458 TRANSFORMATION OF ENERGY 



that the apical region does not always lie horizontally. We have yet to explain 

 the significance of this rotatory movement in the twining. 



Twining commences if we place a more or less upright support beside the 

 greatly overhanging apex of a revolving shoot. The support is enclosed at 

 first by slack, very fiat spirals, which gradually become steeper. This elevation 

 of the apex takes place in obedience to negative geotropism, and if the support 

 be subsequently removed — other conditions being suitable — the spirals are 

 straightened out and the shoot appears twisted ; if the support be not with- 

 drawn the successive spirals become tighter, compressing the support. The char- 

 acteristic of the climbing plant lies in the gradual transition from the diageotropic 

 horizontally placed apex to the negatively geotropic base through the laterally 

 geotropic overhanging region. The twining arises from a combination of the re- 

 volving movement and negative geotropism, the support rendering the otherwise 

 inevitable straightening of the shoot impossible. The twining movements take 

 place, however, without any support, just as they do when it is present, in proof 

 of which we may cite the free twinings which many shoots exhibit when they 

 have grown beyond their supports or when isolated and placed in water (Sachs, 

 1882) ; far too little is known, however, about the origin of such movements 

 to justify us in drawing any conclusions from them. If they also arise as a result 

 of revolution and negative geotropism combined, the capacity for growth must 

 disappear very much earlier in them than in normal shoots. Ordinarily 

 speaking no such free twining is observable at the free ends of the shoots of 

 climbers ; on the contrary, after one complete revolution the apex remains 

 essentially unchanged, exhibiting in specially simple cases a curving almost in 

 one plane. Wortmann (1886) and, earlier, De Vries (1873) tied a fine silk 

 thread round the apex of the shoot, placed it over a pulley, and supported the 

 weight of the overhanging shoot by a small compensating weight. Under such 

 conditions according to Wortmann, the simple revolving movement, such as one 

 sees in the free apex, no longer occurs, but the shoot begins to exhibit twining, 

 the spirals being at first flat but afterwards becoming gradually steeper. The 

 support acts often like the weight in this case, i. e. it prevents the shoot from sink- 

 ing down and permits negative geotropism to have its legitimate effect. Whether 

 the silk thread merely supports the weight of the apex, as Wortmann affirms, 

 without altering the movements in any way cannot be decided ; it would appear 

 to us at least as if the thread must hinder the torsion of the apex on 

 its axis. 



A detailed analysis of the twining phenomena is still unfortunately unavail- 

 able, and by no means all investigators hold the view that twining may be 

 accounted for by revolving movement and negative geotropism alone. Schwen- 

 DENER (1881), for example, postulates as essential a so-called ' grasping move- 

 ment ' in addition to these factors. The apex of the twining shoot must come 

 in contact with the support from time to time, and in consequence of the tensions 

 set up thereby the incurvings of the shoot becomes transformed into permanent 

 twinings. It is impossible to deal with this question here, for details of which 

 reference must be made to the literature. (In addition to the authors already 

 cited see also Ambronn, Noll, and Kolkwitz.) Similarly, we must omit any 

 discussion of the torsions which are of such frequent occurrence in twining 

 stems and which have been so variously interpreted. 



Instead of discussing difficult problems like these we prefer in conclusion to 

 direct attention to certain important phenomena which strongly support our 

 conception of the co-operation of geotropism in twining movements. If we turn 

 a twining shoot of Calystegia upside down some of the youngest spirals first of all 

 begin to open out, then to bend backwards, and proceed to curve upwards in the 

 original direction, viz. to the left. The undoing of the spirals already formed 

 plainly shows that the lateral geotropism is not yet obliterated in them, but when 



