CLIMBING PLANTS. 683 



internoiles have been formed, the nximljer varying according to the species, those 

 upjjerniost bend over laterally, and the whole shoot now consists of a lower erect 

 portion fixed in the soil, and an upper overhanging portion which ends freely. 

 The lower part forms a firm and reliable support, the upper bent portion, waving 

 in the air, undergoes movements the aim of which is to revolve the free end 

 round in a circle or an ellipse. This movement of the nutating portion of the 

 shoot has been compared to that of the hand of a clock; still better, it may be 

 likened to the movement of a pliant switch or whip which is held in the hand 

 above the head and its end swung round in a circle. The nutation of the 

 climber is not so quick as that of the revolving part of the switch, but is accom- 

 plished with a rapidity which astonishes the observer. In warm weather the 

 waving, revolving end of the Hop {Hamulus Lupulus) makes a complete revolu- 

 tion on an average in 2 hours and 8 minutes; the French Bean (Phaseolus vul- 

 garis) in 1 hour and 57 minutes; the Bindweed (Convolvulus) in 1 hour and 

 42 minutes; the Japanese Akebia quinata in 1 hour 38 minutes; and the Chilian 

 climber, Grammcdocarpiis volubilis, in 1 hour and 17 minutes. Since these 

 revolutions are performed by fairly long portions of the shoot, they may, like 

 those of the clock-hand, be seen with the naked eye, especially when a collar 

 of white paper is placed on the shoot in the sunlight below the overarched 

 portion. The shadow of the moving part, like the hand on the dial-plate, is 

 then seen slowly but plainly advancing over the surface of the paper. In other 

 twining plants the motion is of course much slower, and many of them occupy 

 24, or even 48 hours in a revolution. 



Since in most twining stems a twisting of the extended fibrous bundles on 

 the periphery of the stem occurs simultaneously with the circling of the free 

 end, it was formerly supposed that this revolving movement was actually pro- 

 duced by this torsion of strands of fibres there situated. Very careful investi- 

 gations in recent times have, however, demonstr'ated that this is not the case. 

 The cii'cling is produced independently of the torsion, and twining stems exist 

 in which no torsion whatever of these fibrous bundles takes place. 



We shall obtain the most accurate conception of the revolving movement 

 of the tip of a shoot if we still retain our illustration of the movement of a 

 switch swung round in a circle. When the switch, which may be best considered 

 as a cylindrical body whose periphery is striped longitudinally with numei-ous 

 straight lines running parallel to the axis of the cylinder, begins its motion, 

 there is first of all an outward lateral bending. The side which becomes concave 

 experiences a contraction, the convex side an elongation. Thus on the concave 

 side a pressure, and on the convex a tension is set up. At any given moment 

 these opposed strains are greatest along two opposite lines running along the 

 periphery of the switch; in the next moment, however, the greatest strain passes 

 over to the adjacent opposed lines, and since the greatest strain on the periphery 

 of the switch moves in this way, and touches all the lines in succession, that 

 remarkable circular movement of the free end of the switch results which 



