitive Plant, the tentacles of the sundew, the blade of Venus's Fly-trap, the stamens of the barberry, the stigma of the musk, and many other plant structures exhibit exquisite sensitiveness. In the sense of answering- back to stimuli, they feel. How fat: we have got from the aphorism of Linnaeus : Lapides crescunt; vegetabilia crescunt ft vitntnt; animalia crescunt et vivunt et sentiunt. The " nervousness " of plants is now recognized by all, and for driving this conclusion home by persistent and ingenious experiments great credit is due to Sir Jagadis Chunder Bose., When the glowing but not flaming tip of the long thin incense stick of the Chinese is applied to a secondary rib of the beautiful compound _pinnate leaf. of the Sensitive Plant, Mimosa pudica, the result is a spreading impulse, reminding one of the nervous impulse in an animal. The leaflets close together, from below upwards ; the secondary ribs or stalks draw together as in a folding fan ; the leaf as a whole sinks down when the stimulus reaches the motor cushion or pulvinus at the base of the main leaf-stalk ; the impulse passes into the stem and other .leaves collapse; if the stimulus be strong enough, the impulse, whatevei; it is, may ascend. to the apex of the stem and travel down the other side. Of course, there is nothing but convenience in using an incense stick ; a scratching pin will do as well, provided that the stimulus is superficial. It must not injure the surface, nor cause exudation • of sap, nor irritate the enclosed wood. The central problem which Sir Jagadis has tackled is the conduction of the excitation from the place stimulated to a distance in the plant where a motor organ or pulvinus is activated. .Is- the conduction like that which passes along the nerve-fibres of animals ?
Bose .comes to the deliberate conclusion that the higher plants have a well-defined nervous system. Excitation froni part to part is conducted by the phloem or bast of the fibro- vascular bundle. The excitation is not due to the movement of fluid in vessels or cells, as in the sap-currents ; it is of the nature of a protoplasmic thrill, as in an animal nerve. More- over, the idea of a reflex arc is boldly. extended from the animal to the plant. In the animal a stimulus excites the ending of a sensory nerve-fibre, the thrill passes to the sensory nerve-cell, thence to an associative or communicating nerve-cell, thence to a motor nerve cell, and thence by a motor nerve-fibre to the muscles which contract. So, before we can say " reflex _action:' we draw our finger away from a hot object. But there 'is a similar phenomenon, according to Sir Jagadis, in the Sensitive Plant. If a very slight stimulus be applied to one of the secondary ribs of a leaf, it gives rise to an ingoing hnpulse causing the characteristic response 'in a part of the motor cushion or pitM.61141' 'and that is all. But if there he a slight increase in the intensity of the stimulus, a new Phenomenon makes its nppearance ; the ingoing or afferent impulse, reaeliint the Centre (perhaps a question-begging term), becomes reflected along a new path as an efferent impulse. If these Conclusions. are established up to the' hilt, the work of Sir Jagadis is epoch-making, for it means not merely that_ phenomena analogous , to animal reflex action's occur in plants, as has been. often suggested, it means that . plants may have nervous reflex arcs precisely comparable' to those of Whatever conclusion be reached on this crucial question' we cannot but admire the ingenuity of the experimenter'f. methods. Thus by a refinement of the electrical stimulatior he secures a controlled amount' of .siimults. - Thus, again he has improved the methods of recording both the lent. movement and the 'electrical responses, so reaching a' more: exact timing-of the rates of conduction and reaction. . Science,- it has been' said; -begins with measurement ; the advanee-Sir Jagadis 'has made shows how it is continued by measurement.
Among the new facts brought forward in this continuatior • of his always suggestive and brilliant researches, three , may be noted : (1) The motor cushion at the base of. the leaf-stalk • has. four •different 'quadrants which bring -about different movements-:--up and down, left and right; • More than that, there is a definite nerve-connexion between each of the four leaflets and -a corresponding quadrant of tlic motor cushion. This is very striking, not to say strange.. (2)- The movement of the leaf of the Sensitive Plant . is brought about by changes in turgor (or intracellular pressure): in the Motor cushion or pulvinus. When a stimulus is applied.: directly to the'cushion; the leaf falls in about 0.1 of a second,, If the stimulus be indirect; i.e., at some distance from the pulvinus, there is a longer interval before collapse, and during this interval there is a weak upward movement, which , Jagadis. calls .erectile. Generalizing from this, too .quickiy we, think, he defines the effectof direct stimulation as a diminution, of turgor; consequent contraction,' diminished rate of growth; and the negative' responsethe -sinking down of the, leaf+, Distinguished from this is the effect of indirect stimulation of feeble intensity, for it implies increase of turgor, expansion, accelerated rate of growth, and the positive response---the, erection of.the leaf. Here, we confess, the alert experimenter. moves too 'quickly for us. '
(3) Very striking,,we wonder how it has been missed,is the observed fact that an excitation may pass up one side of the stem and down the other.
These are three of the many new facts expounded in this fascinating vohline, the main' result of which is the certainty that the plant is. a more " nervous " organism than has been supposed. But behind the facts is the interpretation.. The conduction of the excitation is probably by the elongated cells of thepi-L][5cm and of the bundle-sheath as it is called, but the analysis of the path does not prove that these elements are in the strict sense nervous.. We cannot 'go further than say that in cur judgment Bose works out a very suggestive parallel between the conducting system in plants and the nervous system in animals. But we do not think that he allows enough for the -alternative interpretation in terms of the' diffusion of fluids and of the hormones which they may contain. The Work 'of Ricca and of-Snow demands. fuller consideration. But even if • Sir Jagadis ;s interpretation fails to convince, his facts are illuminating. He corrects ,Linnaeus Vegetabilia saltiwnt.
J. ARTHUR T110IISON.. .
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