actually attended by change of form, and is, therefore capable of direct observation. He also showed that the disturbance, instead of being single, is of two different kinds—viz., one of expansion (positive) and the other of contraction (negative)—and that, when the stimulus is feeble, the positive is transmitted, and, when the stimulus is stronger, both positive and negative are transmitted, but the negative, however, being more intense, masks the positive. He identified the wave of expansion travelling along the nerve with the tendency to pleasure, and the wave of contraction, with the tendency to pain. It thus appears that all pain contains an element of pleasure, and that pleasure, if carried too far becomes pain—that "the tone of our sensation is determined by the intensity of nervous excitation that reaches the central perceiving organ."

MEMORY IMAGE AND ITS REVIVAL

Dr. Bose next pointed out that there remains, for every response, a certain residual effect. A substance, which has responded to a given stimulus, retains, as an after-effect, a 'latent impression' of that stimulus and this 'latent impression' is capable of subsequent revival by bringing about the original condition of excitation. The impress made by the action of stimulus, though it remains latent and invisible, can be revived by the impact of a fresh excitatory impulse.

Experimenting with a metallic leaf, Dr. Bose demonstrated the revival of a latent impression under the action of diffused stimulus. The investigation by Dr. Bose on the after-effects of stimulus has thrown some light on the obscure phenomenon, of 'memory.' It appears that, when there is a mental revival of past experience, the diffuse impulse of the 'will' acts on the sensory surface, which contains the latent impression and re-awakens the image which appears to have faded out. Memory is concerned, thus, with the after-effect of an impression induced by a stimulus. It differs from ordinary sensation in the fact that the stimulus which evokes the response, instead of being external and objective, is merely psychic and subjective.

Dr. Bose has, by experimental devises, shown the possibility of tracing 'memory-impression' backwards even in inorganic matter, such latent impression being capable of subsequent revival. An investigation of the after-effects of stimulus, on living tissues would open out the great problem of the influence of past events on our present condition.

DEATH-STRUGGLE AND MEMORY REVIVAL

There is a wide-spread belief that, in the case of a sudden death-struggle, as for example, when drowning, the memory, of the past comes in a flash. "Assuming the correctness of this," says Sir Jagadis "certain experimental results which I have obtained may be pertinent to the subject. The experiment consisted in finding whether the plant, near the point of death, gave any signal of the approaching crisis. I found that at this critical moment a sudden electrical spasm sweeps through every part of the organism. Such a strong and diffused stimulation—now involuntary—may be expected in a human subject to crowd into one brief flash a panoramic succession, of all the memory images latent in the organism."²³

"COMPARATIVE ELECTRO-PHYSIOLOGY"

Dr. Bose published the results of these new researches, in 1907, in another remarkable volume, which was styled 'The Comparative Electro-Physiology.'

THIRD SCIENTIFIC DEPUTATION, 1907-08

After the publication of 'The Comparative Electro-Physiology,' the Government of India again sent Dr. Bose on a Scientific Deputation. He went over to England and America and placed the results of his researches before the learned Scientific Bodies. He read a paper 'On Mechanical Response of Plants' at the Liverpool meeting of British Association, in 1907. He then read a paper on 'The Oscillating Recorder for Automatic Tracing of Plant Movements' before the New York Academy of Sciences, and, in December 1908, he gave an address on 'Mechanical and Electrical Response in Plants,' at the Annual Meeting of the American Association for the Advancement of Science, held at Baltimore, and, in January 1909, he delivered a lecture on 'Growth Response of Plants' before the United States Department of Agriculture and, in February 1909, he read a paper on 'Death-spasm in Plants,' before the University of Illinois, and, in March 1909, a paper on 'Multiple and Autonomous Response in Plants' before the Madison University. He also lectured before the New York Botanical Society, the Medical Society of Boston, the Society of Western Electric Engineers at Chicago. He also delivered a series of post-graduate lectures on Electro-Physics and Plant Physiology at the Universities of Wisconsin, Chicago, Ann Arbor. He returned to India, in July 1909.

FURTHER EXPERIMENTAL EXPLORATION

By his new and newer methods of investigation, Dr. Bose got a deep and deeper perception of that underlying unity, for the demonstration of which he had been labouring since 1901. But the dream of his life was not yet realised. No direct method of obtaining response record was yet obtained. Hitherto the response recorder employed was a modification of the optical lever, automatic records being secured by the very inconvenient and tedious process of photography (which again introduced complications by subjecting a plant to darkness and thereby modifying its normal excitability); and the plant was not automatically excited by stimulus, besides the results obtained were liable to be influenced by personal factor. So Dr. Bose set about the invention of an apparatus, which should discard the use of photography and in which the plant (attached to the recording apparatus) should be automatically excited by stimulus absolutely constant, should make its own responsive record, going through its own period of recovery, and embarking on the same cycle over again without assistance at any point on the part of the observer. Great difficulties were encountered in realising these ideal requirements. They appeared, at first, to be insurmountable. But, with continuous toil and persistence, Dr. Bose succeeded in designing a long battery of supersensitive instruments and apparatus, which made the seeming impossible possible. His ingenious "Resonant and Oscillating Recorders" gave a simple and direct method of obtaining the record. The plant, being automatically excited by stimulus, made its own responsive record. The closed doors, at last, opened. The secret of plant life stood revealed by the autographs of the plant itself. The great sadhana of his life now received its fulfilment. "It has been beautifully said—and it is a law of the moral world as unchangeable as physical laws—'Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you; for every one that asketh receiveth; and he that seeketh findeth and to him that knocketh it shall be opened."²⁴

TRANSMISSION OF EXCITATION IN MIMOSA

Dr. Bose had shown that all plants are sensitive—that there is no difference between the so-called 'sensitive' and the supposed 'non-sensitive'—that they gave alike the true excitatory electric response as well