Page:Life Movements in Plants.djvu/172

142 convenience, distinguish the different tonic conditions of the plant as normal, hi/per-tonic and suh-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 suh-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.

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 Dis- similation must be conceived as two closely interwoven pro- cesses, which constitute the metabolism (unknown to us in its intrinsic nature) of the living substance. Excitabi- lity diminishes in proportion with the duration of D-stimu- lus, 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 ditRcult to elucidate is the initial staircase increment of the twitches, more especially in exci ed, bloodless muscle, which seems in direct contradic- tion with the previous theory."

With reference to Herring's theory given above, Bayliss in his " Principles of General Physiology" (1915), page 377