From the investigations that have been described in this and in the previous Papers an insight is obtained into the complexity of response arising from various factors. It has been shown that the sign of response is modified by the intensity of stimulus, by its point of application, and by the tonic condition of the responding tissue. The fundamental reactions have been found to be essentially the same in pulvinated, in growing and non-growing organs. The results described enable us to enunciate general Laws of Effects of Direct and Indirect stimulus on tissues in normal and in sub-tonic condition.
LAWS OF EFFECTS OF DIRECT AND INDIRECT STIMULUS.
1. THE EFFECT OF DIRECT STIMULUS IS NEGATIVE VARIATION OF TURGOR, NEGATIVE MECHANICAL AND ELECTRICAL RESPONSE, NEGATIVE VARIATION, OR RETARDATION OF RATE OF GROWTH.
_a_. SUB-MINIMAL STIMULUS GIVES POSITIVE RESPONSE.
_b_. POSITIVE RESPONSE IS ALSO GIVEN BY A TISSUE IN A SUB-TONIC CONDITION: CONTINUOUS STIMULATION CONVERTS THE ABNORMAL POSITIVE TO NORMAL NEGATIVE RESPONSE.
_c_. AUTONOMOUS ACTIVITY IN A STATE OF STANDSTILL, MAY BE REVIVED BY STIMULUS.
_d_. THE EFFECTS OF STIMULUS AND WARMTH ARE ANTAGONISTIC.
2. THE EFFECT OF INDIRECT STIMULUS IS POSITIVE VARIATION OF TURGOR, POSITIVE MECHANICAL AND ELECTRICAL RESPONSE AND POSITIVE VARIATION OR ACCELERATION OF RATE OF GROWTH.
I have referred to the fact previously demonstrated, that while Direct stimulus induces contraction and retardation of growth, moderate rise of temperature induces the opposite effect of expansion and acceleration of growth. Further demonstration of the antagonistic effects of stimulus and warmth will be given in the next Paper.
The autonomous activity of pulsating leaflet of _Desmodium gyrans_ comes to a stop under depletion of internal energy. A cut leaf isolated from the plant maintains the rhythmic activity of its leaflets for about 48 hours, after which there is an arrest of movement.
In this state of sub-tonicity the arrested autonomous activity is revived under the action of various stimuli. Thus the incidence of light on the pulvinule initiates pulsatory movements, which persists for a time even on the cessation of stimulus. This persistence of autonomous activity increases with the intensity and duration of stimulus to which the leaflet had been subjected.