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 betweenthem. 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 cireumnutation 
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. 
Cireumnutation is exhibited during growth also by the 
hyphe 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 
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