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TEXTBOOK OF PLANT PHYSIOLOGY 



do not manifest any tendency to curve, remaining in the same posi- 



Fig. 131. — A klinostat of Pfeffer's construction {after Benccke-Josi). 



tion in which they had been placed (Fig. 132). This method is an 

 evident proof that geotropic curvatures are connected with growth, 



the fully developed parts of a 

 plant having lost the capacity of 

 forming curvatures. 



A seeming exception to this 

 general rule are the culms of 

 cereals, which are capable of rising 

 after "lodging," by forming a 

 bend just above the node (Fig. 

 133). This phenomenon, which 

 is of great importance in saving 

 lodged grain, is due to the fact 

 that the nodes of cereals preserve 

 the ability of renewing growth 

 for a considerable time. As soon 

 as the stem has been bent into 

 a horizontal position, the lower 

 side of the node starts growing, 

 thus forming a curvature and rais- 

 ing the part of the stem situated 

 above it. 

 If a seedling is placed in a horizontal position and is rotated 



Fig. 132. — Geotropic curvatures of 

 bean roots in humid air. A, the 

 original horizontal position; B, 7 hr. 

 later; C, 24 hr. later. The dark tri- 

 angle is a paper mark fixed on glass 

 (after Sachs). 



