MOVEMENTS OF IRRITATION. 489 



the north, the plane of curvature is as before vertical, but parallel 

 with the plane of our body, and the ink-line is no longer on the 

 convex side, but on the left flank of the shoot. It is thus shown 

 that the zone of most active growth, indicated by the convex 

 side of the shoot, has in our experiment moved 90 to the right. 

 The apex of the stem, however, has in the process described a 

 horizontal arc of 90 to the left. The further behaviour of the 

 shoot is easy to follow. After a further interval of half an hour 

 or three-quarters of an hour the ink-line is found on the concave 

 side of the shoot, and the original position is not again attained 

 until two or three hours from the start. Then begins a fresh 

 revolution. It is further noteworthy that the zone of most vigor- 

 ous growth almost always runs along the upper side of the curved 

 part of the shoot, which results in the plane of curvature being 

 always vertical. The revolving motion of the end bud from what 

 we have seen is due to the fact that the zone of most vigorous 

 growth moves round and round the stem. 



It must now be emphasised that the rate at which a complete 

 revolution of the apex of the shoot is executed is essentially 

 dependent on external factors, particularly on temperature. If 

 nutating shoots are exposed to unilateral illumination, the move- 

 ment towards the light is more rapidly effected owing to helio- 

 tropic processes than the movement away from the source of light. 

 Phaseolus and Calystegia shoots exhibit very active movements, 

 other twining plants only slow ones, but even in one and the same 

 object, and with approximately constant external conditions, the 

 rate of movement may vary in course of time to a notable extent. 

 Variations from the normal behaviour of the shoot also occur in 

 consequence of which- the plane of curvature is not always verti- 

 cal, i.e. tihe zone of vigorous growth does not always lie along the 

 upper side of the curvature, but is displaced. Different parts of the 

 rotating stem may, moreover, be curved in different planes. Atten- 

 tion must be paid to all these points, and it is well to observe a 

 number of plants continuously, and accurately note the results 

 obtained. 



To obtain a further insight into the highly complicated pheno- 

 mena here under consideration, it is necessary first to make the 

 following experiment. Phaseolus plants or Calystegia shoots at a 

 stage of development in which the rotating nutation is just about 

 to begin, are slowly rotated on the clinostat in the horizontal 

 plane, so as to exclude heliotropic nutations. We may also observe 



