TWINING OF CLIMBING PLANTS. 86^ 



The direction of revolving nutation and of torsion is, in all climbing plants, the 



ime as that in which they twine round their support^ If a point in the terminal 

 region exhibiting nutation is prevented from moving by some external cause, as by 

 )eing fixed, the revolving movement of the free part will continue for some time, 



)ut the free part will then grow in a spiral ascending in the direction of nutation. 



'he revolving movement of nutation thus combines with the induced torsion of the 

 lower parts which are already coiled spirally ; but this torsion is opposed in its direc- 

 tion to the revolving nutation, and therefore also to the torsion previously mentioned 



rhich exists in the lower portion of the free part. This latter torsion is probably 



:casioned by the weight of the free overhanging apex of the shoot; at all events 

 causes the concave side of the part in a state of revolving nutation to face from 



lat time the axis of the spiral which has been formed. 



The" most common case in which revolving nutation is hindered in this way is 

 when the apex of a shoot comes, in consequence of this motion, into contact with 

 m erect support. If the support is not too thick, it forms the axis of the spiral 

 curvatures which the climbing stem makes round it ; when the support is very 

 slender, the stem winds in such large coils that they do not touch the support at all, 

 )r only accidentally at a few places. 



But revolving nutation can also be artificially interfered with in various other 



mys ; as, for example, by placing a support on the posterior side of the shoot as 

 respects its revolution, and fastening it by means of gum to the apex of the shoot, 



;hich would otherwise become detached from it. The first spiral coil is in this case 

 formed in precisely the same manner as if the support were in its normal position, 

 )ut the support stands outside the coil which does not therefore embrace it. Spiral 



)ils of this kind, not embracing any support, are frequently produced when the 

 stem rises above its support. 



The youngest coils of a twining stem are not usually in contact with its support ; 

 they are wide and flat ; while the older coils are in close contact with it, and are 

 narrower and more oblique. This shows that the close clinging of climbing stems 

 to their support is a subsequent result, the coils being at first looser and wider, and 

 becoming afterwards narrower and more oblique. This fact, which is of great im- 

 portance in the interpretation of the phenomena of climbing plants, was placed 

 beyond doubt by de Vries, who caused the summits of climbing plants to coil in 

 this manner without having any support in the middle. In this case also the coils 

 were at first wider and flatter, and became narrower and more oblique with increasing 

 age, until at length the piece became quite erect, a revolution of torsion being all 

 that represented each spiral revolution. It is not improbable that geotropism is the 

 cause of the coils — at first flat and sometimes almost horizontal — becoming after- 

 wards more oblique. It is clear that the stronger the force with which the coils 

 become narrower and more oblique, the more closely must they cling to their 

 support. If there is a support in the axis of the coils, the younger parts of the 

 summit will be constantly prevented by it from performing their normal revolution 

 of nutation, and the apex will therefore continue to grow in a spiral, and will climb 

 continually further up the support, the older coils always becoming more oblique 



What follows is from de Vries. 



