38 



THE VEGETATIVE ORGANS 



and that the plumule just as persistently grows upwards 

 into the air and sunlight. 



The stimulus of gravity. Geotropism. What force is at 

 work which determines these directions of growth ? If a 

 growing seedling is placed on its side and attached to a rod 

 which is caused to revolve horizontally by means of clock- 

 work (Fig. 18), the radicle and plumule will continue to grow 

 in that direction. An instrument constructed for the pur- 

 pose of rotating plants in various positions is called a 



3 



Fig. 17. Seeds of Broad Bean sown in Different Positions. 

 In each case the radicle grows downwards. 



klinostat. 1 Further, if seedlings of the Pea are pinned on 

 a vertical disk with their roots pointing towards the centre, 

 and the disk is then revolved rapidly for four or five hours, 

 the growing roots turn outwards away from the centre of 

 rotation, and the plumules turn inwards towards the centre. 

 In this experiment the seedlings have grown under the 

 influence of a force stronger than the attraction of gravity. 

 The influence of centrifugal force affects the radicle and 



1 A simple form of klinostat may be made from a small clock 

 by removing the hands and fixing a short tube to the axle of the 

 minute-hand. Fit up a light bottle as a moist chamber and bore 

 a hole in the cork, into which the tube may be firmly fitted. Seed- 

 lings pinned to the cork may be rotated as in the experiment 

 described. 



