THE GROWTH OF THE PLANT 



97 



earth's centre, to which everything is attracted, is 

 termed the Earth's Centre of Gravity. 



Experiments must now be carried out to determine 

 whether this force of gravity is responsible also for the 

 downward growth of roots. To this end the effect 

 produced on the direction of growth of the root, when 

 the force of gravity is rendered inoperative, must be 

 studied. 



A careful examination of Fig. 48 will show clearly 

 how the force of gravity may be rendered of no effect. 

 The same bean-seedling is shown in eight positions 

 which are arranged symmetrically round a central 

 point. The positions 

 form a series of pairs 

 in which the action of 

 gravity on the root-tip 

 is equal and opposite for 

 the two members of any 

 pair; for instance, the 

 effect of the action of 

 gravity on the seedling 

 when in position (b) 

 would be neutralised by 

 the effect produced when 

 in position (/), and simi- 

 larly for the other pairs. 

 Generally, the root in 

 all the positions shown 

 on the right-hand side of the figure, when acted on by 

 gravity, must be pulled in such a way that the outer 

 side of the root-tip (that drawn with the thickened hue) 

 is in every case pulled down. On the other hand, the 

 root, when on the left side, is affected in such a way 

 that the inner side of the tip (that drawn with a thin 

 line) is pulled down. If then the seedling can be 

 arranged so that it takes up each position for the sapae 

 length of time, the net result, due to any attraction 

 that gravity may have for it, will be nil. 



The simplest way to achieve this is to make the seed- 

 ling revolve slowly and evenly by means of some clock- 



G 



