DIRECTION OF TWINING. NUTATIONS. 669 



twining plants twine to the left — i. e. from the left below to the right above when the 

 plant and its support are looked at from the exterior. The latter is the case, for 

 example, with the Bindweeds {Convolvulus mid Ipomcea), Arisfohchi'a, the Kidney 

 Bean {Phaseolus), and a few less known plants such as Thunbergia, Jasininiiim, 

 Asckpias carnosa, Menispermum canadense, &c. Nevertheless, not all species are con- 

 stant as to the direction in which they twine ; in the case of Blumenbachia lateritia, 

 one of the Loasacese, it is easy to observe that not only different shoots of the same 

 stock twine to the right or left, but it happens here, even commonly, that the same 

 shoot after having twined to the right for some time, grows straight upwards for a bit 

 and then twines to the left, and vice versa. According to Charles Darwin, something 

 of the same kind takes place in the case of Scyphantus chgaiis and Hihhcrtia dentata, 

 though these are rare exceptions. 



The first internodes of twining shoots, whether arising directly from the seed as 

 in the Kidney Bean, or as lateral shoots from root-stocks as in the Bindweed 

 {Convolvulus), or from sub-aerial perennial parts as in Aristolochia, are not as yet 

 capable of twining, but grow erect without supports. , It is not until the following 

 internodes of the same shoot are developed that it is able to twine round a support ; 

 these internodes first elongate considerably, the foliage-leaves meanwhile growing 

 out very slowly, and even 30-50 cm. from the tip the leaves, separated by long 

 internodes, are still in the bud-state. 



In consequence of its own weight the elongated apex of the shoot inclines 

 to one side, and in this position its revolving nutation begins — a rotatory move- 

 ment which is produced, without the co-operation of external stimuli, by growth 

 in length taking place progressively along various longitudinal lines on the surface 

 more rapidly than along corresponding lines on the sides opposite. By these means the 

 freely pendent apex describes a curve, commonly in the form of a drawn out S, but 

 which is properly a portion of a very elongated open spiral. This freely hanging 

 portion is in constant movement, whence the apex is carried round in a circle or 

 elhpse. If a plant which twines to the right, such as the Hop, is taken, and a 

 longitudinal black line painted on this ■ portion of the shoot-axis, so that it lies 

 on the convex side when the bud is pointing to the South, the painted mark is 

 found subsequently, when the bud inclines to the West, to lie laterally on the 

 north flank ; as the bud then proceeds round to the North, the mark comes to lie 

 on the concave side ; and, later still, when the bud points to the East, it again lies 

 laterally on the north flank of the shoot-axis. These revolving nutations are com- 

 pleted in quickly and vigorously growing plants in from i to 2 hours, or sometimes 

 even in half an hour, so that in long pendent shoot-apices, the circular movement on 

 a hot summer's day can be directly seen : in other cases, however, it requires many 

 hours to complete a revolution. As a rule two or three of the younger internodes 

 exhibit revolving nutation at the same time, and since these are all in different phases 

 of growth, the curvature of nutation of the older usually does not coincide with that 

 of the younger one. As new internodes develope from the bud they also begin 

 to nutate, the older ones ceasing the movement ; a new form of movement then 

 takes place in the latter, viz. Torsion — i.e. the angles of the older internodes 

 obtain a spiral twist round the true axis of growth, very much as the in- 

 dividual fibres of a cord are twisted round its long axis. I niav here mention at 



