Chap, xi Sensitivity and Stimulation 491 



sequently one explanation is sufficient for all three. In 

 1871 Sachs carried the idea a little further, and while he 

 showed that the convex side grows faster and the concave 

 one more slowly than the normal rate in an uncurved stem, 

 he advanced the view that this growth depends on variation 

 of the turgor of the cells, increased cell pressure being the 

 first result. Ciesielski, in the same year, observed that the 

 changes in the motor zones of curving roots consist chiefly 

 of a greatly exaggerated increase of size in all directions 

 of the cells of the cortex of the convex side, together with 

 a decreased growth combined with a compression of the 

 cells of the concave side. In the next year he observed 

 that these latter cells become more fully stored with proto- 

 plasm than the cells of the convex side. 



In 1879 De Vries pressed further Sachs' theory of varying 

 turgor in the several regions, and said that the growth- 

 curvatures of multicellular organs are due to increased cell 

 pressure on the convex side, this being set up in consequence 

 of the formation of a larger quantity of osmotic substances 

 in the cell sap of the tissues. Kraus show T ed, however, in 

 1882 that these substances are not present in excess in 

 such cells. 



In 1880 Darwin put forward the view that these move- 

 ments of curvature are all brought about through a modi- 

 fication of the universal natural rhythm of circumnutation, 

 in obedience to the stimuli received in each case. 



Wiesner, in his Heliotropische Erscheinungen (1880), said 

 that the increase of osmotic pressure in the convex side 

 is associated with an increased ductility of the membranes 

 of the same part, and suggested also that the concave tissues 

 are more perfectly elastic than the ductile cell walls of the 

 convex side. This view of a modified ductility of the cell 

 wall was supported by Strasburger in 1882. 



The influence of varying turgor on the change gradually 

 lost hold of current opinion, as the proofs of modi- 



