LECTURE XXXVIII. 



the same time that, in the explanation of twining, too much importance has been attri- 

 buted to these torsions, as follows from the fact that some twining plants (e.g. Boivica 



volnhilis and species of Ciiscuia) exhibit no such 

 phenomenon at all; moreover the torsions referred 

 to only make their appearance as a rule after the 

 shoot-axis has already become closely wound 

 round a support, though in other cases it is 

 true they extend close up to the bud, as in 

 the Hop and the Bindweed [Cotivolvulus). It 

 must be admitted that the study of the me- 

 chanics of the twining of twining plants is ren- 

 dered difficult in the highest degree by these 

 torsions ; but on the other hand it is established 

 that they are to be regarded as a bye-phenome- 

 non only, as an adaptation the better to fix the 

 stem to the support around which it has already 

 become twined, and I shall subsequently refer 

 to yet another useful eff'ect of the torsions in 

 shoots which have not yet twined round a sup- 

 port, as well as to the fact that another torsion, 

 which is usually scarcely noticed, is necessarily 

 and obviously combined with twining itself. It 

 will certainly facilitate the understanding of the 

 matter, however, if I take no further notice of 

 all these torsions for the time being, simply add- 

 ing that the direction of the revolving nutation 

 always coincides with that of the torsions and 

 with the spiral lines described round the support. 

 Let us now return to the revolving nutation 

 of the free pendent apex. It is obvious that this, 

 which is usually very long — not rarely 50-80 cm. 

 — must during its sweeping circular movement 

 occasionally come in contact with a support, a 

 thin stem or a stick, &c. ; Hke a tendril which is 

 still nutating in a similar manner, the free sweeping- 

 apex of the twining plant behaves much as a man 

 whose arm is extended horizontally and groping 

 towards all points of the compass in order to 

 fasten on a support. In fact, the twining plant 

 seeks in this manner to reach a support, and 

 when it meets with such with the anterior portion 

 of the sweeping apex, the part of the apex lying towards the bud curves 

 round it, and grows spirally up it. The uppermost coils of the spiral which 

 the apex of the shoot throws round the supports are usually nearly horizontal ; 

 but as the apex goes on creeping further up the stem in this form, the internodes 

 situated further behind undergo still further elongation. On the one hand the 



Fig. 379,— Shoot-apex ^rif of the Blue Bindweed 

 (Ipoinma purpurea) winding— i. e. twining round the 

 rod a a. Flowers, lateral buds and hairs omitted. 



