500 The Physiology of Plants BOOK in 



side becoming concave which leads gradually to a difference 

 in the thickness of the cell walls. As the latter takes 

 time to develop, the process goes on after the removal oi 

 the stimulus, which in fact produces its primary effect in 

 the migration of the protoplasm. 



The vital theory of irritability received support from 

 the discovery that plants are capable of showing fatigue. 

 In 1861 Cohn found that the excitability of the stamens 

 of the Cynareae is temporarily inhibited by stimulation 

 but soon returns. This was also observed by Bert in 1870. 

 In 1873 Pfeffer showed that the continued application oi 

 mechanical shocks to Mimosa does not prevent its return 

 to its original position, but after such treatment it becomes 

 for a time incapable of responding to further mechanical 

 stimulation. It seems to be due to inhibition of percep- 

 tion, as other forms of stimulation provoke responsive 

 movements. The motor mechanism seems intact. Burdon- 

 Sanderson found a similar capability of inducing fatigue 

 in the sensitive hairs of the leaf of Dionaea in 1882. After 

 a single stimulation the sensitiveness of Mimosa pulvini is 

 inhibited for a short time during the return movement. 

 In 1885 Pfeffer found that he could set up fatigue in the 

 leaflets of Oxalis, but not so readily, for they continue 

 capable of receiving stimulation during the return move- 

 ment. 



The vital theory receives support further from observa- 

 tions which have been made upon the behaviour of plants 

 when exposed to anaesthetics. Bert showed in 1870 that 

 the motility of the leaves of Mimosa is inhibited by chloro- 

 form, as did Pfeffer in 1873. Haeckel, in 1873 and 1874, 

 showed the same thing in the case of irritable stamens and 

 stigmas. He found that i per cent, solution of morphia 

 inhibits the stamens of Berberis. Darwin found ether inhibit 

 the sensory hairs of Dionaea in 1875. Kabsch found 

 chloroform arrest the movements of the lateral leaflets of 

 Hedysamm. 



