CHAP, xi Sensitivity and Stimulation 471 



the distribution of materials of differing specific gravity in 

 the plant ; that the downward growth of roots is purely 

 plastic, while the upward curving of a stem placed hori- 

 zontally is due to the nutrient sap sinking to the under side 

 and so causing more rapid growth of that region. Hof- 

 meister, who wrote in 1859 an< ^ I ^3> restated the old 

 hypothesis of Knight, with little modification, though he 

 advanced no actual evidence. He explained the greater 

 growth of the upper side of a horizontally placed curving 

 root by an accumulation there of the less dense nutritive 

 material which favoured the growth of that side, while he 

 attributed the curving of a stem similarly placed, to the 

 pull of the stretched turgid pith on the tissues of the lower 

 side made yielding ^by excess of water settled there. In 

 1872 Ciesielski, and in 1875 Traube, suggested that the 

 explanation must be looked for in the tensions due to mass 

 attraction and in the thickening of the walls due to more 

 favourable nutrition. 



These mechanical conceptions failed to give any even 

 plausible explanation of the curvatures. Frank's remark- 

 able memoirs formed a starting-point for the views now held, 

 and directed research into a new direction. In 1873 some 

 experiments of Sachs afforded confirmation of Frank's sug- 

 gestions. He showed that if stems are laid horizontally 

 on soil till curvature has begun usually from thirty minutes 

 to two hours and then are placed vertically, they continue 

 to curve in the original direction for two or three hours. 

 By repeated experiments on these lines he showed that the 

 problem is one of response to a stimulus administered to 

 the plant, and that removal of that stimulus does not at 

 once cause the action of the plant to cease. 



Sachs, in 1879, helped very materially the practical study 

 of the attraction of gravitation as the stimulus by the 

 introduction of the Klinostat, an instrument which has 

 been of such great value in the investigation of so many 



