84 Newcomlie, Thigmotropism of terrestrial rooti=!. 



cliffer in their sensitiveness, and that nearly half of tlie 32 spe- 

 cies tested sliow no response in a stream of water. Relying on 

 the former rlieotropic experiments also, we may say that the 

 apical millimeter, the elongating zone, and some extent of root back 

 of the elongating zone are thigmotropic. Whether the apex or 

 the elongating zone is the more sensitive has not been deter- 

 mined. 



The latent period for thigmotropism of roots, as shown by 

 the earlier experiments for rheotrojDism , is long relative to some 

 other tropisms; yet it is no longer than that for the thigmotro- 

 pism of some tendrils. Tliis period for the most sensitive ter- 

 restrial roots is about one hour at optimiim temperature. 



The sensitiveness of roots to pressure is all-sided; no differ- 

 ence could be detected when different sides of the roots were 

 exposed to the pressure of the stream of water. 



Like tendrils, roots require that Stimulation should be over 

 a considerable area of surface and be continued for a consid- 

 erable period of time to produce what may be termed a com- 

 plete reaction. The relatively acute sensitiveness of tendrils as 

 determined by Darwin, de Vries, and others, and very re- 

 cently by Fittig,^) enables them to show a transitory response, 

 as roots will not, when but a few cells are stimulated for a 

 fraction of a second. 



The fact is worth alluding to that in my former work on 

 rheotropism 4 water-planis showed only neutral roots in a water- 

 stream. It may be that this behavior on the part of these plants 

 is a useful and acquired adaptation; for one can unterstand how 

 it would be a distinct disadvantage for the roots to bend out of 

 a straight course down to the bottom of the flowing stream. 



The feeble thigmotropism of roots precludes, to my mind, 

 any possibility of utility to the plant. It may he that some 

 terrestrial roots will be found so sensitive as to be capable of 

 responding to such pressure-stimuli as the roots may meet in 

 natm'e; but it seems certain from the foregoing experiments that 

 none of the roots whose behavior is treated in this paper can 

 so respond. The water-stream is the best Stimulus imaginable 

 for giving a constant and accommodating pressure, and such a 

 Stimulus must be used if one would demonstrate the thigmotro- 

 pism of terrestrial roots. 



ij Fittig, Untersuchungen über den Haptrotropismus der Ranken. 

 (Jalirb. f. wiss. Botan. XXXIII. p. 551); also a continuation of the same 

 subject, ibid. XXXIV. 



