AND IRRITABILITY OF SENSITIVE PLANTS 197 



temperature, sufficient light, a certain amount of oxygen in the sur- 

 rounding atmosphere, and also the health and vigor of the plant used 

 in the experiment, all have a very important bearing on the response 

 to be obtained. (2) given that the environment of the plant is favorable, 

 if a specific stimulus, which acts as so much energ>-, is applied to irrito- 

 contractile parts of the plant, a response follows. (3) The response 

 of the irrito-contractile parts is not immediate, but a time interval, 

 the latent period, elapses before there is a change of position of any of 

 the organs of the plant. During this period, the stimulus is presented 

 to and perceived by the plant parts that are irritable. Owing to vary- 

 ing degrees of irritability in different organs of the same plant, or of 

 different plants, the duration of the latent period varies within quite 

 wide limits— from 1/4 second, or even less under optimum conditions 

 in Mimosa Spegazzini, to an hour or more in the feebly sensitive members 

 of the two families under discussion. (4) Following a certain time 

 interval after the stimulus has been applied response in the irrito-con- 

 tractile organs follows. This may be slow and steady or rapid and 

 extensive in the plant. There can only be response if the stimulus, 

 which has been applied to the plant, was of a certain definite intensity, 

 and if it acted for a sufficiently long period of time. A weak stimulus, 

 presented for a short time, may stimulate to a certain degree, but there 

 may be no response in the irrito-contractile organs. In such case there 

 has been an insufficient intensity of excitation to bring about response. 

 Response follows on applying the weak stimulus for a longer time, or 

 applying it a number of times to the irritable plant parts. 



(5) Hence here a summation of stimuli is necessary to produce a 

 response. In many species of Leguminosae and of Oxalidaceae that are 

 very slightly sensitive a considerable number of stimuli are required to 

 bring about closure of leaflets. (6) The stimulus is propagated from 

 the center of stimulation to other parts of the plant; the distance 

 through which it is carried being determined by the intensity of the 

 stimulus, the time during which it acts, and the relative irritability of 

 the plant used. (7) After contraction of the irrito-contractile parts has 

 ceased, there follows the neutral period, during which there is no evi- 

 dent movement of the parts of the plant involved in the stimulation act. 

 (8) After the neutral period the contracted parts re-expand and assume 

 the original position; this phase is called the re-expansion period. 



The eight phases just given comprise a complete stimulation act, 

 and are identical for the Leguminosae and the Oxalidaceae. 



