g that it is
possible to discover the part of the plant which reads the signal: and
this is not necessarily the part that executes the correlated movement.
In the reflex movement of an animal (for instance, a cough produced by a
crumb going the wrong way), we distinguish the irritation of the throat
and the violent action of the muscles of the chest and abdomen; and
further, the nervous machinery by which the stimulus is reflected or
switched on, by way of the central nervous system, from the throat to the
muscles concerned in coughing. In the plant, too, if we are to compare
its movements to the reflexes of animals (as has been done by Czapek), we
must distinguish a region of percipience, another of motility, and the
transmission of an influence from the percipient to the motor region.

Transmission of a stimulus has long been known in _Mimosa_, but, in the
far more important curvatures which we are now considering, it was not
known to [Picture: Narcissus flowers] exist before the publication of the
_Power of Movement in Plants_. There is an experiment of Rothert's {45}
which we make in class-work at Cambridge, and which only differs from my
father's classical experiment in the fact that a much more perfectly
adapted plant is employed. The plant in question is a grass, _Setaria_,
which has a remarkable form of seedling. When the grain germinates it
does not send up a simple cylindrical sprout like an oat, but a delicate
stem terminating in a pointed swollen part which looks like a little
spear-head. When a group of _Setarias_ is illuminated from one side they
bend strongly over, with their spearheads all pointing straight at the
light. The spearheads do not bend; the whole movement is carried out by
the stalk on which the head is supported. And what is remarkable is,
that the spear-head and not the stalk perceives the light. This is
easily proved by covering the heads of a few _Setarias_ with opaque caps.
For the result is that the blindfolded seedlings remain vertical while
their companions are pointing to the light. Thus the part which bends,
is unaffected by illumination, and the part which _is_ affected does not
bend. The spear-head is the percipient organ, the shaft or stalk is the
motor region, and from head to shaft an influence has clearly been
transmitted.

My father and I made an attempt to prove the same thing for the
gravitation-sense of roots, that is, to prove that the tip of the root is
the re