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 
eylindrical. 
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 
hyphe of many fungi, some of which have a ccenocytic 
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 
