49 6 The Physiology of Plants BOOK in 



did Rothert also. At the end of the century Nemec claimed 

 to have demonstrated the existence of special conducting 

 fibrils in the protoplasm of the cells of stimulated roots. 

 This still awaits confirmation. 



The rapidity of transmission was found to vary con- 

 siderably in different cases. Rothert showed, in 1896, 

 that helio tropic stimulation travels at the rate of 1-2 mm. 

 in five minutes ; Czapek, in 1898, that geotropic stimulation 

 progresses at about the same rate; while Fitting, at the 

 close of the century, found that the contact stimulation 

 of certain tendrils travels 18 mm. in the same time. 



The intensity of the stimulus necessary to cause a response 

 was first investigated by Charles Darwin, who showed that in 

 many cases the sensitiveness of vegetable protoplasm is as 

 acute as that of the sense organs of the animal body, and 

 that in some it even exceeds theirs. Darwin showed how 

 greatly it varies in different cases, and ascertained that in 

 most cases repeated excitation causes a summation of the 

 influences, or a cumulative effect. 



Besides Darwin, references may be made to Musset, who 

 showed, in 1890, that bright moonlight is capable of inducing 

 heliotropic curvature, and to Figdor, who proved in 1893 

 that the hypocotyl of cress, and that of Linaria biennis 

 respond to an intensity equal to -003 of a standard candle. 

 Wiesner showed, in the same year, that the most sensitive 

 plants he tested reacted to a light which did not affect 

 chloride of silver paper. Czapek determined, in 1895, that 

 sensitive radicles curve slightly in response to a centrifugal 

 force which is equal to -ooi g. 



The sensitiveness of plants to different forms of stimulation 

 was shown to vary considerably under changes of the environ- 

 ment. An old observation made by Dutrochet, that it is 

 not manifested in the absence of oxygen, was confirmed 

 by Kabsch in 1862, and by Darwin in 1875. Its depen- 

 dence on temperature was first shown by Sachs in 1863. 



