IRRITABILITY. 5 5 I 



This has been ascertained with regard to extremes of heat 

 and cold, darkness, and drought in the case of Mimosa pudica. 

 The position of the immotile leaf is peculiar and is worthy 

 of note. It somewhat resembles the diurnal position ; but 

 the main petiole is horizontal, instead of being directed 

 obliquely upwards, and the leaflets are incompletely expanded, 

 but the secondary petioles are divaricated. The first effect 

 upon the leaf is that it loses its irritability to mechanical 

 stimulation ; then its daily periodic movement ceases ; and 

 finally its spontaneous movement is arrested. The evidence 

 as to the effect of anaesthetics is somewhat conflicting. Bert 

 states that the spontaneous movements of Mimosa are not 

 arrested by chloroform or ether, but it appears that by 

 " spontaneous movements " he means the periodic movements 

 induced by the daily variations in the intensity of light. 

 Heckel found that the spontaneous movements of the stamens 

 of Ruta were not arrested by chloroform, but it is not clear 

 that the movement of these stamens belongs to the category 

 of movements of variation ; it is more probably a movement 

 of growth. On the other hand Kabsch states that he observed 

 an arrest of the movement of the lateral leaflets of Hedy- 

 sarum under the influence of chloroform. 



The foregoing are not, however, all the manifestations of 

 irritability exhibited by motile organs. Motile foliage-leaves 

 are sensitive also to those influences which determine the 

 position of growing leaves, namely, to the direction of the 

 incident rays of light and to gravity. With regard to the 

 former, it has long been known that the motile leaves of 

 many Leguminosse and Oxalidaceae when exposed to light 

 of moderate intensity place themselves so that their upper 

 surfaces are perpendicular to the direction of the incident 

 rays. We have here then additional instances of Diahelio- 

 tropism. When, however, these leaves a*re exposed to bright 

 sunlight they assume such a position that their surfaces are 

 parallel to the direction of the incident rays. This move- 

 ment has long been known as the "diurnal sleep" of leaves, and 

 Darwin terms it Paraheliotropism. This position, however, 

 differs widely from the nocturnal position. Thus, according 



