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PHYSIOLOGY OP PLANTS. 293
of knowledge as a means of determining the individual
manifestations of this omnipresent will and as a mediator
between the outer world and the changes of such a being ;
finally, in inorganic Nature, it is physical agency in general ;
and when, as here, observation takes place from a higher
to a lower degree, both stimulus and physical agency
present themselves as substitutes for knowledge, therefore
as mere analogues to it. Plants cannot properly be said
to perceive light and the sun ; yet we see them sensitive
in various ways to the presence or absence of both. We
see them incline and turn towards the light ;
and though
this movement no doubt generally coincides with their
growth, just as the moon s rotation on its axis coincides
with its movement round the earth, it nevertheless exists,
as well as that of the moon, and the direction of that
growth is determined and systematically modified by
light, just as an action is determined by a motive, and
as the direction of the growth of creeping and clinging
plants is determined by the shape and position of the sup
ports they may chance to find. Thus because plants on
the whole, still have wants, though not such wants as
demand the luxury of a sensorium and an intellect, some
thing analogous has to take the place of these, in order to
enable the will to lay hold of, if not to seek out, the satis
factions which offer themselves to it. Now, this analogous
substitute is susceptibility for stimuli, and I would express
the difference between knowledge and this susceptibility
as follows : in knowledge, the motive which presents itself
as representation and the act of volition which follows from
it, remain distinctly separate one from the other, this separa
tion moreover being the more distinct, the greater the per
fection of the intellect ; whereas, in mere susceptibility
for stimuli, the feeling of the stimulus can no longer be
distinguished from the volition it occasions, and they
coalesce. In inorganic nature finally, even susceptibility