Newcombe, Thigmotropism of terrestrial rools. 65 



ferred thigniotropisin. None of my Vicia faha roots lient in any 

 direction, tliougli the droplets of plaster adhered well to one 

 side of tlie tips for 30 hours. 



The apparent discrej^ancy between my results obtained wlien 

 the plaster of Paris was attached after it had hardened, and wlien 

 it was allowed to harden after it was applied to the root-tip, 

 may be explained in one of three ways: Because different spe- 

 cies of plants were used in the two experiments , because too 

 small a number of seedlings of Vicia faha was used, or because 

 the plaster whicli hardened on the roots did not allow the cap- 

 illary accumulation of water which the plaster plates allowed 

 between themselves and the roots. 



From all of the foregoing experiments this conclusion can 

 certainly be drawn: The thigmotropism of roots is not to be 

 demonstrated by attaching objects to the roots when in a damp- 

 chamber. Such curves as were produced were negative and were 

 more likely to follow the application of a body capable of ab- 

 sorbing water than one not possessing such capability. The 

 curves therefore were not thigmotropic, but were probably due 

 to some disturbance of the water equilibrium of the root. It 

 should be noted also that a body adhering to the flank of a root 

 exerts a pull and not a pressure on the cells. There is probably 

 in a damp-cham])er never any contact between the root and the 

 adhering l)ody, since a Stratum of water intervenes. We should 

 expect thigmotropic curves to be positive; we know of no neg- 

 ative thigmotropic reactions in plants. All of the curves noted 

 above were negative. I have not cared to follow further Ne- 

 mec's method by which he obtained ]50sitive curves in Vicia. 

 faha, because the results would be worthless when obtained. 

 Who shall say when a curve follows the hardening of a droplet 

 of plaster on the root-ti}) whether the response is mechanical, is 

 hydrotropic , chemotropic, thigmotropic or something eise? The 

 fact that Nemec when he applied his plaster to the growing 

 point fomid the droplet of plaster subsequently lying in the 

 apex of the angle of curvature looks suspicious. The motor zone 

 of the root for all known irritable reactions is some distance bac 

 of the growing point, and it is diflicult to see how even after 

 24 hours the relations described by Nemec should obtain, un- 

 less the plaster were causing a mechanical bend by its hold 

 on the cells beneath. Why in a similar experiment my 9 roots 

 gave no such bend, I have no means of knowing. 



2. Preparations with roots immersod in water. 



Belle ving that experimentation with roots of seedlings in 

 damp-chambers is rendered precarious by the great liability to 



1) Nemec. Über die Wahniehmimg des Schwerkraftreizes bei den 

 Pflanzen. (Jahrb. f. wiss. Botau. XXXVL 1901. p. 87.) 



Beihefte Bot. Ceutralbl. Bd. XVin. 1904. 5 



