CHAP, xi Sensitivity and Stimulation 465 



and channelled on their upper edges under the same con- 

 ditions. Vochting observed the same peculiarity of growth 

 under the two conditions by the peduncles of species of 

 Erodium and Taraxacum ; while Wiesner, in 1880, saw 

 the runners of Fragaria and Nepeta exhibit the same be- 

 haviour. Rauwenhoff, in 1878, showed that in many plants 

 the lateral branches which grow obliquely in the light 

 point vertically upwards in darkness. Frank's results were 

 confirmed and their accuracy established by the observa- 

 tions of F. Darwin in 1880, and by those of Pfeffer in 

 1881. F. Darwin studied the positions of leaves while 

 rotating on a klinostat and found they are diaheliotropic. 

 Vochting, in 1888, and Krabbe, in 1889, adduced further 

 evidence in support of Frank's views. 



The so-called mid-day sleep of plants, investigated in 

 1875 by Pfeffer, and studied by Wiesner almost at the 

 same time, was also examined by Darwin in 1880. He 

 gave it the name of paraheliotropism. 



The interpretation of the movements of heliotropic cur- 

 vature was essayed by the older observers, with little 

 appreciation of their real meaning in the life of the plant. 

 True, Dutrochet in 1824 held that the action of light is 

 not a directly mechanical one, but really an inducing 

 cause of the curvature, but his ideas were obscured by the 

 theories of others. The retarding influence of light on 

 growth, advanced as an explanation by de Candolle, in- 

 volving a partial etiolation of the shaded side, was accepted 

 for some time, despite its inadequacy to explain the curva- 

 ture of negative heliotropism. After Frank's researches 

 a better view replaced it, and investigations, carried out in 

 Sachs' laboratory after 1874, completely demonstrated the 

 insufficiency of the old ideas. The first experiments, 

 those of Miiller-Thurgau, showed that negatively helio- 

 tropic roots grow more rapidly in darkness than in light. 

 Even more conclusive researches on the subject were 



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