Sense of Touch 391 
they thought the whole thing a delusion. And the story ended with 
his triumph when Mr Huxley cried out, “It ¢s moving.” 
Darwin’s work on tendrils has led to some interesting investigations 
on the mechanisms by which plants perceive stimuli. Thus Pfeffer? 
showed that certain epidermic cells occurring in tendrils are probably 
organs of touch. In these cells the protoplasm burrows as it were 
into cavities in the thickness of the external cell-walls and thus 
comes close to the surface, being separated from an object touching 
the tendril merely by a very thin layer of cell-wall substance. 
Haberlandt? has greatly extended our knowledge of vegetable 
structure in relation to mechanical stimulation. He defines a sense- 
organ as a contrivance by which the deformation or forcible change 
of form in the protoplasm—on which mechanical stimulation depends 
—is rendered rapid and considerable in amplitude (Simnesorgane, 
p. 10). He has shown that in certain papillose and _ 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 Némec?® published independently and simul- 
taneously 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, Némec and Haberlandt be- 
lieve that special loose and freely movable starch-grains play the 
part of the otoliths or statoliths of the crustacea, while the proto- 
plasm 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 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’ that a Passion-Flower tendril moved distinctly within 25 
1 Tiibingen Untersuchungen, 3. p. 524, 
? Physiologische Pflanzenanatomie, Ed. 111. Leipzig, 1904. Sinnesorgane im Pflanzen- 
reich, Leipzig, 1901, and other publications. 
3 Ber. d. Deutschen bot. Gesellschaft, xv. 1900. See F. Darwin, Presidential Address 
to Section K, British Association, 1904, 
4 The original conception was due to Noll (Heterogene Induction, Leipzig, 1892), but 
his view differed in essential points from those here given. 
5 Climbing Plants, p. 155. Others have observed movement after about 6”. 
