THEKMO-GEOTROPISM 515 



ment of closure. In these cases the movement is deter- 

 mined solely I)y the natural anisotropy of the organ, and 

 not by the paratonic action of a directive external force. 

 Thus the inner side of the perianth leaves undergoes an 

 expansion with rise of temperature attended by the open- 

 ing of the flower ; this movement of opening does not 

 undergo any change on holding the flower in an inverted 

 position. 



But the torsional movement of the leaf of Mimosri, and 

 the induced variation of torsion under change of tempera- 

 ture are not solely determined by the natural anistropy of 

 the organ ; it is, on the contrary, regulated by the direc- 

 tive action of the stimulus of gravity. The pulvinus in 

 normal position does not exhibit any geotropic torsion and 

 in the absence of an antecedent torsion change of tempera- 

 ture cannot induce any variation in it. It is only' after 

 the pulvinus had become torsioned under the lateral action 

 of geotropic stimulus that a responsive variation is induced 

 in it by the action of changing temperature. 



The change in torsion is, moreover, determined in 

 reference to the paratonic action of incident geotropic 

 stimulus. This will be clearly understood from the tabular 

 statement given below. 



TABLE XLVI. — SHUUIXG THK EPKECT UK RISE (»K TEMl'EBA ILRE UN 

 GEOTROPIC TORSION. 



r. •.• r , i /^ X • ,,. Effect of rise of 



Pusitiori ot t!'e organ. ; Geotropic effect. temperature. 



Right flank above : | Rij^ht-handed toision. Left-handed torsional 



(a) position. I movement (untwist). 



Left flank aliove : ! Left-handed torsion, j Right-handed torsional 



(6) position. movement (untvist). 



By right flank in the above table is meant the side of 

 the pulvinus to the right of the observer facing the leaf 

 of the plant held in the normal position. When the plant 



