Page:Life Movements in Plants.djvu/255

Rh obtained under feeble electric stimulation. The response is reversed to normal negative by increasing the intensity or duration of stimulus. Very feeble stimulus thus in- duces an acceleration and strong stimulus a retardation of growth. Ihave frequently obtained p )sitive mechanical and electrical responses under sub-minimal stimulation. As chemical substances often act as stimulating agents, the opposite effects of the same drug in small and large doses may perhaps prove to be a parallel phenomenon.

It. has been shown that stimulus induces simultane- ously both A- and D-etl'ect‘s, with the attendant positive and negative responsive reactions, alike in pulvinated and in growing organs. A tissue, in an Optimum condition, exhi- bits only the resultant negative response; the comparatively feeble positive is imperceptible, being masked by the pre- dominant negative; but with the decline of its tone exci- tability diminishes, with it the ll-ell'eet, and we get, the A-eti'eCL unmasked, resulting response then becomes diphasie.’ In extreme sub-tonic condition, it exhibits only the positive. The sequenee is reversed when we begin with a tissue in a state of extreme sub—tonicity, which lirst exhibits only the positive. Successive stimulations eontinually exalt the tonic condition, the subsequent responses becoming, diphasie, and, with the attainment. of optimum tone, a resultant. negative response, As a further verification of the simultaneous existence of both A-and D-eil'eets, it has been shown that in ordinary tonic eomlitiou a sub—mininml stimulus gives rise only to positive response; this becomes converted into normal negative under mo lerate stinmlation.

I have described the action of stimulus on tissues in which, on account. of sub-tonicity, growth has become en- feebled. I shall next take up the question of cll‘cct of