658 



PLANT RESPONSE 



vertical up and down movement, if there be any, is prevented 

 by a lateral support, which has at its end a smooth bent rod 

 of glass, in the concavity of which the free petiole rests. If 

 there be any responsive torsional movement, a spot of light 

 reflected from the mirror will now undergo a vertical deflec- 

 tion. This is, for convenience of the record, converted into 

 lateral by reflection from a second mirror suitably inclined. 

 The response record is then taken in the usual manner on 



Fig. 266. Torsion-recorder 



M, mirror slipped over petiole by light aluminium clip behind; G, bent 

 glass piece supporting petiole to prevent vertical movement. 



a revolving drum. The absolute value of the angular 

 movement shown in the record can be determined from a 

 knowledge of the distance from the mirror of the recording 

 drum. 



Torsional response under the lateral action of 

 light. — The pulvinus of the leaf, say of Mimosa, is now 

 stimulated by throwing upon it a horizontal beam of light, 

 which strikes it laterally. By the word lateral is here meant 

 either the right or left flank of the pulvinus or pulvinoid, 

 consisting of part of the upper and part of the lower aniso- 

 tropic halves. This causes a responsive torsion, by which the 

 petiole is rotated, the tendency being for the upper, or less 

 excitable, half to face the stimulating agent. The up curve, 

 then, in this and the following records will represent a torsional 

 movement by which the less excitable upper side is made to 



