316 VEGETABLE PHYSIOLOGY 



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 round 

 the growing zone. The apex 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 growing organs. The passage of the maximum tur- 

 gidity 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. 



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 

 incident to growth, and proceeding altogether from internal 

 causes many advantages are secured by the plant. In 

 the case of a climbing stem, the circumnutation enables it 

 to reach a support, round which it twines, so that with 

 but little expenditure of substance it can secure access to 



