GROWTH. 361 



to grow will form an angle at a point in the region in which 

 the unequal growth is taking place. 



Coming now to the actual phenomena of nutation, we find 

 that it presents itself in different forms, the form being de- 

 pendent upon the peculiar organisation of the plant which we 

 may attempt to explain in the following manner. The sim- 

 plest form of nutation is that in which the axis of the growing 

 portion travels in one plane from one side to the other of 

 the prolongation of the axis of the portion which has ceased 

 to grow, forming an angle with it first on this side and then 

 on that. It is this form of nutation which is usually termed 

 Simple Citation, or merely Nutation. It is produced by the 

 alternate and more rapid growth of two opposite longitudinal 

 halves of the growing region of the organ, the two longitu- 

 dinal halves occupying permanently the same relative posi- 

 tions. A more complex form of nutation is that which is 

 known as Revolving Nutation. In this, the axis of the 

 growing portion does not oscillate from side to side of the 

 prolongation of the axis of the portion which has ceased to 

 grow, but describes an orbit about it. This, like simple 

 nutation, is produced by the more rapid growth of one side 

 of the organ as compared with that of the side opposite to 

 it, but the differences in the rapidity of growth are not con- 

 fined in this case to two particular sides. A wave of more 

 rapid growth travels, as it were, round the growing organ 

 from segment to segment. Let us take illustrative cases. 

 A growing organ exhibits simple nutation in any one plane, 

 say a plane running east and west ; this is due to the 

 alternate more rapid growth of the west and east sides of 

 'the organ, and the differences in the rate of growth are 

 exhibited only by these two sides. In an organ which 

 exhibits revolving nutation, the nutation is not confined to 

 any one plane, but takes place from east to west, from north 

 to south, and in all intermediate planes as well ; all sides take 

 .on in succession a period of most rapid growth. 



According to Baranetzky revolving nutation is not spontaneous but 

 is induced by the action of gravity. Stems which are withdrawn from 



