IRRITABILITY. 513 



these circumstances there is a conflict between the geotropic 

 effect and the tendency to twine induced by contact with the 

 support. Of these the former is clearly the more powerful, so 

 much so that internodes which have already twined, but which 

 are still growing, unroll themselves and straighten themselves 

 out. The fact that stems will not twine round supports 

 which are inclined at a considerable angle to the vertical 

 is to be explained in the same way ; it is the result of the 

 preponderance of the geotropic effect over the effect of con- 

 tact. With tendrils this is not so ; their irritability to contact 

 is so great that they twine round supports in any position. 



The view that the twining of stems depends upon their 

 irritability receives indirect support from some observations 

 of Darwin's, which shew that the nature of the support is 

 of some importance in determining twining. He found, for 

 instance, that when a plant of Hibbertia dentata was sur- 

 rounded by branched twigs, its shoots did not twine around 

 them, but immediately did so when the plant was surrounded 

 with thin upright sticks. Again, he observed that the stem 

 of Solatium Dulcamara will only twine round thin and 

 flexible supports. It is clear that if the twining of stems 

 depended on purely mechanical conditions, the nature of the 

 support, provided it fulfil the necessary requirements as to 

 thickness, ought not to influence the twining. These obser- 

 vations of Darwin's are only intelligible on the assumption 

 that twining stems are sensitive. 



The conclusion to be drawn as to the process of twining 

 appears, then, to be this. By its circumnutation a twining 

 stem is brought into contact with a support ; the effect of 

 contact is to cause a concavity of the stem at the point of 

 contact ; this concavity causes fresh portions of the stem to 

 touch the support, and thus, supposing that its position and 

 its thickness are appropriate, the stem twines round it. It 

 is not improbable that the spirals formed by the free ends of 

 twining stems which have outgrown their supports is due to 

 some extent, as in the case of tendrils, to a conduction of 

 the stimulus from the parts below which are in contact with 

 the support. 



v- 33 



