By

Sir J. C. Bose

Assisted by

Guruprasanna Das.

In experiments with different pulvinated organs, great difference is noticed as regards their ex­cit­abil­ity. If electric shock of increasing intensity from a secondary coil be passed through the pulvini of Mimosa, Neptunia, and Erythrina arranged in series, it would be found that Mimosa would be the first to respond; a nearer approach of the secondary coil to the primary would be necessary for Neptunia to show sign of excitation. Erythrina would require a far greater intensity of electric shock to induce excitatory movement. Organs of different plants may thus be arranged, according to their ex­cit­abil­ity, in a vertical series, the one at the top being the most excitable. The specific ex­cit­abil­ity of a given organ is different in different species.

In addition to this char­ac­ter­is­tic difference, an identical organ may, on account of favourable or unfavourable conditions, exhibit wide variation in ex­cit­abil­ity. Thus under favourable conditions of light, warmth and other factors, the ex­cit­abil­ity of an organ is greatly enhanced. In the absence of these favourable tonic conditions the ex­cit­abil­ity is depressed or even abolished. I shall, for convenience, distinguish the different tonic conditions of the plant as normal, hyper-tonic and sub-tonic. In the first case, stimulus of moderate intensity will induce excitation; in the second, the ex­cit­abil­ity being exceptionally high, very feeble stimulus will be found to precipitate excitatory reaction. But a tissue in a sub-tonic condition will require a very strong stimulus to bring about excitation. The ex­cit­abil­ity of an organ is thus determined by two factors: the specific ex­cit­abil­ity, and the tonic condition of the tissue.

THEORY OF ASSIMILATION AND DISSIMILATION.

A muscle contracts under stimulus; this is assumed to be due to some explosive chemical change which leaves the tissue in a condition less capable of functioning, or in a condition below par. Herring designates this as a process of dissimilation. The ex­cit­abil­ity of the muscle is restored after suitable periods of rest, by the opposite metabolic change of assimilation. “Assimilation and Dissimilation must be conceived as two closely interwoven processes, which constitute the metabolism (unknown to us in its intrinsic nature) of the living substance. Excitability diminishes in proportion with the duration of D-stimulus, or, as it is usually expressed, the substance fatigues itself. It is perfectly intelligible that a progressive fatigue and decrement of the magnitude of contraction must ensue. The only point that is difficult to elucidate is the initial staircase increment of the twitches, more especially in excised, bloodless muscle, which seems in direct contradiction with the previous theory.”[Q]

With reference to Herring’s theory given above, Bayliss in his “Principles of General Physiology” (1915), page 377 says, “In the phenomenon of metabolism, two processes must be distinguished, the building up of a complex system or substance of high potential energy, ‘anabolism,’ and the breaking down of such a system, ‘catabolism,’ giving off energy in other forms. The tendency of much recent work, however, is to throw doubt on the universality of this opposition of anabolism and catabolism as explanatory of physio­logic­al activity in general.”

The results obtained with the response of plants to stimulus may perhaps throw some light on the obscurities that surround the subject. They show that the two processes may be present simultaneously, and that the ‘down’ change induced by stimulus may, in certain instances, be more than compensated by the ‘up’ change.[R] I shall, for convenience, designate the physico-chemical modi­fi­ca­tion, associated with the excitatory negative mechanical and electrical response of plants, as the “D” change; this is attended by run down of energy. The positive mechanical and electrical response must therefore connote opposite physico-chemical change, with increase of potential energy. This I shall designate as the “A” change, which by increasing the latent energy, enhances the functional activity of the tissue. That stimulus may give rise simultaneously to both A, and D, effects, finds strong support in the dual reactions exhibited in plant-response. Under indirect stimulus, the two responses are seen separately, the more intense negative following the feeble positive. When by the reduction of the intervening distance, stimulus is made direct, the resultant response, as previously stated, is negative; and this is due not to the total absence of the positive but to its being masked by the predominant negative. Let us next consider the question of unmasking this positive element in the resultant negative response.