49 The Physiology of Plants BOOK in 



special character, have been referred to already, but certain 

 questions still claim attention. 



The cause of curvature in growing organs was investi- 

 gated by many botanists, who studied it as it occurs in 

 response to various forms of stimulation. At the com- 

 mencement of the period under review the older views of 

 Knight were again advanced and extended by Hofmeister. 

 He explained the geotropic curvature of roots as a plastic 

 bending of the organ caused by its weight, and he ad- 

 vanced an ingenious theory to account for the upward 

 curving of a stem. He said that when an apogeotropic 

 organ is placed horizontally, the resistance of the tissue 

 of the lower side to curvature decreases because the cells 

 become water-logged as the fluid in the shoot settles to 

 that side, and their walls increase in extensibility ; in this 

 condition they yield more readily than the upper ones to 

 the longitudinal stretching by the turgid pith. He sug- 

 gested that in coenocytic structures the cause of curving 

 is, in like manner, a strain on the convex side, but in 

 these plants its seat is the common cell wall, whose cuticle 

 is passive and resistant to the more active growth of the 

 inner layers. 



This view was not overthrown till the appearance of 

 Frank's work ; it met with support, though with certain 

 modifications, from Sachs in 1865. Frank, however, 

 brought out the idea that these curvatures are vital 

 phenomena, associated with changes in the environ- 

 ment, and so diverted thought and research into a 

 different direction. The old mechanical views hence- 

 forward gave way to biological theories. Frank himself 

 attributed the curvatures to alteration of growth set up 

 by the organism ; he supposed the distribution of longi- 

 tudinal growth to be made unequal, and showed that such 

 a hypothesis can account for all geotropic curvatures, 

 whether positive, negative, or transverse, and that con- 



