ANTONOMOUS TORSION 673 



minute. This Was reduced in the horizontal position to 

 minus fourteen divisions per minute, and in the vertically 

 downward position it was found to have undergone reversal 

 \.o plus nine divisions per minute. 



In the next series of experiments' I took records from 

 specimens held alternately up and down and up again. 

 This was done in order to eliminate the effect of any chance 

 variation. The specimen employed was Jpoi/uva. The tor- 

 sional response in the first up position was at the rate of 

 minus sixteen divisions per minute. When held in the in- 

 verted position, the rate was found reduced to minus ten 

 divisions per minute, and when once more placed in its 

 normal vertical position, the Ipomoea exhibited an increased 

 rate of movement — that is to say, the torsion now took place 

 at the rate of minus twenty divisions of the scale per minute. 



All these results tend to show that the action of stimulus 

 of gravity is to retard the autonomous torsion, this effect 

 being at a maximum when the specimen is in a position at 

 180° from the vertically upright. 



The twining of stems. — The twining movements of 

 certain stems are the result of various contributing factors, 

 the relative values of which may differ in different cases. 

 One such factor, suggested by Von Mohl and denied by 

 others, may lie in the irritability of the stem to the contact 

 of its support. Such response to unilateral pressure was found 

 to occur in various organs (p. 497), and probably plays an 

 important part, in some cases, in the phenomenon of twining. 



Some connection would also seem to exist, in many in- 

 stances, between autonomous torsion and twining. In the 

 first place, most of the twining plants also exhibit autonomous 

 torsion. Again, just as we have various types of torsion ing 

 organs — some characterised by positive, others negative, and 

 others again by positive alternating with negative, or negative 

 with positive, torsions — so in twining stems also, we see some 

 which exhibit positive-directioned twining, others negative, 

 and others again alternately one and the other. A plant, 

 moreover, which has twined, shows, if inverted, a tendency 



