436 Newcombe. — The Sensory Zone of Roots. 
from the light also at all times, except for the few minutes 
when observations were being made. 
Table II. Brassica alba with root-tips shielded from 
water-stream by glass-tubes. Temperature of the water = 
22° to 24 0 ; duration of experiments = from 7-5 to 11-75 hours ; 
velocities = from 200 cm. to 700 cm. per minute. 
No. 
Seedlings. 
Extent of 
Apex shielded. 
Positive Cu-f'ves. 
Neutral Roots. 
Negative Curves. 
16 
10 mm. 
T2 = 75 % 
4 
0 
33 
15 mm. 
28 = 85% 
4 
1 
18 
20 mm. 
9 = 50 % 
9 
0 
22 
25 mm. 
0 
LQ 
II 
9 
2 
This summary, if it teaches what it seems to, is remarkable 
in showing that this plant is sensitive rheotropicaily at a 
place at least 25 mm. distant from the apex of the root. 
One must not, however, put too much faith in summaries. 
Let us examine the individual experiments to see whether 
they tend toward a constant result : there were three sets of 
seedlings prepared and carried through the experiment with 
25 mm. of the apex shielded. One with ten seedlings gave 
seven positive curves, one with nine seedlings gave four 
positive curves, and one with three gave one positive curve. 
None of the curves counted were less than io°, and the average 
was 22 0 . Inasmuch as the roots did not become contorted, 
and there were only two negative bends, I see no escape 
from the conclusion that the most of these positive curves were 
caused by the water-stream. Should we consider the experi- 
ments in which 20 mm. of the apex were covered by the tube, 
we should find the same relations. When, however, we 
consider the cases in which only 15 mm. of the apex were 
shielded by the tube, even the most sceptical must admit the 
evidence. The total of thirty-three seedlings used were 
distributed in six experiments, and in every experiment two- 
thirds or more of the roots responded with positive curves. 
