RIGOR DUE TO DARKNESS, DROUGHT, ETC. 595 



positions of the leaves in rigor due to darkness resemble the diurnal position much 

 more than the nocturnal position of those of phototonic plants. 



(4) Temporary drought-rigor. I have only observed this in plants of Mimosa 

 pudica. If the soil in the pots in which they are growing is left for some time 

 without watering, the irritability of the motile organs visibly diminishes as the 

 drought increases, and an almost complete rigor then sets in, the primary petioles 

 stand horizontally, and the leaflets are expanded. In this case the leaves which have 

 become non-irritable to stimuli are neither withered nor drooping ; but watering the 

 soil causes a return of irritability within two or three hours. 



(5) Temporary rigor due to chemical influences. In this category I include 

 particularly the condition termed by Dutrochet Asphyxia, which occurs in Mimosse 

 when they remain for a time in the vacuum of the air-pump. During the exhaustion 

 the leaves become folded together, probably in consequence of the shaking ; then the 

 leaflets expand, the petioles become erect, and while the leaves assume a position 

 similar to that in rigor due to darkness, they remain rigid, and are neither periodically 

 motile nor irritable to shaking. When brought into the air the plant again becomes 

 motile. It can scarcely be doubted that the vacuum causes the rigor essentially 

 by withdrawing the oxygen of the air and therefore suspending respiration. 



Kabsch confirmed these statements, and showed that the stamens of Berberis, 

 Mahonia, and Helianthe?}ium also lose their irritability in a vacuum, regaining it 

 in the air. 



The disappearance of irritability of the stamens mentioned, in nitrogen 

 and hydrogen, is according to Kabsch probably also to be referred to the 

 mere suspension of respiration: the irritability returns on the access of air. It 

 must, on the contrary, be, regarded as a positively injurious chemical eff'ect — 

 poisoning — when the irritability of the stamens of Berberis disappears in pure 

 carbon dioxide, or in air containing more than 40 °l^ of that gas, as the same 

 observer states to be the case. If they remain for three or four hours in carbon 

 dioxide the irritability returns only after several hours in the air. Carbonic oxide 

 gas mixed with air to the extent of 20-25 % 'annihilates' the irritabiHty; whereas 

 nitrous oxide gas behaves indifferently. The stamens placed in nitric oxide, 

 on the contrary, become bent after i| to 2 minutes towards the pistil, and lose 

 their irritability. Ammonia gas appears to produce a transitory condition of rigor 

 after a few minutes. 



Even in pure oxygen, according to Kabsch, a condition of rigor sets in 

 after ^-i hour, from which the stamens recover subsequently in the air. 



The vapours of chloroform and ether suspend the irritability of the motile 

 organs (for variations of light also?) without destroying the hfe, if the eff'ect 

 does not continue too long. If entire plants or separated branches of IMimosa 

 are placed in an atmosphere highly charged with these vapours, the irritability 

 may disappear even in a few minutes. If the organs have been previously 

 stimulated, they nevertheless now rise up (without being irritable), at the same 

 time becoming more rigid. The action of the vapours of chloroform or ether 

 is a purely local one, only the organs directly exposed to them losing their 

 irritability. 



(6) By means of frequent stimulations (vibrations) repeated at short intervals 



Q q 2 



