Curvature of Roots. 445 
It has been shown (experiments 12, 13, and others) that 
traumatropic curvature follows an injury that extends to 
the growing-point, but fails to take place when even extensive 
injury is inflicted in which the growing-point is not involved. 
Thus we have seen that the root-cap may be wounded to 
a considerable depth without apparent effect, and that a wound 
which just opposite the punctum vegetationis is followed 
by deflection, produces no such result a millimetre, or even 
less than a millimetre, higher. On exclusively mechanical 
principles these facts remain unexplained, but become in- 
telligible upon the supposition that the tissue of the growing- 
point is sensitive, and that the application of a stimulus here 
is followed by induction, the effects of which are manifested 
in the zone of rapid growth. 
The behaviour of aerial roots (experiments 8-11) presents 
further evidence in the same direction. In these, precisely as 
in the radicles of seedlings, traumatropic curvature promptly 
follows injury, of whatever kind, that reaches the growing- 
point. Thus the action is the same whether the tip is branded 
and many cells destroyed in the process, or the injury is 
produced by the penetration of a sharp instrument driven into 
the growing-point of the root by its own elongation, with 
a minimum destruction of adjacent tissue. The inadequacy 
of Detlefsen’s explanation, in view of the fact that the aerial 
roots employed are practically destitute of a root-cap, has 
already been pointed out. It is equally clear that the 
relatively very slight destruction of tissue caused by the 
introduction of the point of a needle leaves very little room 
for Wiesner’s hypothesis. 
Experiment 14, in which it was shown that the removal of 
the root-cap is still followed by traumatropic curvature after 
branding, has also an important bearing. The disturbance 
of the natural equilibrium must of necessity be so great, 
no matter how skilfully the root-cap is removed, that one 
might well doubt whether definite results could be obtained in 
stand, since Wiesner’s term ‘ Darwin’s curvature ’ is extended by him to include 
what Darwin, so far as appears from his works, never observed. 
