92 Chapters in Modern Botany CHAP. 



vex side grows faster, while the concave side grows slower, 

 than if the organ had remained vertical and uncurved. 



" Then it began to be established, through Sachs's work 

 (1871), that turgescence is a necessary condition of growth." 

 "De Vries (1879) maintained that growth-curvatures in 

 multicellular organs are due to increased cell-pressure on 

 the convex side : the rise in hydrostatic pressure being 

 put down to increase of osmotic substances in the cell-sap 

 of the tissues in question." But, as Francis Darwin points 

 out, there are many serious objections to this explanation. 



Many suggestions followed. Sachs directed attention 

 to the changes in the extensibility of cell-walls. " Wiesner 

 held that the curvature of multicellular organs is due both 

 to an increase of osmotic force on the convex side, and to 

 increased ductility of the membranes of the same part." 

 " Strasburger suggested that growth-curvatures are due to 

 increased ductility of the convex membranes." 



More detailed explanations in similar lines are those of 

 Noll and Wortmann, which differ in this : " The former 

 lays the greater stress on the increased extensibility of the 

 convex side, the latter on the diminution of that of the 

 concave side. Again, Wortmann explains the difference 

 in extensibility as due to differences in thickness of the 

 cell-walls. Noll gives no mechanical explanation, but 

 assumes that the outer layer of protoplasm has the power 

 of producing changes in the quality of the cell-wall in some 

 unknown way." 



Francis Darwin concludes "there is a focussing of 

 speculation from many sides in favour of ' active ' surface- 

 growth, or, what is perhaps a better way of putting it. in 

 favour of the belief that the extension of cell-membranes 

 depends on physiological rather than physical properties, 

 that it is in some way under the immediate control of the 

 protoplasm." Beyond that we may choose between rival 



