MOBILE AND IMMOBILE CONDITION. 789 



requires consideration from a mechanical point of view, will yet in general need for 

 its answer a proof that chemical or molecular changes of the contents and walls of 

 the cells have produced abnormal conditions, by which they have become immobile, 

 or cease to be capable of the normal curving upwards and downwards. The 

 mechanism of a watch which is going correctly may be accurately known to the 

 watchmaker ; but if it begins to go wrong or stops altogether a special examination 

 is needed, not of the mechanism of the watch, but of the causes which have pre- 

 vented the motive forces from acting. These causes may be of a purely chemical 

 nature, as from injury to the spring by a drop of acid which has diminished its 

 elasticity ; or they may be of a purely mechanical nature, as when the watch has 

 been exposed to too high or too low a temperature, or has lain within reach of a 

 powerful magnet. 



The investigation of the transitory condition of immobility and of its external 

 causes does not therefore in the first place concern the mechanics of the arrangement 

 of the motile part necessary for every separate movement, but leads to questions 

 which concern the molecular structure and chemical nature of the tissue. The fact 

 that poisonous substances destroy the motility of the tissue, teaches us nothing as to 

 the mechanics of its movements in the normal condition. It may be assumed that 

 transitory conditions of immobility are caused by chemical and molecular changes 

 in the cells, which when they are strongly pronounced would kill them ; it is only 

 because the injurious influences are interrupted in time, and the internal changes 

 have not reached the point at which they cause death, that they can be neutralised, 

 and the normal internal condition of the tissue, and together with this its motility, be 

 restored under favourable external conditions. 



Transitory conditions of immobility ensue from a low temperature above the 

 freezing point, and a high one below 50° C. if not lasting too long ; in the case of 

 leaves also from darkness lasting for two or more days, a deep shade for a longer 

 time ; with irritable leaves from want of water but not sufficient to cause withering. 

 In the case apparently of all irritable organs a transitory condition of immobility is 

 caused by placing the plant for a time hi vacuo or in an atmosphere devoid of 

 oxygen or strongly impregnated with carbon dioxide or certain vapours as that of 

 chloroform. In all these cases death is the final result of a long continuance or 

 increase of the injurious influence. 



The following particulars are taken from the detailed illustrations in my work already 

 quoted. 



(i) Transitory rigidity from cold occurs in the leaves of Mimosa pudica when the in- 

 fluences are otherwise favourable if the temperature of the surrounding air remains 

 for some hours below 15° C; the lower the temperature falls below this point, the 

 more quickly does the rigidity set in ; the irritability to touch and concussion disap- 

 pears first, then that to the action of light, and finally also the spontaneous periodic 

 movement. The lateral leaflets of Desmodium gyrans, are, according to Kabsch, immotile 

 when the temperature of the air is below 22° C. 



(2) Transitory rigidity from cold occurs in Mimosa within an hour in damp air of 

 40" C, within half an hour in air of 45° C, in a few minutes in air of 49° to 50° C. ; the 

 sensitiveness returns after exposure for some hours to warmer air. In water the rigidity 

 from cold of Mimosa sets in at a higher temperature, viz. in a quarter of an hour be- 

 tween 16° and 17° C, and the rigidity from heat at a lower temperature than in air, viz. 



