Newcombe . — The Sensory Zone of Roots, 443 
IV. Summary and Conclusion. 
As far as we know, there is no reason in the nature of the case 
why the sensitiveness of roots to stimuli should be confined 
to the region capable of elongation. A grass-stem receives and 
responds to the gravitation stimulus when under ordinary con- 
ditions its growth has ceased 1 ; the cotyledon of Panicum 2 is 
sensitive to light after growth has ceased ; and the leaves of 
Mimosa and of numerous other plants continue to receive and 
respond to stimuli long after growth has ended. The region 
of roots shown in the foregoing pages to be sensitive to stimuli, 
though no longer capable of elongation, may still grow in dia- 
meter. And it may be said here that the assumption of 
the limitation of sensitiveness of roots for certain stimuli to 
certain regions is not in all cases justified. Czapek 3 appears 
to have tenable ground for limiting the perception of gravita- 
tion to the root-apex. But that moisture stimulates only the 
apex, cannot be maintained until the elongating zone and the 
part back of it are tested in the neutralization of gravitation. 
So also with thermotropism, aerotropism, and the like ; for 
aught we know, the stimuli for these responses may be re- 
ceived beyond as well as in the elongating zone. 
In carrying out the experiments detailed in the foregoing 
pages, there are various difficulties involved and various objec- 
tions which may be raised. These will now be briefly con- 
sidered. In the first place, I may admit that the employment 
of glass-tubes is but an imperfect method ; the tubes necessarily 
obstruct the flow of water. If, however, fibres of cotton are 
attached to the glass-tubes, they are seen to trail constantly in 
the direction at right angles to the diameter of the revolving 
basin of water, showing that a current approximately constant 
in direction is maintained. The current, after being disturbed 
by the tubes and seedlings in one-half of the diameter, is re- 
composed by its undisturbed progress through the following 
1 De Vries, Ueber die Aufrichtung des gelagerten Getreides. Landw. Jahr- 
biicher, ix, 1880, p. 473. 
2 Rothert, Ueber Heliotropismus. Cohn’s Beitr. z. Biol., vii, p. 176. 
3 Czapek, Jahrb. f. wiss. Botanik, xxvii, p. 255; ibid, xxxv, p. 317. 
