IRRITABILITY. 515 



is not essential to twining. When a young internode twines 

 round a smooth support of appropriate thickness, it may 

 do so just in the same way as it performs its circumnutation, 

 that is, that any one side may always face the same point 

 of the compass (see Fig. 41, p. 364). Under these circum- 

 stances a line drawn longitudinally down any one side 

 remains all the time parallel to the axis of the stem. But, 

 sooner or later, the internode begins to twist on its own axis, 

 so that a longitudinal line on any one side no longer remains 

 parallel to the axis, but describes a spiral about it. There 

 are two sets of influences at work to produce torsion, the 

 internal and the external. The internal cause, which is by 

 no means peculiar to twining internodes, is that more pro- 

 longed growth of the peripheral as compared with the central 

 tissues to which we have alluded in a previous lecture (p. 353), 

 and it tends to cause a twisting of the internode round its 

 own axis in the same direction as that of twining, to pro- 

 duce, that is, homodromous torsion.- The external causes are 

 various ; the negative geotropism of the internode, the weight 

 of the terminal bud or of the leaves, the alteration in position 

 of the leaves, as Dutrochet and Wiesner point out, in the 

 taking up by them of the most favourable fixed light-position, 

 and lastly, the friction of the stem against the support. With 

 regard to the nature of the torsion produced by these 

 external causes, it depends upon circumstances, in most cases, 

 whether it is homodromous or antidromous, that is, with 

 or against the direction of twining. The effect of the last- 

 named cause, the friction against the support, is constant ; 

 in all cases it produces antidromous torsion. Von Mohl 

 first observed, namely, that when a stem twines round a 

 smooth cylindrical support the torsion is not well marked, 

 whereas when the support is rough it is considerable, an 

 observation which has been confirmed by all subsequent 

 researches. The torsion which a twining stem exhibits is 

 the algebraical sum of the action of these various causes ; it 

 will depend upon the conditions under which it has been 

 growing whether its torsion is homodromous or antidromous. 

 In some cases, as Leon has observed, successive internodes 



332 



