A young Mimosa standing straight in its pot, the stem held by a clamp without excessive pressure. Bose brings up a handle from which an adjustable pin point protrudes, standing out by only a fraction of a millimetre. He scratches the stem on the left, between the second and the fourth leaf. Both fall, the fourth after ten seconds for thirty-seven millimetres travelled upwards, the second after twenty seconds for twenty-eight millimetres downwards. And the leaves on the right do not move. He shifts the scratch to the opposite side, and it is the leaves on the right that droop, the left staying still.
That is the opening of the book, and it is also its whole programme. A scratch that does not cut into the wood, that draws no drop of sap, and that nonetheless sends something travelling along a channel, with a side and a laterality. Bose insists, in a note and in the text: « The scratches, it should be remembered, are purely superficial. » One experiment further on, the impulse reaches the highest leaf on the right, crosses the top of the plant and comes back down on the left, from top to bottom. Bose concludes, before he even picks up his microscope, that there are two opposite conducting cords which meet at the summit.
A borehole, not a continent
The Nervous Mechanism of Plants was published in London by Longmans, Green and Co. in 1926, seventeen chapters, two hundred and eighteen pages, eighty-two illustrations, ninety-nine numbered experiments. On the title page, Bose is no longer the Presidency College professor he had been twenty years earlier: he is Sir Jagadis Chunder Bose, director of the Bose Research Institute in Calcutta, and the preface is dated October 1925. At the head of the volume, a single dedication, without a word of comment: To my life-long friend Rabindra Nath Tagore.
It is a short book, and that is what makes it readable. Where Comparative Electro-Physiology swept through forty-eight subjects in seven hundred and thirty-four pages, this one pursues only one: to establish that the plant has a nervous system, and that this system is differentiated. The order of the chapters is an argument, not a catalogue. It is not hydro-mechanical, it is not the sap, therefore it is nervous; here is the anatomy of that nerve, here is how you block it, here is its speed, here is the reflex arc, here are the two distinct nerves, here is strychnine. Each chapter closes on a one-page summary, and the seventeenth takes up the other sixteen with cross-references. A book designed to be checked.
The demolition
Chapter II is the most polemical and probably the best. In it Bose takes on two rival theories. That of Pfeffer and Haberlandt, for whom the excitation is only a pressure wave, since it crosses a stretch of petiole that has been chloroformed or killed with boiling water. That of Ricca and Snow, more recent, for whom a wound in the wood releases a stimulating substance that the transpiration current then carries along.
His answer to Pfeffer fits into a comparison: « The task would almost be as difficult as narcotising a nerve-trunk lying between muscles, by the application of chloroform on the skin outside! » The chloroform reaches the external phloem, the xylem bars its way to the internal phloem, and that one goes on conducting. Against Snow, Bose brings out the figures. Fifty specimens of Mimosa pudica grown in the grounds of his institute, cut, both ends dipped into a tube of water and put back in contact: the stimulus, pushed to twenty seconds at most, never crosses the water. Twenty specimens for the internode extract applied to the cut end, without the slightest effect. And above all the speed. Snow gives three millimetres per second for the rise of sap. Bose measures the excitation at four hundred millimetres per second in the thinnest petioles, on an April day at thirty-three degrees in Calcutta.
The three dye experiments, for their part, have an elegance that commands admiration, because they compare the two speeds on the same specimen. On Biophytum sensitivum, methylene blue is both stimulant and tracer: the impulse travels sixty millimetres down in thirty seconds, against the direction of the sap, and closes the leaflets from the centre outwards. Longitudinal section under the microscope: the dye itself has migrated three tenths of a millimetre at most. Two hundred times slower. On Mimosa pudica, a drop of hydrochloric acid makes seven leaves fall in forty seconds over one hundred and twenty millimetres, and silver nitrate reveals that the acid has not moved from its point of application.
The instruments
Bose is first of all a builder of apparatus, and the book presents two that deserve to be known. The Resonant Recorder settles a friction problem: a stylus dragging on the plate falsifies short times, so Bose makes his vibrate by electromagnet, tuned to ten, a hundred or two hundred times per second, so that it touches the surface only through intermittent contacts. The trace becomes a series of dots, each interval worth five thousandths of a second, and Bose notes that it is not difficult to read a fifth of one. Hence his formula: « The plant-record or phytogram is also its own chronogram. » The curve times itself.
The other one is the electric probe, and it is perhaps the prettiest idea in the volume. A needle insulated except at the very tip, connected to a galvanometer, driven into the petiole in steps of five hundredths of a millimetre, while a sub-petiole is stimulated periodically. Bose spins out the analogy himself: the petiole is a cable, the conductor lies inside an insulating sheath, so you go looking for the messages by pushing the probe in. A point of protocol, the puncture itself excites, you have to wait a quarter of an hour for that to subside. And it is by accident that the anatomical discovery of the book arrives: the probe gives two maxima, one outside the xylem, one inside. Bose confirms it by staining, haematoxylin then safranin, the two phloems turning deep violet while the xylem turns red. The bundle is bicollateral, there is an internal phloem, and nobody expected it there.
The reflex arc
The result Bose presents as the reason for his book comes in chapter XV. The pulvinus, that motor swelling at the base of the leaf, does not have two functional halves as was believed, but four quadrants, hence four effectors, each nervously connected to a specific sub-petiole. Bose tetanises the middle of the first sub-petiole for one second, taking care to slip a cotton-covered rod five millimetres under the petiole to cushion the fall and avoid a parasitic excitation. The afferent impulse travels up, the leaf falls. Then, at the centre, it sets off again by another route towards the second sub-petiole, whose leaflets close in series.
The two figures are the key to everything. Afferent journey, eighteen seconds. Efferent journey, over the same distance, three seconds. Six times faster after reflection. Bose writes that everything happens as if a discharge of energy took place at the centre, the outgoing impulse becoming far more intense than the weak signal that provoked it. The next chapter brings direct proof that the paths are separate: on the same nerve, a superficial scratch reaching only the external phloem takes twenty-two seconds, a pin pushed as far as the internal phloem takes three point six, averages established on ten specimens. The external phloem is the sensory nerve, the internal one is the motor nerve.
Then comes strychnine, and Bose admits his own surprise: « The results of some of the successful experiments on the action of strychnine on plant-nerve were as astonishing as they were unexpected. » At a dilution of one in ten thousand, applied for twenty minutes, the block between nerve terminations is entirely lifted, and the three other sub-petioles respond simultaneously, with no lost time. Some twenty successful experiments, « too consistent to be ignored », and many failures which he acknowledges.
What the book does not do
No mysticism, no plant consciousness, no emotion. The vocabulary of sensitivity is strictly technical there, sensory designating an afferent impulse and nothing else. Only two departures, the dedication to Tagore, and one page where Bose wonders what the fall of the leaves is for. He rejects the received explanation, that of cattle being frightened off, on the grounds that « the cow is not sufficiently intelligent to take notice of the movement of the leaf ». He proposes something else: on a long creeping stem, the impulse runs through the whole plant, and a patch of bright green turns all at once into thin lines of a dull grey, invisible against the dark ground. A camouflage.
The final paragraph of the book has a caution that does him credit. Yes, vascular plants possess a well-defined nervous system, yes the excitation is conducted by the phloem and is modified by the same means as in the animal, yes one can follow the transformation of the sensory into the motor. But no structure comparable to the animal nerve ganglion has been discovered in the pulvinus, and Bose merely hopes that the physiological facts will one day receive their histological verification. He was not wrong to put it that way.
A word about the reception, because it teaches something. The main charge brought against Bose in his lifetime, by John Burdon-Sanderson, was terminological: he wrote response where he should have written reaction, which granted too much agency to the plant. Augustus Waller called for one of his manuscripts to be rejected, and the Royal Society refused to publish his work on the electric response of plants for more than a decade. The same institution elected him a Fellow in 1920, the first Indian admitted in the sciences. The man was honoured while his thesis was refused for a long time. The most interesting point is that the quarrel has not changed in nature: speaking of plant electrical signalling has raised no problem since a paper in Nature established it in 1992 on the wounded tomato, but speaking of a nervous system remains contested, exactly as in Burdon-Sanderson’s day.
The French edition, « Le Mécanisme nerveux des plantes », came out with Gauthier-Villars in 1931, five years after the original, in a translation by Édouard Monod-Herzen. A curiosity worth pointing out, the translator was not a scientist but a decorator, a specialist in metal engraving and librarian of the École des arts décoratifs. He carried three of the four French volumes of Bose, and his son joined him on the last one.
What I take from it for my own part fits into one sentence of Bose’s, the one he places in chapter XV and which overturns the whole usual hierarchy: the plant impulse being a thousand times slower than the animal one, you see things in it that the frog nerve moves too fast to show. Slowness as an instrument. That is perhaps the best thing the plant has to teach us.
Read in November 2016. Note written in 2026.
