d bristle-like contrivances,
plants possess such sense-organs; and moreover that these contrivances
show a remarkable similarity to corresponding sense-organs in animals.

Haberlandt and Nemec ("Ber. d. Deutschen bot. Gesellschaft", XVIII.
1900. See F. Darwin, Presidential Address to Section K, British
Association, 1904.) published independently and simultaneously a
theory of the mechanism by which plants are orientated in relation
to gravitation. And here again we find an arrangement identical in
principle with that by which certain animals recognise the vertical,
namely the pressure of free particles on the irritable wall of a cavity.
In the higher plants, Nemec and Haberlandt believe that special loose
and freely movable starch-grains play the part of the otoliths or
statoliths of the crustacea, while the protoplasm lining the cells in
which they are contained corresponds to the sensitive membrane lining
the otocyst of the animal. What is of special interest in our present
connection is that according to this ingenious theory (The original
conception was due to Noll ("Heterogene Induction", Leipzig, 1892), but
his view differed in essential points from those here given.) the
sense of verticality in a plant is a form of contact-irritability. The
vertical position is distinguished from the horizontal by the fact that,
in the latter case, the loose starch-grains rest on the lateral walls
of the cells instead of on the terminal walls as occurs in the normal
upright position. It should be added that the statolith theory is
still sub judice; personally I cannot doubt that it is in the main a
satisfactory explanation of the facts.

With regard to the RAPIDITY of the reaction of tendrils, Darwin records
("Climbing Plants", page 155. Others have observed movement after about
6".) that a Passion-Flower tendril moved distinctly within 25 seconds of
stimulation. It was this fact, more than any other, that made him doubt
the current explanation, viz. that the movement is due to unequal
growth on the two sides of the tendril. The interesting work of Fitting
(Pringsheim's "Jahrb." XXXVIII. 1903, page 545.) has shown, however,
that the primary cause is not (as Darwin supposed) contraction on the
concave, but an astonishingly rapid increase in growth-rate on the
convex side.

On the last page of "Climbing Plants" Darwin wrote: "It has often been
vaguely asserted that plants are distinguished from animals by not
having the power of