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 Oxalis^ 
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 Phototonus 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 stifl 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 ^ 
Trifolium incarnatum exhibited similar phenomena, with only immaterial differences. 
^ [Bert (Bull, de la Soc. bot. de France, vol. XVII, 1871, p. 107) found that the irritability of 
the leaves of Mimo&a 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,] 
