488 PHYSIOLOGY OF GKOWTH. 



round the sticks, but the bean is a left-winding plant, not 

 like the hop a right-winding one. The spiral extends from left 

 below to right above. Fig. 163 clearly shows this ; and it in- 

 dicates further, what we shall consider carefully in 189, that the 

 older turns of the spiral are tighter and steeper than the younger 

 ones. This is seen with especial clearness when the supports 

 used are thin, not exceeding, in experiments with Phaseolus for 

 example, a thickness of a few millimetres. It is not uninteresting 

 to observe a number of twining plants, which have been allowed 

 to wind round supports of different thicknesses. I let bean stems 

 wind round stretched thread, wires 1 mm. in diameter, and sticks 

 4, 16 and even 30 mm. in diameter. 



187. Rotating Nutation. 



In carefully studying the winding of twining plants, it is of the 

 utmost importance to make ourselves acquainted with the rotating 

 nutation performed by shoots capable of twining. We lay seeds 

 of Phaseolus multiflorus in garden soil contained in fairly large 

 flower-pots. The first internodes of the young plants do not 

 exhibit the phenomenon of rotating nutation, or not clearly, but 

 the following ones do. The long terminal bud nods sideways, 

 owing to its own weight, and if we observe it accurately it will be 

 found that it is not at rest, but in a state of uninterrupted move- 

 ment, by which it is carried round in a circle. 



To study more closely the remarkable phenomenon of rotating 

 nutation, we employ other plants besides those of Phaseolus, e.g. 

 shoots of Calystegia 30 cm. in length, or the shoots of other twin- 

 ing plants. It is very important, however, for the plants to be in 

 a state of active growth, and exposed to very favourable condi- 

 tions. The basal part of the shoot is erect, but the summit of the 

 shoot is bent over in a wide arc, so that the apex is horizontal or 

 even directed a little downwards. We now make an ink-line along 

 the convex side of the shoot, running parallel with its axis, and 

 place the shoot with its convex side towards us (see Wortmann, 

 Botan. Zeitung, 1886). The plane of curvature is then vertical to 

 the plane of our body, the apex of the shoot is turned away from 

 us, and points, say, to the east. If we look at the shoot again, 

 after half an hour or three-quarters of an hour, we perceive 

 striking changes in it. The horizontally hanging apex points to 



