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PRINCIPLES OF GENERAL PHYSIOLOGY 



That it is essentially similar to that in the animal is indicated by the fact that 

 vegetable protoplasm, in a state of excitation, is electrically negative to that at 

 rest. This is shown by the observations of Hormann on Nitdla (1898, pp. 69-79). 

 A stimulus at a point sets a wave of excitation in progress, accompanied by a state 

 of electrical negativity. This electrical state, so far as could be ascertained, 

 precedes the cessation of movement caused by the stimulus. In these cells, the 

 streaming movement corresponds to the contractile properties of a muscle fibre and 

 is, as we have seen, something added on to the simple excitatory process as it shows 

 itself in nerve. Similar conclusions are to be drawn from the work of Burdon- 

 Sanderson on the leaf of Dioncea muscipula (1888), whose leaves shut up rapidly 



Fio. 130. ELECTRICAL CHANGES IN THE LEAF OF DIONJEA. Corresponding points on the 

 under surface of each lobe led off to capillary electrometer. 



a, Stimulated mechanically four times on right side. Diphasic effect due to excitatory process arriving 



at each electrode in turn. 



b, Stimulated on right and left sides, alternately. 



c, Stimulated on left side, four times. 



Time signal twenty per second. 



(Burdon-Sanderson, 1888, Figs. 8, 9, and 10.) 



when a fly touches certain sensitive hairs on the upper surface. By placing 

 electrodes on opposite lobes, and stimulating the neighbourhood of each electrode 

 in turn, it was shown that the excited spot becomes first negative, then positive, 

 as the wave of excitation reaches the other electrode (see Fig. 130); just as was 

 described above for nerve and muscle. It is pointed out that this wave of 

 excitation precedes the change of form and travels at a much faster rate. Visible 

 change of form was prevented by a cross-bar fixed between the lobes. 



It is a familiar fact that plants in general, root, stem, leaves, and flowers, 

 respond to gravity and to light, and in various ways. Certain of them, as the 

 climbing plants, and especially the " sensitive plant," and Dioncea, respond to 

 touch by more or less rapid movements. 



Now this response is not confined to the part actually stimulated, but the 

 excitation is conducted to more distant parts. For a description of the various 

 phenomena concerned, the reader is referred to the book of Pringsheim (1912). 

 We are chiefly interested here in the excitatory process itself. 



