338 
PHYSIOLOGY. 
BOOK II. 
incontestably that attraction only influenced its nervous powers, 
and not its ponderable matter : and the same is undoubtedly 
the case with terrestrial plants. The unknown power of at- 
traction is only the accidental cause of the ascent of the stem, 
and of the descent of the roots, and not the immediate cause : 
in this case, attraction only operates as an agent for exciting 
nervous action. Other evidence exists to confirm most in- 
contestably this important conclusion, that the visible move- 
ments of vegetation are all spontaneous ; being brought into 
action by the influence of an external agent, but not move- 
ments originating with that agent. 
Light is another cause of no less power than that just de- 
scribed : it is well known that a plant, placed in a room from 
which the light is excluded, except at a single aperture, 
directs its stem and leaves towards that aperture, and no 
longer takes a perpendicular position. This is accounted for 
by De Candolle as follows : — Let any one expose a green 
branch in such a manner that light strikes it only on one 
side ; the tissue of that side will fix most carbon, will become 
harder, and will lengthen less ; while the opposite will fix less 
carbon, be less hard, and will lengthen more ; the consequence 
of which will be, that the illuminated side will contract, and 
pull the branch towards the light. This, if rightly considered, 
will of itself explain the uniform tendency of the green parts 
of plants to turn towards the light ; and if it will not account 
for such phenomena as that of the sunflower turning its flowers 
constantly to the sun, and following him in his course, as we 
find repeated by author after author, that circumstance is 
ascribable not to any defect in the explanations that have just 
been given, but to the alleged phenomenon having no existence in 
nature. The same tendency of the stems towards the light 
takes place in the open air. As light is diffused nearly 
equally around all bodies exposed to it, carbonic acid will be 
decomposed equally on all sides, and the various parts will 
naturally assume a direction towards the heavens ; so that 
light thus becomes an aid to gravitation. It might even be 
believed that light alone was a sufficient cause of the perpen- 
dicular position of the stems of vegetables, if experience did 
not prove the contrary. Dutrochet laid horizontally on the 
