UNEQUAL GROWTH. 
usually connected a difference in the growth of the two dissimilar sides, which causes 
curvatures and hence changes in the position of the apex. The two dissimilar sides 
of bilateral organs must also be acted on differently by external agencies which affect 
growth, such as light, gravitation, and pressure. We do not attempt here to solve the 
question of the causes which produce the bilateral structure in any particular case ; 
it need only be shown incidentally that this structure of lateral organs (as we have 
already seen in Book 1. Sect. 27) is probably always brought about by internal 
causes, and is independent of the action of external circumstances. This is in 
general evident from the fact that the median plane of bilateral appendicular organs 
has always a perfectly definite geometrical relation to the axial structure which bears 
them, and that moreover in the dark and under the influence of slow rotation round 
a horizontal axis, which eliminates the effect of gravitation, the bilateral structure and 
relation to the axis remain unchanged. 
But before we proceed to the consideration of the growth of bilateral organs, it 
must be premised that even in multilateral erect stems and vertically descending 
roots growth does not always proceed equally and with equal rapidity on all sides of 
the longitudinal axis ; it is much more common for first one side and then another 
of the organ to grow more rapidly than the rest, curvatures being thus caused the 
convexity of which always indicates the side that is at the time growing most rapidly. 
If another side then grows more rapidly, it becomes convex, and the curvature 
changes its direction. Curvatures of this kind caused by the unequal growth of 
different sides of an organ may be called Nutations, and in so far as they are pro- 
duced entirely by internal causes they may be said to be spontaneous. They occur 
most commonly and evidently when growth is very rapid, and consequently in 
organs of considerable length, and are produced under the influence of a high 
temperature either in darkness or when the amount of light is very small. 
When two opposite sides of an organ grow alternately more and less rapidly, 
curvatures are caused first on one side and then on the other ; it will, for example, 
bend first to the left, then become erect, and then bend to the right side ; as occurs, 
e.g. in the long flower-scapes of Allium Porrum, which finally take an erect position 
when their growth is ended. It is much more common for the apices of erect stems 
above the curved growing part to move round in a circle or ellipse, the region of 
most active growth moving gradually, as it were, round the axis : it lies, for instance, 
at one time towards the north, then towards the west, south, and east in succession 
until it comes again to lie towards the north. This kind of nutation may be termed 
a Revolving Nutation \ Since the apex of the stem is constantly rising higher 
during the nutation owing to the elongation of the part below it, its revolving motion 
does not take place in a plane, but describes an ascending spiral line. This form of 
nutation occurs in many flower-stalks before the unfolding of the flowers, as in those 
^ [Darwin is of opinion (Movements of Plants) that all nutation is revolving nutation, or, as he 
terms it, circnmmitation. He regards all the movements connected with growth (heliotropism, 
geotropism, hyponasty, epinasty) as well as those of mature parts (spontaneous or induced move- 
ments) as being modified forms of circumnutation. 
The unequal growth is of course the expression of an unequal turgidity of different parts of the 
growing organ (see de Vries, Ueb. die inneren Vorgänge bei den Wachsthumskrümmungen mehr- 
zelliger Organe, Bot. Zeitg. 1879.] 
