Under favourable conditions of the environment, the excitability of the organs is at its maximum. A given stimulus will bring about an intense excitation, and the ‘down’ D-change will therefore be very much greater than the A-change. Let us now consider the case at the opposite extreme where, owing to unfavourable condition, the excitability is at its lowest. Under stimulus the excitatory D-change will now be relatively feeble compared to the A-change, by which the potential energy of the system becomes increased. In such a case successive stimuli will increase the functional activity of the tissue, and bring about staircase response. Biedermann mentions the staircase response of _excised bloodless muscle_ as offering difficulty of explanation. It is obvious that the physiological condition of the excised muscle must have fallen below par. The staircase response in such a tissue is thus explained from considerations that have just been adduced. The results obtained with _Mimosa_ not only corroborate them, but add incontestable proof of the simultaneous existence of both A and D changes. The physiological condition of a plant, _Mimosa_ for example, is greatly modified by the favourable or unfavourable condition of the environment. In a hyper-tonic condition its excitability becomes very great; in this condition the plant responds to its maximum even under very feeble stimulus. Here the D-change is relatively great, and successive responses are apt to show sign of fatigue. [Illustration: FIG. 53.--Record showing the effect of stimulus modifying tonicity and producing staircase effect. (_Mimosa_)] But the plant in a sub-tonic condition will exhibit feeble or no excitation. The D-change will be absent while the A-change will take place under the action of stimulus. This, by increasing the potential energy, will enhance the functional activity of the tissue. _Staircase response in Mimosa: Experiment 48._--The theoretical considerations will be found experimentally verified in the record obtained with a specimen of _Mimosa_ in a sub-tonic condition (Fig. 53). Owing to the lack of favourable ‘tone’ the leaf was relaxing as seen in the first part of the curve. The stimulus of electric shock, applied at the thick dot in the curve slanting downwards, gave no response but raised the tone of the tissue by arresting the growing relaxation. Subsequent stimuli gave rise to staircase responses. Stimulus has, through the A-effect, raised the functional activity of the tissue to a maximum. ARTIFICIAL DEPRESSION OF TONIC CONDITION AND MODIFICATION OF RESPONSE.