Sense of Touch 391 



they thought the whole thing a delusion. And the story ended with 

 his triumph when Mr Huxley cried out, " It is moving." 



Darwin's work on tendrils has led to some interesting investigations 

 on the mechanisms by which plants perceive stimuli. Thus Pfeffer 1 

 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 

 conies close to the surface, being separated from an object touching 

 the tendril merely by a very thin layer of cell-wall substance. 

 Haberlandt 2 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 (Sinnesorgane, 

 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 Nemec 3 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, Nemec 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 4 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 5 that a Passion-Flower tendril moved distinctly within 25 



1 Tubingen Untersuehungen, i. p. 524. 



2 Physiologischc PJlanzcnanatomie , Ed. in. Leipzig, 1904. Sirmesorgane im J'thinzen- 

 reich, Leipzig, 1901, and other publications. 



3 Ber. d. Deutschen hot. Gesellschaft, xvm. 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". 



