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PART IV. THE PHYSIOLOGY OF PLANTS. 



Fie. 478. Diagram illustrating nutation of a cylindrical stem, as seen from above: 

 N, S, W, E, indicate the points of the compass. 



The small upper circle divided into segments represents an ideal transverse section of 

 the natating region : the larger circle below represents the orbit of nutation, and the small 

 circles upon it represent different positions of the apex of the stem in its orbit ; the shaded 

 segment indicates always the position of one and the same side of the stem; the smull 

 circle in the centre of the orbit represents the position of the apex of the stem were it not 

 nutating, but were it in a straight line with the older parts which have ceased to grow. 

 Assuming, to begin with, that the rate of growth is uniform in all the segments of the 

 elongating region, then the position of the apex will coincide with the small circle in the 

 centre; but the growth is not uniform, being more active on one side than on the others, 

 say in segment 1; then segment 4 will be the region of least active growth, and, as a con. 

 sequence, the apex of the stem will be displaced southwards till it takes up a position / in 

 the orbit ; the wave of rapid growth then travels from segment 1 to segment 2, and the apex 

 travels from position I to II in the orbit, and so on, until the apex has completed its orbit 

 and returned to position I. It will be observed that the shaded segment always faces the 

 same side (south) in all positions in the orbit; nutation is thus not accompanied by 

 torsion, that is, by twisting of the member about its own axis. In this case the direction 

 of nutation is that of the sun, or of the Lands of a watch. 



