Stimulus & Mechanism as Factors in Organisation .223 
not the sole means through which movements can be affected. 
For in them the mechanical conditions for a one-sided alteration of 
a turgescent system are wanting, and it seems certain that the 
stimulus provokes the action of a different kind of mechanism. It 
may be observed, in some cases at least, that the nuclei of the 
stimulated hyphae become aggregated on the convex side of the 
filament, and perhaps this indicates some definite metabolic change 
by means of which the protoplasm is able to affect the elasticity of 
the cell-wall, further investigations are needed before this suggestion 
can be regarded as anything but merely tentative. 
Many movements are observed to exhibit the pecularity of 
rhythm or periodicity, and this is specially the ease with organs 
that are subject to intermittent stimulation. A remarkable example 
of induced rhythm has recently been described by Mr. Darwin, 
who exposed opposite surfaces of axial organs to periodically 
reversed stimuli given by gravitation. The result was that not only 
did a reversal of the movements accompany the changed conditions 
of stimulus; but this periodicity was observed to continue for a 
while after the application of the stimulus-reversals had been 
arrested. As the existence of rhythm is one of the most general, 
as well as perhaps one of the most remarkable facts associated 
with irritability, the subject urgently needs further investigations. 
May it be that the causes underlying it are correlated with those that 
also underlie growth ? Certainly nutation is a very common rhythmical 
accompaniment of growth phenomena. Growth of a structure 
means rapid metabolism. We have direct evidence of this in the 
rapid changes that go on in the cells of actively growing zones, 
of stems and roots. Now the first action of a stimulus is certainly 
to modify metabolic relations. This in many cases is known to take 
the form of Katabolism, perhaps initially it always does so. But 
be this as it may, a local disturbance of metabolism means a 
corresponding disturbance of the energy relations of the system 
owing to the accumulation of chemical compounds of a more or 
less saturated character than those in the near vicinity. This may 
mean, if their diffusion is in any way opposed, or if its rate be 
relatively slow, that there is established a flow of nutriment or 
renovating material to the spot, followed by a reconstruction of 
the explosive material. Is it possible to imagine that there might be 
oscillations about the mean composition, due to self-distruction and 
subsequent renovation before relative uniformity is established 
after the discontinuance of the excitations ? If the movement is 
