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] [Q] BIEDERMANN--Electro-Physiology (English Translation), Vol 1, pp. 83, 84, 85; Macmillan & Co.