New combe . — The Sensory Zone of Roots. 44.1 
the rheotropic stimulus at least 8 mm. back of the limit of 
the elongating zone. 
Zea Mays , L. (the variety commonly called white popcorn ), 
unlike its near relatives, the sweet and the field corn> produces 
mostly straight roots in water, and shows a good degree of 
rheotropic sensitiveness. In seventy-nine seedlings whose 
naked roots were subjected to a stream of water, 72 per cent, 
showed positive curves. This plant was selected therefore as 
promising results by the employment of glass-tubes. In the 
experiments with 15 mm. of the apex of the root shielded 
from the water-stream, so many roots responded, that it was 
thought unnecessary to experiment with but 10 mm. of the 
apex covered. 
The roots of this plant are so large, that larger glass-tubes 
were used, viz. tubes of 4 mm. internal diameter. 
Table V. Zea Mays (white popcorn) with root-tips 
shielded from the water-stream by glass-tubes. Temperature 
of the water =24° ; duration of the experiments = 10 hours to 
12 hours ; velocities = from 200 cm. to 700 cm. per minute. 
No. 
Seedlings. 
Extent of 
Apex shielded. 
Positive Curves. 
Neutral Roots. 
Negative Curves. 
30 
15 mm. 
23 = 76% 
5 
2 
3° 
20 mm. 
0 
II 
c* 
8 
1 
3° 
25 mm. 
11 = 37% 
13 
6 
These ninety seedlings were used in nine sets, each set 
having ten seedlings. The results for the various sets were 
remarkably consistent, and each group of ten showed about 
the same proportionate number of curves and neutral roots 
as shown by the total of its class in the table. 
Measured with the horizontal microscope as they were 
growing in water, one seedling of Zea (popcorn) showed the 
elongating zone of its root to be confined to the first 8 mm., 
three showed growth in the first 9 mm., and a fifth showed 
growth in the first 10 mm. 10 mm. may be assumed as the 
