886 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 
It is worth tioting that in the plants observed by me the positions of the leaves in- 
duced by the rigidity caused by darkness resemble the diurnal more than the nocturnal 
position of phototonic plants. Rigidity produced by darkness is apparently only ex- 
hibited by organs containing chlorophyll, for, according to a communication from 
Pfeffer, the stamens of Cynaraceae, though developed in the dark, are irritable. This, 
as well as the fact that individual organs of a plant may be made rigid by darkness^, 
shows that this condition is not due to an excessive accumulation of carbonic acid 
in the tissues. 
(4) Transitory rigidity from drought I have observed only in Mimosa pudica. If the 
earth in the pot in which a plant is growing is left unwatered for a considerable time, 
the irritability of leaves perceptibly diminishes with the increasing dryness, and an almost 
complete rigidity ensues, causing the common petiole to assume a horizontal position, 
and the leaflets to expand. Leaves which have lost their irritability are not withered 
nor flaccid ; but the watering of the soil causes a return of the irritability within two or 
three hours. 
(5) Transitory rigidity resulting from chemical influences. In this category I include 
especially the condition termed by Dutrochet^ Asphyxia, which occurs in Mimosa when 
placed in the receiver of an air-pump. While the air is being pumped out, the leaves 
fold up, no doubt in consequence of the concussion ; but the leaflets then expand, the 
petiole becomes erect, and while the leaves assume the same position as after pro- 
longed withdrawal of light, they become rigid and possess neither periodic motility 
nor irritability to concussion. When brought into the air the plant again becomes 
motile. It can scarcely be doubted that the effect of the vacuum is essentially a result 
of the removal of the atmospheric oxygen, and therefore causes rigidity by suspending 
the respiration. 
Kabsch^ confirmed these statements, and showed that the stamens of Berberts, Ma- 
hoiiia, and Helianthemum also lose their irritability in- 'vacuo, regaining it in the air. 
The cessation of the irritability of the stamens of these plants which Kabsch states to 
take place when they are placed in nitrogen or hydrogen gas may also be ascribed to a 
simple suspension of respiration, the irritability returning on access of air. The destruc- 
tion of the irritability which takes place, on the authority of the same observer, in the 
stamens of Berberis in pure carbon dioxide or in air containing more than 40 p. c. of this 
gas must, on the contrary, be considered a positively injurious chemical action of the 
nature of poisoning. If they remain from three to four hours in carbon dioxide, the 
irritability returns only after some hours on replacing them in air. Carbonic oxide 
mixed with air in the proportion of from 20 to 25 p. c. destroys irritability, while nitrous 
oxide produces no effect. The stamens, on the other hand, bend towards the pistil in 
nitric oxide, and lose their irritability in or 2 minutes. Ammoniacal gas appears 
to cause transitory rigidity after a few minutes*. 
Kabsch states that rigidity ensues after from i§ to 2 hours in pure oxygen, the 
stamens again recovering in the air. 
The vapours of chloroform or of ether destroy the irritability (also for variations 
of light ?) of motile organs, without however causing death unless the action be too 
prolonged. If entire plants of Mimoseae or branches which have been cut off be 
^ Pfeffer, Physiol. Unters., Leipzig 1873, p. 66. 
^ Dutrochet, Mem. pour servir, vol. 1. p. 562. 
2 Kabsch, Bot. Zeit. 1862, p. 342. 
* [J. B. Schnetzler (Bull, de la Societe vaudoise des Sciences naturelles, 1869) points out that 
the substances which destroy the contractility of animal ' sarcode ' also destroy the irritability of the 
stamens of Berberts and the leaves of Mifnosa. Curare has no prejudicial effect in either case ; 
while nicotine, alcohol, and mineral acids destroy both. In the Comptes rendus for April 23rd, 
1870, is a record of a series of experiments on the effect of chloroform on the irritability of the 
stamens of Maho/da.'] 
