GROWTH 321 



Cylindrical organs may exhibit similar phenomena. 

 One side of the cortex of a stem may be more turgid than 

 the rest, and the maximum turgidity with its consequent 

 growth may pass a little later to the opposite side and 

 subsequently alternate between them. The greater turgidity 

 of the cells is often accompanied by an increased extensi- 

 bility of the cell-walls of the temporarily more turgid region. 

 The growing apex will thus alternately incline first to 

 one side and then to the other, exhibiting a kind of nodding 

 movement in the two directions. This, known as nutation, 

 is of frequent occurrence, particularly in such stems as are 

 slightly flattened instead of being truly cylindrical. 



The region of maximum turgidity in the cortex instead 

 of occurring alternately on two opposite sides may pass 

 gradually and regularly round the growing zone. The 

 apex of a truly cylindrical stem in this case will describe 

 a circle, or rather a spiral, as it is elongating all the time, 

 pointing to all points of the compass in succession. This 

 continuous change of position has been described by Darwin 

 as circumnutation, and has been said by him to be universal 

 in all cylindrical growing organs. The passage of the 

 maximum turgidity round the stem may vary in rapidity 

 at different places, causing the circle to be replaced by an 

 ellipse. Indeed the simplest nutation spoken of above may 

 be regarded as only an extreme instance of the latter. 



The variations of turgidity which cause circumnutation 

 only affect the zone of active growth. They are not observ- 

 able towards the base of this, so that the adult part becomes 

 straight, and growth is ultimately in a straight line. 



Circumnutation is exhibited during growth also by the 

 hyphse of many fungi, some of which have a coenocytic 

 structure. In these cases the movement appears to be due 

 to a rhythmic variation in the extensibility of the membrane, 

 induced probably by the protoplasm. It cannot be caused 

 by differences of turgidity on the two sides of the hypha as 

 this contains only one cavity. 



By these movements of the growing apices movements 



