492 



PHYSIOLOGY OF GROWTH. 



188. Free Winding. 



From plants of Ipomcea purpurea or Phaseolus growing in the 

 open, we cut very vigorous shoot ends about 20-30' cm. long, as 

 straight as possible, and not yet grasping any support. These are 

 now placed with their lower ends in a small glass vessel, filled 

 with water, and covered with a large bell-glass or put into a large 

 glass cylinder, whose mouth is covered with a sheet of cardboard. 

 In order to keep the air surrounding the objects very moist, we 

 wet the sides of the bell-glass or glass cylinder 

 with water. Under these conditions the shoots 

 continue their growth, and after two or three 

 days have formed a number of free turns. l 

 I have obtained this result in experiments 

 with shoot ends of Phaseolus and Ipomoea 

 purpurea. The latter plant serves particu- 

 larly well, and I saw free turns originate 

 whether the plants were left in the dark or 

 were exposed to diffuse daylight. The draw- 

 ing at the side (Fig. 164) represents a shoot 

 of Ipomoea which has formed free turns. It 

 is shown, and I observed this still more 

 clearly in other cases, that the lower and 

 therefore older parts of the coiled stem are 

 directed more steeply than the younger 

 parts, a fact to which we shall return later. 



1 See Sachs, Lectures on Plant PJiysiology. 



FIG. 164. Shoot o 

 Ipomoaa purpurea, with 

 free turns. 



189. The Mechanics of the Winding of 

 Twining Plants. 



If the end of the stem of a twining plant 

 is being carried round in space by rotating 

 nutation, it is obvious that it may easily encounter a suitable 

 support. 



To understand the phenomena which are to be observed in 

 twining, it is of the first importance to consider the rotating 

 nutation, in the causation of which, as we saw in 187, the per- 

 sistent negatively geotropic behaviour of the stem plays so im- 



