672 PLANT RESPONSE 



by moderate vigour and uniformity of natural torsion during 

 a considerable length of time. 



In studying the effect of geotropic stimulus on radial 

 organs, we have seen that its effective intensity was greater 

 when the specimen was inclined at an angle of 135 to the 

 vertical than when at 90 (p. 501). It occurred to me, 

 then, that the effect of geotropic stimulus might possibly be 

 detected, by means of variations induced in natural torsional 

 response at different angles of inclination. Thus, we might 

 first take a record of the normal rate of torsion, occurring 

 when the tip of the organ was in its natural position upwards. 

 We might next take a record of torsion, the stem being held 

 horizontal ; and we might finally take a record with the tip 

 held vertically downwards. 1 The effect of geotropic stimulus 

 might then be seen in the retardation induced in the normal 

 torsion. I give here a summary of results obtained from two 

 different specimens of P or ana paniculata, and from a specimen 

 of Ipomoea. 



In the first case, the record taken in the normal upright 

 position gave a rate of movement of negative torsion through 

 minus twenty-three divisions of the scale per minute. The 

 record of torsion in the horizontal position was found to give 

 a very much diminished rate, being now only minus seven divi- 

 sions per minute ; and finally, with the tip held downwards, 

 the torsional movement of the specimen was found to have 

 undergone an absolute reversal to positive — that is to say, the 

 tip now moved with the hands of a watch at a rate of plus 

 five divisions of the scale per minute. 



In the second case, the normal negative torsion in the 

 erect position was at the rate of minus twenty divisions per 



1 Here we must bear ill mind, with regard to the effective angle of inclination, 

 a certain difference as between radial and torsioning organs. In the former, the 

 line of growth may be taken as vertical, and coincident with the axis, whereas in 

 the latter it is spiral, or inclined at some undetermined angle to the axis of the 

 organ. Hence any inclination of the growth-line of a radial organ is measured 

 by the angle between the organ and the vertical. In the case of the torsioning 

 organ, however, when it is held vertically downwards, its growth-line makes an 

 angle— not of l8o° to the vertical, but — of l8o° less by the degree of its inclination 

 to the axis. 



