MOVEMENTS <>F IRRITATION. 493 



portant a part, and also the resistance of the supports. These 

 two factors cause winding stems to run round the supports in 

 a spiral line, and they also afford us an explanation of the fact 

 which we observed, that the uppermost turns made round a 

 support are flat and relatively wide, while the lower ones are 

 steeper. We must bear in mind, viz., that the stems of twin- 

 ing plants continue to grow for some time after coiling, and 

 consequently they become elevated under the influence of 

 gravity. If a support is present, the older parts of the stem 

 cannot completely straighten themselves, because of course the 

 support always stands in the way. The winding stems now 

 closely apply themselves to the supports in a spiral line, and 

 the ultimate angle of inclination of the shoot axis will be the 

 less, the thinner the support. With thick supports, the appli- 

 cation of the older parts of the stem to the supports takes 

 place early ; the erection of the internodes is soon arrested, and 

 the completed spirals therefore appear comparatively flat. 



It is perhaps at first sight surprising that free turns only 

 seldom appear in a typical form, in the overhanging shoot 

 ends of twining plants growing under perfectly normal conditions 

 in the open, whereas they readily develop, as we have seen, in 

 pieces of stern cut off. The case soon becomes clear, however, on 

 careful consideration. The vigour of growth in cut shoots is at 

 any rate considerably reduced. Free turns can, it is true, be pro- 

 duced, owing to the rotating nutation associated with the growth, 

 but the geotropic elevation of the internodes is only imperfectly 

 exhibited. The shoot ends of twining plants vigorously growing 

 in the open and projecting beyond the supports, react for the 

 most part so well to the action of gravity, that usually their inter- 

 nodes straighten out almost completely, and consequently no 

 permanent free turns can be developed. 



We now proceed to various experiments, the results of which 

 are calculated to afford us a deeper insight into the mechanics of 

 the winding of twining plants. 



Beside a young, very vigorous plant of Phaseolus multiflorus, 

 grown either in a flower-pot or in the ground, is placed a support 

 30 mm. in diameter. After the stem has wound round it a few 

 times, the support is rapidly removed, and replaced by a thin 

 one, only a few millimetres in diameter. The turns of the stem, 

 naturally, are not at first applied to this thin support. We now 

 observe that the upper advancing end of the stem forms new 



