504 LIFE MOVEMENTS IN PLANTS 



Mimosa is taken as the typical dorsiventral organ. For 

 lateral stimulation, the plan! is placed on its side, 

 so that the vertical lines of gravity impinge on one 

 of the two flanks of the organ. In regard to this, I 

 shall distinguish two different positions, a and h. In the 

 a-position, the apex of the stem and the upper half of the 

 pulvinus are to the left of the observer, and in h-pnsi- 

 lion, the apex of the stem and the less excitable upper 

 lialf of the pulvinus are to the right. The arrangement 

 for obtaining record of the torsional response under 

 a-position is shown ia figure 179. 



Torsional response in a- and b-positions : Experiment 

 niS. — When the leaf is in a-position, the geotropic tor- 

 sion is found to be with the movement of the hands of 

 a clock. In the fj-posUion, on the other hand, the tor- 

 sion is against the hands of a clock. In both these cases 

 the geotropic torsion makes the less e.rcitahle nijper half of 

 the pulvinus face the vertical lines of gravity. The in- 

 cident stimulus is vertical, and it is the upper flank, con- 

 sisting of the upper and lower halves of the pulvinus (on 

 which the vertical lines of gravity impinge) that under- 

 goes effective stimulation. 



Algebraical summation of geotropic and phototropic effects : 

 Exjjeriment 194. — We are, however, able to adduce further 

 tests in confirmation of the above. If the rlirection of the 

 incident geotropic stimulus is vertical, and should it 

 act more efl:"ectively on the upper flank, it follows that 

 stimulus of light acting from above would enhance the 

 })revious torsional response due to geotropisra. In the above 

 case, the lines of gravity and the rays of light coincide. The 

 effect of rays of light acting from below should, on the 

 other hand, oppose the geotropic torsion. The additive 

 effect of stimulus of light and gravity is seen illustrated 

 in figure 180. The first part of the curve is the record of 

 pure geotropic torsional movement. Light from above is 



