IRRITABILITY. 509 



number of cycles of circumnutation performed in a given 

 time is not the same, as in the case of the rope, as the number 

 of turns round the support, but the former is much larger 

 than the latter. Darwin himself noticed this last difficulty, 

 and he accounts for it by assuming that it is due to the 

 retarding effect of contact with the support on the circum- 

 nutation of the part of the shoot touching it. 



According to Dutrochet, the twining of climbing stems is 

 due to an " internal and vital force the action of which is 

 revolving round the central axis of the stem." This finds its 

 expression in that that portion of the stem which forms the 

 outer side of the spiral described grows more actively than 

 that which forms the inner side, the difference of growth 

 never being compensated ; the result is necessarily the forma- 

 tion of a spiral. He points out that this difference is not due 

 to the contact of the inner side of the spiral with the support, 

 for he had observed that spirals were in many cases formed 

 by twining stems which were not in contact with a support. 

 There is, in fact, in these stems, a natural tendency to grow 

 spirally. Nevertheless, he admits that the support has some 

 influence in inducing twining, for in most cases no spiral is 

 formed unless the stem is in contact with a support. But 

 even admitting the universality of Dutrochet's assumed 

 natural tendency, his view is no explanation but a mere 

 statement of fact. 



De Vries regards the prevention of circumnutation as the 

 cause of the spiral growth of twining stems. Thus, for 

 instance, when any point of the overhanging free end of a 

 stem is held fast, let us say the highest point of the curvature, 

 the normal circumnutation is arrested. The free end now 

 endeavours to nutate round an axis which is a straight line 

 produced through the axis of the stem at the fixed point. 

 It raises itself until its plane of curvature is above the hori- 

 zontal. The described curve may, at this moment, be 

 regarded as a portion of a spiral, the axis of which is vertical, 

 which has the same direction as the circumnutation, and the 

 apex continues to travel upward in this spiral. When, as is 

 usually the case in nature, the apex itself is the fixed point, 



