MOTILE AND IRRITABLE PARTS OF PLANTS. 885 



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

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

 are placed in the dark, the spontaneous periodic movements take place without the 

 changes in position caused by the action of light, and therefore 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 ren- 

 dered rigid by exposure to darkness is again placed in the light, the motile condition is 

 not restored for some hours or even for some days. 



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 darkness, I have applied the term Phototoniis to the 

 normal motile condition resulting from the alternation of day and night. 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 during the night. In the same 

 manner a plant which has become rigid in continued darkness 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 darkness, the irritability of Mimosa to con- 

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

 also different from that produced by rigidity due to 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 effect as irritants, but only on 

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

 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 window, where within two 

 hours it directed its leaflets strongly downwards, the sky being cloudy, and 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 

 expanded, 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 

 or nine leaves closed ; the next morning all the leaves again expanded into their normal 

 diurnal position \ 



Irifoliiim incarnatum exhibited similar phenomena, with only immaterial differences. 



* [Bert (Bull, de la Soc. hot. de France, vol. XVII, 1871, p. 107) found that the irritability of 

 the leaves of Mimosa was destroyed by placing them under bell-glasses of green glass almost to the 

 same extent as if placed in the dark ; the plants being entirely killed in twelve days under blackened, 

 in sixteen days under green glass ; plants placed in the same manner beneath white, red, yellow, 

 violet, and blue glasses being still perfectly healthy and sensitive, though varying in the rapidity 

 of their growth.] 



