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1900] MECHANISM OF ROOT CURVATURE 17 
PATH OF TRANSMISSION OF STIMULUS. 
As has already been stated, in these experiments it is 
regarded as settled that only the tip of the root is capable of 
receiving the stimulus, and that an impulse is transmitted from 
the tip to the zone of curvature which in the most typical cases 
has a region of maximum curvature from 3—5™™ from the tip. 
The whole zone of curvature is somewhat longer but seldom 
extends more than §™™ from the tip at first. Afterward, by 
growth in length of the root, the curve may come to lie farther 
from the tip. Under favorable conditions the tip of the root 
may describe an arc of go° in from three to four hours from the 
time of the branding; and in these cases, where the curve is 
rapid, it is found by careful measurement that the zone of maxi- 
mum growth does not coincide with the zone of maximum curva- 
ture, but the latter lies nearer the tip than the former. Later, as 
the zone of maximum growth advances the two may coincide. 
These results confirm those of Weisner (78). The method used 
for ascertaining the path of transmission of the stimulus was 
suggested by Kohl’s notch experiments on stems. The objection 
urged against his experiments, that the cutting itself acts as a 
stimulus, does not hold with roots, for as the tip alone is sensi- 
tive, cuts made back of the sensitive part do not appreciably 
stimulate it. 
The method consisted in cutting off certain parts of the root 
before branding, and ascertaining whether the stimulus was 
transmitted in the part that was left, past the cut, or, to put 
it in another way, whether the root curved both above and 
below the cut. 
In one set of experiments the root was cut with a sharp, thin 
bladed razor, ina plane exactly at right angles to the long axis, 
generally at a distance of from 2-3™" from the tip, though 
Sometimes from 3-4™™ from the tip. It was very noticeable 
that the nearer to the tip the cut was made the larger the number 
of responses to the stimulus in which the curve was on both 
sides of the cut or gash. In many of those in which the curve 
