GROWTH. 365 



the apex of the stem in its orbit : the shaded segment in each of the 

 small circles serves to indicate the position throughout the orbit of one 

 and the same side of the stem : the small circle in the centre of the orbit 

 represents the position of the apex when the axis of the growing portion 

 coincides with the prolongation of the axis of the portion which has 

 ceased to grow. Let us assume, to begin with, that the rate of growth 

 is uniform in all the segments of the transverse section of the growing 

 region ; the position of the apex of the stem at this time is indicated 

 by the small circle about the centre of the orbit. But let us suppose 

 that the growth in length is not uniform and that a portion of the 

 growing region, segment i for instance, is the seat of the most rapid 

 growth ; then, as explained above, segment 4, which is opposite to 

 segment I, will be the portion which is growing in length at the slowest 

 rate : the effect of this unequal growth is that the north side of the organ 

 becomes convex and the south side concave, and in consequence of this 

 curvature the apex is removed from its first position to a position which 

 we may call I. The wave of rapid growth then travels from segment I 

 to segment 2, and the apex travels from position I to position 1 1 ; each 

 pair of opposite segments exhibit in succession a kind of polarity, such 

 that when one of the segments is growing the most rapidly, the one 

 diametrically opposite to it is growing the least rapidly : at length 

 segment I again becomes the seat of the most rapid growth and the 

 apex again comes to lie in position I. 



It will be observed that the shaded segment faces towards the same 

 side, the south, in all positions of the orbit. This illustrates the fact that 

 circumnutation is not accompanied by any twisting of the organ about 

 its own axis. In this case the direction of nutation is that of the sun 

 or of the hands of a watch, and the orbit has been assumed to be a 

 circle. 



The nutation of a growing organ is the more conspicuous 

 the greater its activity of growth : the nutation of stems, for 

 instance, is more marked than that of roots. With regard to 

 stems it must be remembered that since several internodes 

 may be growing at the same time, they may be also nutating 

 at the same time. Darwin observed in the plants with which 

 he experimented that two, and sometimes three, internodes 

 exhibited circumnutation simultaneously. But it by no means 

 follows that they nutate synchronously; on the contrary, they 

 do not; for, as we have seen, the rate of nutation varies with 

 the age of the organ. In a case of this kind the movement 

 of the apex is the resultant effect of the nutation of the inter- 

 nodes which are still growing. 



