512 LECTURE XIX. 



(p. 491), is on all hands admitted to be irritable, it is not 

 surprising that irritability should be found to exist in others 

 also. 



But it may be objected that, as has been frequently ob- 

 served, the stems of twining plants form spirals when they are 

 not in contact with a support. If, however, we clearly dis- 

 tinguish between spiral growth and twining, this objection 

 will be at once perceived not to be a real one. It is not at 

 all surprising that a flexible shoot should, in consequence of 

 its negative geotropism, of its own weight, of its spontaneous 

 torsion, and of its circumnutation, assume a spiral growth 

 in its endeavours to grow upwards. Schwendener has rightly 

 pointed out that spiral growth under these conditions has 

 nothing whatever to do with twining. He mentions other 

 cases of other organs which exhibit spiral growth, such as 

 tendrils which have not grasped a support, and the stalks 

 of the female flowers of Vallisneria. Sachs also mentions 

 cases of this kind, such as primary roots of Vicia Faba and 

 shoots of Enteromorpha mesenterica, which he had found to 

 have grown spirally. Moreover, as was pointed out above, 

 all organs which exhibit torsion must have grown spirally, 

 though in this case the axis of the spiral is the longitudinal 

 axis of the organ. The fact that twining stems may form 

 spirals independently of a support, does not affect the view 

 that the twining round a support is due to irritability to 

 contact. 



Another objection to the assumption of irritability to 

 contact in climbing stems might be based upon Schwendener's 

 and Baranetski's observations (see p. 510), that a stem will 

 not twine when it is rotated horizontally, together with its 

 support, on a clinostat, and that, under these circumstances, 

 one or more of the last-formed turns round the support will 

 become unwound. But this objection is by no means fatal, 

 and the fact is capable of a simple explanation. When the 

 stem is rotating on the clinostat, each side of it in turn 

 tends, in virtue of its negative geotropism, to grow so as 

 to produce an upward curvature, the resultant effect being 

 to cause the stem to grow straight horizontally. Under 



