CH. VIIl] TENDRILS. 201 



(231) Tendrils: Pfeffer s contact experiment. 



Pfeffer 1 has shown that only rough bodies which 

 produce discontinuous pressure serve to stimulate tendrils, 

 while perfectly smooth homogeneous bodies are not 

 irritating. 



Melt 2 sheets of Marshall's " Leaf Gelatine " in half a 

 cupful of warm water 2 : dip into it while still hot a 

 smooth wooden or glass rod about 3 mm. in diameter. In 

 this way a length of 4 or 5 cm. must be thickly coated and 

 allowed to cool thoroughly before use. The gelatine- 

 coated rod, which must be kept as free from dust as 

 possible, is now to be employed to touch the tendrils of 

 Bryonia as described in exp. 230. It will be found that 

 the tendrils show no signs of curving ; they must now be 

 touched with the uncoated part of the rod, to prove 

 that the absence of movement is not due to want of 

 sensitiveness. 



On a windy day (if the experiment is made out-of- 

 doors) it may be a little difficult to make sure that, in the 

 first part of the experiment, the tendril does not come into 

 contact with the uncoated part of the rod : it is for this 

 reason that a good length of gelatine coating is recom- 

 mended. The tendril can however easily be held still by 

 touching its convex (insensitive) side with another gela- 

 tine rod; the slight stickiness of the gelatine fixes the 

 tendril while the observer manipulates with the first rod. 



1 Untersuchungen aus d. bot. Institut zu Tubingen, i. 1885, p. 483. 



2 Pfeffer uses solutions containing from 5 to 14 per cent, of air-dry 

 gelatine. 



