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grows in the opposite direction, away from the centre of the 

 earth, is negatively geotropic. Those parts of plants which grow 

 horizontally, i.e. in a direction at right angles to that along 

 which gravity acts, are said to be dia-geotropic, such as are 

 often the primary branches of the stem and root. It is also 

 found that if the seedling, after being placed in a horizontal 

 position, is continuously and evenly rotated so that no side 

 of the root or stem is allowed to remain stationary, no geotropic 

 curvature takes place, inasmuch as no sooner does any part of 

 the root or stem receive a stimulus to grow faster than the side 

 opposite to it than it almost immediately receives a contrary 

 stimulus which neutralises it. In these movements, as in the 

 case of those of the leaves of Mimosa pudica, we see that the 

 response of the plant is out of proportion to the stimulus applied 

 and the result is quite different to that obtained when the 

 same stimulus is applied to lifeless matter. Thus in this case 

 the shoot moves in a direction exactly opposite to that followed 

 by a lifeless body acted on by gravity, and which falls down- 

 wards towards the centre of the earth merely by its own 

 weight. Moreover a primary root growing geotropically down- 

 wards is able to penetrate and force its way into mercury which 

 is specifically much heavier than itself, which would not be the 

 case if the root was composed of lifeless substance and was acted 

 on by gravity. The irritability of a plant organ may vary at 

 different periods of its existence, as is shown by the fact of a 

 dia-geotropic lateral root becoming positively geotropic and 

 taking the place of a tap-root which has been cut off, or injured, 

 and the same thing occurs when a lateral branch replaces a 

 damaged leading shoot. This question of the movements of 

 plant organs is complicated by the fact that in nature one 

 and the same organ is frequently exposed to the influence of 

 several factors, or stimuli, to each one of which individually it 

 may be able to respond in a particular way. Thus the move- 

 ments of such an organ would be the result of the combined 

 influence of all the stimuli affecting it at one and the same time. 

 Hydrotro- Thus roots are also found to be sensitive to the presence of water 

 and are able to regulate their growth with reference to the 

 source of moisture, i.e. they are hydrotropic. If a primary 

 root is growing in soil with plenty of moisture available on all 

 sides, it will proceed downwards, following the direction 

 suggested by gravity, but if it is situated in a very dry soil, in 

 which moist patches occasionally occur, the root will be found 

 to grow towards the moist areas, it thus being positively hy- 

 drotropic, irrespective of whether these areas lie directly below it, 



