546 LECTURE XXXII. 



only, it is likewise possible to employ certain equably distributed sculpture markings 

 on the cell-wall for the criticism of the ways and means by which the growth between 

 them advances. Nevertheless these natural aids have only been used now and again 

 so far. 



Assuming that in the entire length of the growing region of a shoot-axis or root, 

 or even in any one portion of this length, at any time whatever, the growth takes place 

 a little more rapidly on the one side than on the opposite side, it is obvious that 

 a curvature must thereby be produced, and that this is the more pronounced 

 the greater the difference in the elongation of the two opposite sides. Such 

 phenomena now are as a matter of fact to be observed very commonly during the 

 process of growth in length : they are distinguished as Nutations, and are perceived 

 the more easily the longer the growing portion and the more energetic the elongation 

 itself is. Hence nutations are particularly evident in the case of rapidly growing 

 erect flower - stalks. The leafless flower -scapes of species of Allium — e.g. the 

 common kitchen onion — are found, so long as they are still growing in length, to be 

 always bent over towards one side, often to such an extent that they describe more 

 than a semicircle, and the thin flowering scapes of Allium rotundum and other species 

 even make loops in this manner of more than a whole circle. Finally, however, 

 when the growth is ceasing they straighten themselves completely, and stand quite 

 erect. On observing such nutating flower-stalks from hour to hour, now, and 

 if possible in the dark in order to exclude the heliotropic curvatures due to light, it 

 is soon noticed that the curvature does not always remain the same, but alters in 

 such a manner that the side which was previously concave becomes convex after 

 several or many hours, and therefore, of course, the pendent apex bends over towards 

 the opposite side. In the intervals also, sometimes one, sometimes another side 

 of the stem becomes convex, so that the apex is gradually turned towards all 

 parts of the horizon; and in particularly exquisite cases the change proceeds so 

 that the apex is gradually carried round in a circle, or to put it better and more 

 accurately, it moves in the form of an ascending spiral line, because during this 

 rotating nutation a continuous elongation, and therefore an ascent of the apex in 

 space, takes place. 



After what has been said above, the phenomenon finds its explanation in the 

 fact that first one and then another side of the organ elongates more rapidly 

 than the rest. If this takes place alternately on two opposite sides, the apex therefore 

 bends over at one time to the left, at another to the right ; but ifat the circumference 

 of the organ different sides in succession gradually take their turns in the process, 

 then the pendent apex must rotate in space. ' The latter case, however, is regularly 

 observed to any considerable extent only in strictly radially constructed shoot-axes. 

 I found such to be the case, for example, in the scapes of Allium, in the 

 flower stem of Brassica napus (Rape), and in the stem of Linum usiiatissimum 

 (Flax), as mentioned in my ' Handbuch der Experimental-Physiologic' (1865), p. 514'. 



» The first communication, so far .as I know, made on the phenomenon which I subsequently 

 mentioned in my ' Text-book ' as revolving nutation, and which has been lately extended by Darwin 

 to excessive importance as circumnntation, is found expressed in m^' Experimental- Physiologie; 1865, 

 p. 5 1 4, among the movements due to the process of growth itself, in the words :- ' Here also belongs the 

 hanging over of rapidly growing leaf and flower shoots,- which, without exhibiting torsion, become 



