790 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



in a quarter of an hour between 36^ and 40"^ C} During the rigidity from heat, whether 

 in air or water the leaflets are closed, as after irritation, but the petiole is erect, while 

 ^vhen irritated it turns downwards. 



(3) 'Transitory rigidity from darkness. If plants whose leaves are periodically motile 

 and irritable to light and concussion, as Mimosa, Acacia, Trifolium, Phaseolus, and 

 Oxalis, are placed in the dark, the spontaneous periodic movements tiike place without 

 the changes in position caused by irritability to light, but all the more clearly, and the 

 irritability to touch is also not at first injured. But this motile condition disappears 

 completely when the darkness lasts for one day or more. If a plant rendered rigid by 

 exposure to dark is again placed in the light, the motile condition is restored after some 

 hours or even a day. 



Perfect darkness is however by no means necessary in order to produce rigidity. It 

 may be brought about by placing a plant that is very dependent on light, like Mimosa, 

 for some days in a deficient light, as in an ordinary dwelling room, at some distance from 

 the window. 



In contrast to the rigidity caused by dark, I have applied the term Pbototonus to the 

 motile condition resulting from permanent exposure to light. A plant in this condition, 

 if placed in the dark, will, as we have seen, remain for some time (hours or even days) in 

 a state of phototonus, which then disappears gradually ; the plant is therefore, under 

 normal conditions, in a state of phototonus even in the dark. In the same manner a 

 plant which has become rigid from the dark retains its rigidity for some time (hours or 

 even days) after being exposed to light. The two conditions therefore pass over into 

 one another only slowly. 



In the case also of rigidity caused by dark, the irritability of Mimosa to concus- 

 sion disappears first, and then the spontaneous periodic motion. In the same manner a 

 plant which has thus become rigid reassumes first of all its periodic movement, then its 

 irritability. 



The position of the various parts of the leaves of Mimosa when in a state of rigidity 

 caused by dark is different from that caused by darkness in phototonic plants, and 

 also different from that under rigidity caused by heat. In the first case the leaves 

 remain quite expanded, the petiolules directed downwards, the common petiole almost 

 horizontal. 



Changes in the intensity of the light produce the same eftect as irritants, but only on 

 healthy phototonic plants ; leaves which have become rigid from exposure to the dark 

 show no irritability to variations in its intensity until they have again become photo- 

 tonic from long-continued exposure to light. A plant of Acacia lophantha, left for five 

 days in the dark, was found to have lost during the last forty-eight hours every trace 

 of its spontaneous movements. It was then placed in a \vindow, where within two 

 hours it directed its leaflets strongly downwards, the sky being cloudy, ar.d other small 

 changes of position took place in the petiolules. In this condition the plant was still 

 rigid ; when it was then placed about noon in the dark with another phototonic plant 

 of the same species, the position of its leaves did not change, the leaflets remained ex- 

 panded, while the other plant within an hour closed its leaflets and assumed the most 

 complete nocturnal position. Both plants were then once more placed in the window, 

 when the first again retained the position of its leaves unchanged, while the normal 

 phototonic plant expanded its closed leaflets in an hour, the sky being still cloudy. By 

 the evening the lowest six leaves still remained rigid and expanded, but the upper eight 



^ A plant of Mimosa immersed in water of from 19 to 2i-5°C. remains sensitive to impact 

 and light for eighteen hours or more. Bert's statement (Recherches sur le mouvement de la 

 sensitive; Paris, 1867, p. 20) that Mimosa remains irritable up to 56 or even 6o^C. is not suffici- 

 ently confirmed, and is opposed to all that we knuv/ about the su|^erior limits of temperature for 

 vegetation. 



