164 REGION OF CURVATURE. [CH. VII 



position of the greatest curvature. It is however better 

 to let the root grow in damp sawdust behind glass. 

 The glass wall is not vertical but slopes at an angle of 

 about 80 ; the advantage of the sloping wall is that the 

 root, in its attempt to grow vertically down, is closely 

 pressed against the glass, and is visible from the outside. 

 The marks must be made on the side which comes against 

 the glass. The figure p. 387 of Sachs' paper (Arbeiten, I.) 

 should be consulted. 



(192) Region of curvature. 



In summer any rapidly growing vertical shoots will 

 serve. Cabbage-shoots answer well, also the stems of 

 valerian 1 . They should be marked at intervals of 10 

 mm. and may be either fixed by means of bored and split 

 corks in bottles of water, or into an embankment of wet 

 sand in the angle of a tin box, the air being kept 

 thoroughly damp by wet sand sprinkled over the whole of 

 the bottom. At a temperature of about 20 C. curvature 

 will be well marked in 3 hours, when the form of curve 

 should be noted and the distance between the marks 

 measured. A strip of thin sheet-lead makes a useful 

 scale by which to measure the distance between the 

 marks, since it can be easily applied to the curved surface, 

 and retains its curvature. Rothert 2 recommends strips 

 cut from the paper used for Richard's Thermograph. The 

 lines are said to be fine and at accurately equal distances. 



1 In winter young sunflower seedlings may be used. 



2 Ueber Heliotropismus. Breslau, 1894, p. 29. 



