go Chapters in Modern Botany CHAP. 



2. "In what way are curvatures which bring the plant 

 into the vertical line executed?" a question of the 

 mechanism of movement. 



The history of the answers to these questions may 

 conveniently begin with Hofmeister's researches (1859) 

 on the effects of bending or striking a turgescent shoot. 

 He showed that when a shoot is violently bent the 

 elasticity of the passive tissues (cortical and vascular con- 

 stituents) on the convex side is injured by over-stretching. 

 " The system must assume a new position of equilibrium ; 

 the turgescent pith (the active or erectile tissue) stretches 

 the cortex ; but as the passive tissues are now no longer 

 equally resisting on the two sides, the shoot must assume 

 a curvature towards that side on which the passive tissues 

 are most resisting." Applying the same conception to a 

 cell, Francis Darwin says : " As pith is to cortex, so is 

 cell-pressure to cell-membrane." 



When a shoot is laid horizontally there is, according to 

 Hofmeister, a tendency for the resisting passive tissue 

 along the lower side to become water-logged, and therefore 

 more extensible. Therefore the shoot bends upwards. 

 So Knight, in 1806, supposed that roots penetrated down- 

 wards, because of the sinking downwards of the juices. 

 But both these explanations are crude; they are too 

 mechanical. 



As far back as 1824, Dutrochet, who was, however, by 

 no means consistent, had recognised the fundamental 

 biological fact that growth -curvatures were provoked by 

 external influences acting as stimuli, but "the botanical 

 mind took more than fifty years to assimilate Dutrochet's 

 view." 



In 1868 Frank attacked the problem with true physio- 

 logical insight, showing that earth - seeking is an active 

 curvature, and that it depends, like other growth-curvatures, 



