Page:Life Movements in Plants.djvu/174

144 consider the question of unmasking this positive element in the resultant negative response.

Under favourable conditions of the environment, the ex- citability 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 consiJer the case at the opposite extreme where, owing to unfavourable condition, the excita- bility 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 nof only corroborate them, but add incontestable proof of the simultaneous exis- tence 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.

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.