106 GENERAL BOTANY 



a stick, we shall see that the lower coils slope steeply 

 upwards while the last few are flatter and the tip 

 itself stands almost, if not quite, horizontal. And 

 if we take a plant growing in a small pot and fix 

 it horizontally to a clock-work mechanism so that 

 it may be rotated at a uniform speed, the last few 

 coils will untwist and the growing end become free 

 and no longer twine. And, if a plant is put upside 

 down, the last few coils will again untwist and twine 

 upwards in the same direction, clockwise or anti- 

 clockwise as the case may be. 



3. Now all that the rotation on a horizontal axis 

 does is to neutralize the influence of gravity by con- 

 stantly changing the direction of its action on any part 

 of the plant. No other condition need be altered, for 

 it is the same if the light be from above or from 

 one side. We must conclude, therefore, that the twin- 

 ing is due neither to light, nor to any independent 

 tendency of the plant, nor to a sensitiveness to contact 

 as in the case of tendrils (see p. Ill), but to a pe- 

 culiar reaction to gravity whereby the growing end 

 is kept horizontal like a dia-geotropic organ, and also 

 made to bend sideways, unlike any other part. A 

 little behind the growing point the shoot is weakly 

 apo-geotropic, like an ordinary stem, and under the 

 stimulus of gravity bends upwards so that the coil- 

 ing is made steeper and at Jrtie same time, as may 

 easily be seen by coiling a narrow strip of paper 

 round a pencil and pulling it out, much tighter. When 

 a part of the stem has finished growing in length, it 

 becomes thicker and woody, so that the coils are not 

 easily unwound. 



