GROWTH 315 



Cylindrical organs may exhibit similar phenomena. 

 One side of a stem may be more turgid than another, and 

 the maximum turgidity with its consequent growth may 

 alternate between two opposite sides. The greater turgidity 

 of the cells is often accompanied by an increased extensi- 

 bility of the cell- walls of the turgid region. The growing 

 apex of such a stem will alternately incline first to one side 

 and then to the other, exhibiting a kind of nodding move- 

 ment in the two directions. This is known as nutation, 

 and is of very frequent occurrence, particularly in such 

 stems as are slightly flattened instead of being truly 

 cylindrical. 



The region of maximum turgidity 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 simple 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 

 observable 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 

 hyphae 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 



