RESPONSE OF DIGESTIVE ORGANS 331 



excitable lower half is exhibited in a marked manner by 

 downward mechanical movement, magnified as this is by the 

 long petiolar index. The intact organ, moreover, is invested 

 with an impervious skin, hence the excitatory exudation, or 

 expulsion, of water, being internal, is not seen outwardly. 

 Thus a single identical reaction may appear from different 

 points of view, as either secretory or contractile. 



The occurrence of contraction is thus most easily demon- 

 strable when it is accompanied by conspicuous movement. 

 This, however, demands considerable physiological aniso- 

 tropy, the differential contraction then giving rise to a very 

 marked lateral movement, as in Mimosa. In radial organs 

 of plants, on the other hand, owing to balanced contractions 

 of opposite sides, there is no marked responsive movement. 

 Hence ordinary plant-organs have hitherto been regarded as 

 non-contractile and insensitive. But I have shown that all 

 these radial organs exhibit longitudinal contraction, to be 

 detected and recorded by means of suitable magnifying 

 devices. All motile responses are brought about, it must be 

 remembered, by transference or redistribution of fluids. Now, 

 in organs invested with impervious membranes the effect of 

 fluid-transference is manifested by mechanical movement ; 

 whereas, in naked tissues, the fluid-transference is directly 

 visible as secretion. 



From the very important series of researches carried out 

 by Darwin, on the excitatory reactions in the tentacles of 

 Drosera, we know that the pedicel, carrying the gland on its 

 summit, is somewhat flattened, and that it is this anisotropic 

 lower part which is alone capable of movement. The gland- 

 cells on the head of the tentacle have been shown by 

 Gardiner to be provided with delicate uncuticularised cell- 

 walls, which are curiously pitted on their upper or free 

 surfaces. The terminal organ, or head, which is radial, 

 would thus seem to be peculiarly fitted for the exudation of 

 liquid on excitation. In the anisotropic motile portion of 

 the pedicel, on the other hand, the responsive reaction mani- 

 fests itself by bending. It would thus appear that the same 



