VIIl] TWINING PLANTS 105 



the bud at the end is observed to become more and more 

 curved, as if nodding under the weight of the bud. In 

 addition to this, this curved or arched upper stretch is 

 seen to be acting in a very peculiar manner. The plane 

 of the ax'ch lies at a given moment, say, north and south, 

 so that the bud points to the north ; but if we examine it 

 some time afterwards, say half an hour to several hours, 

 the bud will be found directed towards the east or 

 towards the west, and after a similar period towards 

 the next point of the compass — the south — and so on. 

 In other words, the bud is being directed to every 

 point of the compass in succession, either passing round 

 in the direction of the hands of a watch or the reverse. 



But since the internodes do not stop their elongation 

 during this circumnutation\ as the circulating nodding 

 is called, it follows that the real direction in space of the 

 bud describes an ascending spiral. 



Since the internodes and bud do not revolve around 

 the axis of the pith, and since the convex side of the 

 arch is longer than the concave side, we see that the 

 circuranutation is due to the growth at any moment 

 being most rapid along the vertical line which crests the 

 convex side, and least so along the line parallel to this on 

 the concave side : this causes the nodding. The moving 

 round is due to this longitudinal line of quickest growth 

 slowly passing round the stem, either from left to right or 

 conversely. 



As the arched shoot gets longer and describes bigger 

 sweeps, it usually comes at last in contact with a vertical 

 support, and then its free swing is stopped. But another 

 event now comes into play. While growth as a whole — 

 i.e. the elongation of the internodes — still goes on, that 



' [This phenomenon in twining plants is, by some botanists, regarded 

 aa not being of the nature of circumnutation.] 



