49 6 The Physiology of Plants BOOK n 



did Rothert also. At the end of the century Nemec claime* 

 to have demonstrated the existence of special conductin, 

 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 heliotropic stimulation travels at the rate of 1-2 mm 

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

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

 close of the century, found that the contact stimulatioi 

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



The intensity of the stimulus necessary to cause a responsi 

 was first investigated by Charles Darwin, who showed thatii 

 many cases the sensitiveness of vegetable protoplasm is a 

 acute as that of the sense organs of the animal body, an< 

 that in some it even exceeds theirs. Darwin showed hov 

 greatly it varies in different cases, and ascertained that ii 

 most cases repeated excitation causes a summation of th< 

 influences, or a cumulative effect. 



Besides Darwin, references may be made to Musset, wh( 

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

 heliotropic curvature, and to Figdor, who proved in 189- 

 that the hypocotyl of cress, and that of Linaria bienni, 

 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 affed 

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

 sensitive radicles curve slightly in response to a centrifuga 

 force which is equal to -ooi g. 



The sensitiveness of plants to different forms of stimulatior 

 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 confirmee 

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

 dence on temperature was first shown by Sachs in 1863. 



