GEOTROPISM. II 



451 



a stronger centrifugal force (50 g.), by increasing the geotropic excitation, we 

 can induce the same reaction in the chief root in moist air, and re-estabhsh 

 the same orthotropic behaviour as we see in roots grown in the soil and subject 

 to gravity only. 



We have now to speak of a fourth external factor which may affect the 

 way in which the organ reacts, the amount of centrifugal force. We know 

 from Sachs's experi- 

 ments (1874) that the 

 angles formed by 

 secondary roots may be 

 altered by centrifugal 

 force and the roots made 

 to exhibit more nearly 

 orthotropic reactions, 

 and, according to 

 CzAPEK (1895), the same 

 is true of the rhizome. 

 The last-mentioned fac- 

 tor has certainlyno effect 

 in nature ; it is owing to 

 the combined influence of 

 light, temperature, and 

 the nature of the medium 

 that the rhizome main- 

 tains a certain constant 

 depth of position in the 

 soil; we may indeed 

 affirm that its existence 

 would not be possible 

 unless its geotropic re- 

 action could be altered 

 and its ' disposition ' be 

 changed. 



To speak of a 

 ' change of disposition ' 

 presupposes of course a 

 ' geotropic disposition ' 

 in the plant. We are 

 unable, however, to dis- 

 cover at what point in 

 the chain of processes 

 this makes its appear- 

 ance. Whether it makes 

 itself evident in con- 

 nexion with the actual 

 perception, in the struc- 

 ture of the perceiving 

 apparatus (Noll, 1896), 

 or later on, between the 

 act of perception and the 

 reaction (Pfeffer, 1893; 

 CzAPEK, 1898) is still a 



disputed point. Noll's view, viz. that the ' disposition ' is governed by the 

 structure of the perceiving apparatus has the great advantage of simplicity and 

 ease of conception, but the assumption of the close relations existing between 



G g 2 



Roots of tlie seedling of PAaseo/us i 

 diffuse light ; //, in darkness. 



tilliflorns, grown behind 



