434 



TRANSFORMATION OF ENERGY 



at the boundary between the completely grown and still growing regions, 

 and that the entire apical region becomes perfectly perpendicular. 



In order that we may clearly appreciate the distribution of growth in 

 the shoot we will study more in detail SACHS'S numerical results from experi- 

 mental research on the stem of Cephalaria, as figured at Fig. 134. The letter 

 U indicates the increase in length on the under side, O that on the upper side, 

 both in mm. ; and it must be noted that the uppermost zone at the commence- 

 ment is only half as long as the others ; R represents the radius of the arc of 

 the curvature in cm. 



Since we have no data available as to growth in an uncurved control 



3. 



Fig. 134. Cephalaria procera. a, stem laid 

 horizontally at noon, showing divisions; zones 

 -5 -/i 3-, and 2 each 10 cm. long, zone i. 5 cm. long; 

 A, the same 34 hours later ; c> 2 i- hours later than b , 

 d, 1 6 hours later thane. After SACHS (1888). About 

 one-tenth nat. size. 



Fig. 135. Geotropic curvature in Allium atropurpureum 

 in various stages (/-/). After SACHS ^1882, p. 839). 



experiment it is impossible to say here, as in the case of the root, whether an 

 elongation takes place over all (measured on an axial line), and we can, therefore, 

 only affirm that, during curvature, growth on the concave side frequently 

 remains stationary, or that it shows a distinct retardation. We must not 

 attempt to generalize on these results, since growth takes place on the concave 

 side, not merely in the example we are considering, but in other organs also, 

 and the principal fact is that there is*a differential growth on both sides. As 

 a second illustration we may take the measurements made by NOLL (1888). 

 This author established the fact that in Hippuris, a geotropic curvature occurred 

 when the increase on the under side was (in twelve hours) 5 mm. and on the upper 

 side 0-25 mm. In the same time the axis increased about 2-6 mm. Comparing 

 these figures with growth in an erect shoot of Hippuris we find growth in the 

 latter to amount only to i-o mm. Here, therefore, we have to deal with an 

 acceleration of growth and not, as in the root, with a retardation. [LUXBURG 

 (1905) was unable to confirm this growth acceleration in Hippuris ; on the con- 

 trary, this author has shown that both here and in other plants during the 

 curving a retardation of growth occurs.] 



