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Darwin was fond of telling how when he demonstrated the sensitiveness of Drosera to Mr Huxley and (I think) to Sir John Burdon Sanderson, he could perceive (in spite of their courtesy) that 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 (Tubingen "Untersuchungen" I. page 524.) 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 ("Physiologische Pflanzenanatomie", Edition III. Leipzig, 1904. "Sinnesorgane im Pflanzenreich", Leipzig, 1901, and other publications.) 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", page 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 ("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.