BEHAVIOUR OF TWINING STEMS ON THE SUPPORT. 671 



uppermost coils of thfe shoot are passively driven up the support by these lower 

 internodes, since they only lie loosely on it; on the other hand the older coils thus 

 become steeper and more erect. Hence it is commonly found, especially with thin 

 smooth supports, that the lower already completely developed parts of the shoot-axis, 

 run round the support in steep long-drawn coils, whereas the uppermost coils lie 

 nearly horizontally, or obliquely with a slight gradient. Subsequently, as they become 

 older, they also ascend more steeply. This is particularly clear in the case of 

 thick shoot-axes, such as those of the Hop or the Blue Bindweed [Tpomma) when 

 twining round a cord or a wire : where , the support is thick, it is due to purely 

 mechanical causes that even the fully developed older turns are not very steep. 



The terminal bud of the twining shoot is often applied continuously close to 

 the support when the latter is sufficiently thick ; but where the support is thinner it 

 often happens that the uppermost coil of the stem is twined quite loosely round the 

 support, or presses on it only at one point. In other cases again the bud may be 

 curved sharply downwards close to the support, or even outwards and away from it, or 

 upwards, evidently in consequence of nutations and torsions occurring in the upper- 

 most parts of the twining stem. 



As regards the true cause of twining and its mechanism, no completely clear 

 insight into the matter has as yet been obtained. The twining of twining plants 

 is not so well understood as the coiling of tendrils, and the views on the 'matter do 

 not agree as to whether irritability is in twining plants likewise the important factor. 

 Where the circumstances are so involved it is perhaps better to put together the most 

 important results of observation, without adopting any theory. 



I lay especial stress in the first place on the fact that in the twining of twining 

 plants some form of geotropism — i. e. an influence due to gravitation — very evidently 

 plays a conspicuous part : this results from the following observations. On growing 

 a twining plant — e. g. a Kidney Bean, Bindweed, or Hop — in a pot until it has 

 climbed up a rod, and then invejting the whole plant together with its pot so that the 

 pot is uppermost and the twining apex of the plant lowermost, the youngest two or 

 three coils of the shoot loosen themselves from the rod, and the terminal bud, first 

 becoming free, turns sideways, and then erects itself and again grows upwards close 

 to the rod. Mere up-turning thus reverses the twining round the support which has 

 already been accomphshed so far as concerns parts of the shoot which are stUl grow- 

 ing, though the parts which are fully developed and wound round the support are 

 not further affected. 



If a twining plant grown in a pot and provided with a support, is laid horizontally, 

 the terminal bud in like manner loosens itself from the support and directs itself at 

 once vertically upwards, and if .the pot and support are rotated for several hours in a 

 direction opposite to that of the coils of the support, those parts of the shoot which 

 are still capable of growth gradually uncoil themselves. To remark it by the way, it 

 obviously follows from these observations that twining plants (apart from special 

 circumstances) are unable to twine round a support either in a horizontal or in aa 

 inverted position, and there is certainly no doubt that this behaviour depends 

 upon an influence of gravitation which cannot be immediately id^ntified with the 

 ordinary geotropism of orthotropic shoot-axes. 



There is a second series of facts of, if possible, still greater significance, 



