9© 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 w-ater-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 w^ere 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, 



