RIGOR IN IRRITABLE ORGANS. 593 



I have quoted these remarkable processes occurring in crystals in illustration 

 of the fact that the same chemical substance is able, according to circumstances, 

 to have its molecules arranged either in unstable or stable equilibrium, and that 

 by means of certain external impulses the unstable arrangement may pass over 

 into the stable one. The majority of the phenomena of irritability, however, give the 

 impression that in their case also conditions of unstable equilibrium are transformed 

 into conditions of stable equilibrium by means of small external influences. This 

 comes out with especial clearness in such cases as the irritable leaves of the Mimosae, 

 where a light touch or other mechanically insignificant influence suffices to transform 

 the state of unstable equilibrium of the organ suddenly into a new state, which 

 we might regard as the stable one ; only of course with the difference already men- 

 tioned, that in this case, when the stimulus has ceased to act, the new condition returns 

 again to the previous one — the stable condition returns to the unstable one. There 

 is, however, yet another series of facts which support this view. Just as the unstable 

 crystalline form of certain salts continues only within narrow limits of external 

 conditions, passing over into the stable form when they are transgressed, so also 

 the irritable condition of an organ exists only within certain Hmits of external 

 influences, during specially favourable circumstances ; if these are transgressed the 

 irritable — i.e. the unstable — condition ceases to be, and the organ loses its irritability, 

 i.e. it assumes a condition of stable equilibrium. It is of special interest for an insight 

 into the phenomena of irritability to take these facts somewhat closely into con- 

 sideration: this I did in the year 1863 in a treatise entitled ' Vorübergehende Slarre- 

 zustände periodisch beweglicher und reizbarer Pflanzenorgmie! 



From among the details in the work just mentioned I may take the following state- 

 ments as examples : — 



(i) Temporary cold-rigor occurs in the motile organs of Alimosa pudica, the 

 conditions being otherwise favourable, when the temperature of the surrounding air 

 remains for some hours below 15° C. The lower the temperature falls below i5°C. 

 the more rapidly the rigor sets in ; the first to disappear is the irritability for contact 

 and shaking, later that for the influence of light, and finally even the spontaneous 

 periodic movement. When the temperature of the air is below 2 2° C. the lateral 

 leaflets of Hcdysarum gyrans are rigid, according to Kabsch. 



(2) Temporary heat-rigor occurs in Mmosa, in moist air at 40° C. within 

 I hour; in air at 45° C. within 30 minutes; in air at 49-50° C. within a few minutes. 

 The irritability returns after a few hours in air at a favourable temperature. In 

 water the cold-rigor of Mimosa occurs even at a higher temperature (viz. within 

 15 minutes at 16-17° C.) and the heat-rigor even at a lower temperature (viz. at 

 36-40° in 15 minutes) than in the air. During the heat-rigor, in air as well as 

 in water, the leaflets are closed, as after stimulation ; but the stalks are directed 

 steeply upwards, whereas in the stimulated condition they point downwards. 



(3) Temporary dark-rigor. If plants with periodically motile leaves, which 

 are also irritable for light or shaking, such as Mimosa, Acacia, Trifolium, Phascolus, 

 Oxalis, are placed in darkness, the spontaneously periodic movements occur, apart 

 from the changes in position effected by the light stimulus, only so much the more 

 distinctly, and also the irritability for contact remains at first undisturbed. But this 

 motile condition disappears completely when the darkness continues for one or several 



[3] Qq 



