The Dia-Heliotropic Attitude of Leaves. 



159 



employed another device which is even more perfect. Instead of the 

 tubular support, the petiole is slightly stretched in a horizontal direction 

 by an attached thin elastic string of indiarubber tied to a rod. The up 

 or down movement is thus prevented, whereas the string offers but feeble 

 resistance to torsion. The torsion is magnified by an L-shaped piece of 

 aluminium wire appropriately tied to the petiole, so that the long arm is 

 at right angles to the petiole. The end of the arm is attached by a silk 

 thread to the short arm of a recording lever, E ; there is thus a compound 



Fig. 2. — Diagrammatic representation for obtaining record of torsional response : 

 H, hooked glass rod to secure pure torsional effect ; L, bent piece of metal for 

 magnification of torsional movement ; R, recording lever ; G, oscillating smoked 

 glass plate. Direct stimulation of the right flank, r, or indirect stimulation of the 

 right sub-petiole, /, induces right-handed torsion. 



magnification of the torsional movement, a right-handed torsion producing an 

 up-curve, and a left-handed torsion a down-curve. The oscillating recorder 

 gives successive dots at definite intervals of time, which may be varied, 

 according to requirements, from twenty to sixty seconds. Time relations 

 of response may thus be obtained from the dotted record. 



Fig. 2 gives a representation of the experimental method for studying the 

 torsional response of various " sensitive " and ordinary leaves. Diverse stimuli 

 are applied at one flank of the organ, which is the junction of the unequally 

 excitable upper and lower halves of the pulvinus or petiole. We shall 



