Newcombe, Thigmofcropism of teiTestrial roots. 77 



yei presented for belief in tliigmotropism. Unless it ean be 

 f^liown tliat tlie paper liood would not di.striljute the water 

 equally about the root, the excess of positive curves must have 

 been due to the j^ressure of the glass-post. Of course in the 

 employment of j)aper about the root-tip it must be seen to that 

 the paper makes even contact on both sides of the root. It is 

 beUeved that this condition was complied with in the foregoing 

 experiments. 



It is not inconceivable that the pressui-e of an object Uke 

 the glass-post used in the foregomg experiments miglit exert a 

 traumatic effect on the cells, retarding tlieir growth, and thus 

 producing a positive, but non-irri table, curve. Numerous tests 

 were made with seedlings, giving some a very strong, others 

 the usual pressure on one side of the elongating zone of the 

 root, while the seedlings were revolved as before on the kli- 

 nostat. There was no perceivable difference in the number of 

 responses or degree of curvature in the 2 cases. 



It is apparent that it would be desirable, if possible, while 

 plants are revolved so as to neutralize the effect of gravitation 

 to bring pressure to bear on the elongating zone of the roots by 

 the employment of some object which would not, like the glass- 

 rods already used, offer a surface so near the apex of the root 

 for the possible diffusion of moisture against its sloping side. 

 Such an object has been found in the use of lu'istles. Woak 

 l)ristles were selected and cut into lengths of about 3 cm. Each 

 piece was then cemented at one end to a wire arm, making a 

 right angle with the arm, and the arm was fastened to a pin. 

 When in use, 5 of these bristles were made to press as weak 

 Springs against the elongating zone of each root, by thrusting 

 the supporting pin into the proper place in the cork floor of 

 the damp-chaml)er. Fifteen roots of Zea may.s (popcorn) were 

 so tested while revolved in a damp-chamber on the klinostat, 

 their axes being in the vertical plane, and none curved. Ten 

 roots of Baplianus satiims in a similar experiment gave one root 

 positive, one negative, and 8 neutral. Loops of collodion about 

 3 mm wide Avere made to pull laterally on the elongating zones 

 of 17 roots of Baphanus safivus during revolution on the kli- 

 nostat. Two roots became negative, and 15 grew straight. 

 These results cannot help but weaken the faith which had be- 

 gun to found itself on the results of the experiments with the 

 glass-posts. The only explanation which Avould seem to recon- 

 cile the results in the two cases is that the loops and the bris- 

 tles did not exert as great a pressure in as sensitive a position 

 as did the glass-posts. Happily we can now turn to more satis- 

 fying results. 



III. Effect of aStream of Water oii Roots coveredby Collodion. 



If sensitive to pressm^e, the terrestrial roots of plants, as 

 the preceding experiments have sliown, are but feebly so; and 

 to bring this sensitiveness to expression, pressure long continued 



